7023069: G1: Introduce symmetric locking in the slow allocation path

7023151: G1: refactor the code that operates on _cur_alloc_region to be re-used for allocs by the GC threads
7018286: G1: humongous allocation attempts should take the GC locker into account
Summary: First, this change replaces the asymmetric locking scheme in the G1 slow alloc path by a summetric one. Second, it factors out the code that operates on _cur_alloc_region so that it can be re-used for allocations by the GC threads in the future.
Reviewed-by: stefank, brutisso, johnc

src/share/vm/gc_implementation/g1/g1AllocRegion.cpp

+/*
+ * Copyright (c) 2011, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#include "precompiled.hpp"
+#include "gc_implementation/g1/g1AllocRegion.inline.hpp"
+#include "gc_implementation/g1/g1CollectedHeap.inline.hpp"
+
+G1CollectedHeap* G1AllocRegion::_g1h = NULL;
+HeapRegion* G1AllocRegion::_dummy_region = NULL;
+
+void G1AllocRegion::setup(G1CollectedHeap* g1h, HeapRegion* dummy_region) {
+  assert(_dummy_region == NULL, "should be set once");
+  assert(dummy_region != NULL, "pre-condition");
+  assert(dummy_region->free() == 0, "pre-condition");
+
+  // Make sure that any allocation attempt on this region will fail
+  // and will not trigger any asserts.
+  assert(allocate(dummy_region, 1, false) == NULL, "should fail");
+  assert(par_allocate(dummy_region, 1, false) == NULL, "should fail");
+  assert(allocate(dummy_region, 1, true) == NULL, "should fail");
+  assert(par_allocate(dummy_region, 1, true) == NULL, "should fail");
+
+  _g1h = g1h;
+  _dummy_region = dummy_region;
+}
+
+void G1AllocRegion::fill_up_remaining_space(HeapRegion* alloc_region,
+                                            bool bot_updates) {
+  assert(alloc_region != NULL && alloc_region != _dummy_region,
+         "pre-condition");
+
+  // Other threads might still be trying to allocate using a CAS out
+  // of the region we are trying to retire, as they can do so without
+  // holding the lock. So, we first have to make sure that noone else
+  // can allocate out of it by doing a maximal allocation. Even if our
+  // CAS attempt fails a few times, we'll succeed sooner or later
+  // given that failed CAS attempts mean that the region is getting
+  // closed to being full.
+  size_t free_word_size = alloc_region->free() / HeapWordSize;
+
+  // This is the minimum free chunk we can turn into a dummy
+  // object. If the free space falls below this, then noone can
+  // allocate in this region anyway (all allocation requests will be
+  // of a size larger than this) so we won't have to perform the dummy
+  // allocation.
+  size_t min_word_size_to_fill = CollectedHeap::min_fill_size();
+
+  while (free_word_size >= min_word_size_to_fill) {
+    HeapWord* dummy = par_allocate(alloc_region, free_word_size, bot_updates);
+    if (dummy != NULL) {
+      // If the allocation was successful we should fill in the space.
+      CollectedHeap::fill_with_object(dummy, free_word_size);
+      alloc_region->set_pre_dummy_top(dummy);
+      break;
+    }
+
+    free_word_size = alloc_region->free() / HeapWordSize;
+    // It's also possible that someone else beats us to the
+    // allocation and they fill up the region. In that case, we can
+    // just get out of the loop.
+  }
+  assert(alloc_region->free() / HeapWordSize < min_word_size_to_fill,
+         "post-condition");
+}
+
+void G1AllocRegion::retire(bool fill_up) {
+  assert(_alloc_region != NULL, ar_ext_msg(this, "not initialized properly"));
+
+  trace("retiring");
+  HeapRegion* alloc_region = _alloc_region;
+  if (alloc_region != _dummy_region) {
+    // We never have to check whether the active region is empty or not,
+    // and potentially free it if it is, given that it's guaranteed that
+    // it will never be empty.
+    assert(!alloc_region->is_empty(),
+           ar_ext_msg(this, "the alloc region should never be empty"));
+
+    if (fill_up) {
+      fill_up_remaining_space(alloc_region, _bot_updates);
+    }
+
+    assert(alloc_region->used() >= _used_bytes_before,
+           ar_ext_msg(this, "invariant"));
+    size_t allocated_bytes = alloc_region->used() - _used_bytes_before;
+    retire_region(alloc_region, allocated_bytes);
+    _used_bytes_before = 0;
+    _alloc_region = _dummy_region;
+  }
+  trace("retired");
+}
+
+HeapWord* G1AllocRegion::new_alloc_region_and_allocate(size_t word_size,
+                                                       bool force) {
+  assert(_alloc_region == _dummy_region, ar_ext_msg(this, "pre-condition"));
+  assert(_used_bytes_before == 0, ar_ext_msg(this, "pre-condition"));
+
+  trace("attempting region allocation");
+  HeapRegion* new_alloc_region = allocate_new_region(word_size, force);
+  if (new_alloc_region != NULL) {
+    new_alloc_region->reset_pre_dummy_top();
+    // Need to do this before the allocation
+    _used_bytes_before = new_alloc_region->used();
+    HeapWord* result = allocate(new_alloc_region, word_size, _bot_updates);
+    assert(result != NULL, ar_ext_msg(this, "the allocation should succeeded"));
+
+    OrderAccess::storestore();
+    // Note that we first perform the allocation and then we store the
+    // region in _alloc_region. This is the reason why an active region
+    // can never be empty.
+    _alloc_region = new_alloc_region;
+    trace("region allocation successful");
+    return result;
+  } else {
+    trace("region allocation failed");
+    return NULL;
+  }
+  ShouldNotReachHere();
+}
+
+void G1AllocRegion::fill_in_ext_msg(ar_ext_msg* msg, const char* message) {
+  msg->append("[%s] %s b: %s r: "PTR_FORMAT" u: "SIZE_FORMAT,
+              _name, message, BOOL_TO_STR(_bot_updates),
+              _alloc_region, _used_bytes_before);
+}
+
+void G1AllocRegion::init() {
+  trace("initializing");
+  assert(_alloc_region == NULL && _used_bytes_before == 0,
+         ar_ext_msg(this, "pre-condition"));
+  assert(_dummy_region != NULL, "should have been set");
+  _alloc_region = _dummy_region;
+  trace("initialized");
+}
+
+HeapRegion* G1AllocRegion::release() {
+  trace("releasing");
+  HeapRegion* alloc_region = _alloc_region;
+  retire(false /* fill_up */);
+  assert(_alloc_region == _dummy_region, "post-condition of retire()");
+  _alloc_region = NULL;
+  trace("released");
+  return (alloc_region == _dummy_region) ? NULL : alloc_region;
+}
+
+#if G1_ALLOC_REGION_TRACING
+void G1AllocRegion::trace(const char* str, size_t word_size, HeapWord* result) {
+  // All the calls to trace that set either just the size or the size
+  // and the result are considered part of level 2 tracing and are
+  // skipped during level 1 tracing.
+  if ((word_size == 0 && result == NULL) || (G1_ALLOC_REGION_TRACING > 1)) {
+    const size_t buffer_length = 128;
+    char hr_buffer[buffer_length];
+    char rest_buffer[buffer_length];
+
+    HeapRegion* alloc_region = _alloc_region;
+    if (alloc_region == NULL) {
+      jio_snprintf(hr_buffer, buffer_length, "NULL");
+    } else if (alloc_region == _dummy_region) {
+      jio_snprintf(hr_buffer, buffer_length, "DUMMY");
+    } else {
+      jio_snprintf(hr_buffer, buffer_length,
+                   HR_FORMAT, HR_FORMAT_PARAMS(alloc_region));
+    }
+
+    if (G1_ALLOC_REGION_TRACING > 1) {
+      if (result != NULL) {
+        jio_snprintf(rest_buffer, buffer_length, SIZE_FORMAT" "PTR_FORMAT,
+                     word_size, result);
+      } else if (word_size != 0) {
+        jio_snprintf(rest_buffer, buffer_length, SIZE_FORMAT, word_size);
+      } else {
+        jio_snprintf(rest_buffer, buffer_length, "");
+      }
+    } else {
+      jio_snprintf(rest_buffer, buffer_length, "");
+    }
+
+    tty->print_cr("[%s] %s : %s %s", _name, hr_buffer, str, rest_buffer);
+  }
+}
+#endif // G1_ALLOC_REGION_TRACING
+
+G1AllocRegion::G1AllocRegion(const char* name,
+                             bool bot_updates)
+  : _name(name), _bot_updates(bot_updates),
+    _alloc_region(NULL), _used_bytes_before(0) { }
+

src/share/vm/gc_implementation/g1/g1AllocRegion.hpp

+/*
+ * Copyright (c) 2011, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1ALLOCREGION_HPP
+#define SHARE_VM_GC_IMPLEMENTATION_G1_G1ALLOCREGION_HPP
+
+#include "gc_implementation/g1/heapRegion.hpp"
+
+class G1CollectedHeap;
+
+// 0 -> no tracing, 1 -> basic tracing, 2 -> basic + allocation tracing
+#define G1_ALLOC_REGION_TRACING 0
+
+class ar_ext_msg;
+
+// A class that holds a region that is active in satisfying allocation
+// requests, potentially issued in parallel. When the active region is
+// full it will be retired it replaced with a new one. The
+// implementation assumes that fast-path allocations will be lock-free
+// and a lock will need to be taken when the active region needs to be
+// replaced.
+
+class G1AllocRegion VALUE_OBJ_CLASS_SPEC {
+  friend class ar_ext_msg;
+
+private:
+  // The active allocating region we are currently allocating out
+  // of. The invariant is that if this object is initialized (i.e.,
+  // init() has been called and release() has not) then _alloc_region
+  // is either an active allocating region or the dummy region (i.e.,
+  // it can never be NULL) and this object can be used to satisfy
+  // allocation requests. If this object is not initialized
+  // (i.e. init() has not been called or release() has been called)
+  // then _alloc_region is NULL and this object should not be used to
+  // satisfy allocation requests (it was done this way to force the
+  // correct use of init() and release()).
+  HeapRegion* _alloc_region;
+
+  // When we set up a new active region we save its used bytes in this
+  // field so that, when we retire it, we can calculate how much space
+  // we allocated in it.
+  size_t _used_bytes_before;
+
+  // Specifies whether the allocate calls will do BOT updates or not.
+  bool _bot_updates;
+
+  // Useful for debugging and tracing.
+  const char* _name;
+
+  // A dummy region (i.e., it's been allocated specially for this
+  // purpose and it is not part of the heap) that is full (i.e., top()
+  // == end()). When we don't have a valid active region we make
+  // _alloc_region point to this. This allows us to skip checking
+  // whether the _alloc_region is NULL or not.
+  static HeapRegion* _dummy_region;
+
+  // Some of the methods below take a bot_updates parameter. Its value
+  // should be the same as the _bot_updates field. The idea is that
+  // the parameter will be a constant for a particular alloc region
+  // and, given that these methods will be hopefully inlined, the
+  // compiler should compile out the test.
+
+  // Perform a non-MT-safe allocation out of the given region.
+  static inline HeapWord* allocate(HeapRegion* alloc_region,
+                                   size_t word_size,
+                                   bool bot_updates);
+
+  // Perform a MT-safe allocation out of the given region.
+  static inline HeapWord* par_allocate(HeapRegion* alloc_region,
+                                       size_t word_size,
+                                       bool bot_updates);
+
+  // Ensure that the region passed as a parameter has been filled up
+  // so that noone else can allocate out of it any more.
+  static void fill_up_remaining_space(HeapRegion* alloc_region,
+                                      bool bot_updates);
+
+  // Retire the active allocating region. If fill_up is true then make
+  // sure that the region is full before we retire it so that noone
+  // else can allocate out of it.
+  void retire(bool fill_up);
+
+  // Allocate a new active region and use it to perform a word_size
+  // allocation. The force parameter will be passed on to
+  // G1CollectedHeap::allocate_new_alloc_region() and tells it to try
+  // to allocate a new region even if the max has been reached.
+  HeapWord* new_alloc_region_and_allocate(size_t word_size, bool force);
+
+  void fill_in_ext_msg(ar_ext_msg* msg, const char* message);
+
+protected:
+  // For convenience as subclasses use it.
+  static G1CollectedHeap* _g1h;
+
+  virtual HeapRegion* allocate_new_region(size_t word_size, bool force) = 0;
+  virtual void retire_region(HeapRegion* alloc_region,
+                             size_t allocated_bytes) = 0;
+
+  G1AllocRegion(const char* name, bool bot_updates);
+
+public:
+  static void setup(G1CollectedHeap* g1h, HeapRegion* dummy_region);
+
+  HeapRegion* get() const {
+    // Make sure that the dummy region does not escape this class.
+    return (_alloc_region == _dummy_region) ? NULL : _alloc_region;
+  }
+
+  // The following two are the building blocks for the allocation method.
+
+  // First-level allocation: Should be called without holding a
+  // lock. It will try to allocate lock-free out of the active region,
+  // or return NULL if it was unable to.
+  inline HeapWord* attempt_allocation(size_t word_size, bool bot_updates);
+
+  // Second-level allocation: Should be called while holding a
+  // lock. It will try to first allocate lock-free out of the active
+  // region or, if it's unable to, it will try to replace the active
+  // alloc region with a new one. We require that the caller takes the
+  // appropriate lock before calling this so that it is easier to make
+  // it conform to its locking protocol.
+  inline HeapWord* attempt_allocation_locked(size_t word_size,
+                                             bool bot_updates);
+
+  // Should be called to allocate a new region even if the max of this
+  // type of regions has been reached. Should only be called if other
+  // allocation attempts have failed and we are not holding a valid
+  // active region.
+  inline HeapWord* attempt_allocation_force(size_t word_size,
+                                            bool bot_updates);
+
+  // Should be called before we start using this object.
+  void init();
+
+  // Should be called when we want to release the active region which
+  // is returned after it's been retired.
+  HeapRegion* release();
+
+#if G1_ALLOC_REGION_TRACING
+  void trace(const char* str, size_t word_size = 0, HeapWord* result = NULL);
+#else // G1_ALLOC_REGION_TRACING
+  void trace(const char* str, size_t word_size = 0, HeapWord* result = NULL) { }
+#endif // G1_ALLOC_REGION_TRACING
+};
+
+class ar_ext_msg : public err_msg {
+public:
+  ar_ext_msg(G1AllocRegion* alloc_region, const char *message) : err_msg("") {
+    alloc_region->fill_in_ext_msg(this, message);
+  }
+};
+
+#endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1ALLOCREGION_HPP

src/share/vm/gc_implementation/g1/g1AllocRegion.inline.hpp

+/*
+ * Copyright (c) 2011, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1ALLOCREGION_INLINE_HPP
+#define SHARE_VM_GC_IMPLEMENTATION_G1_G1ALLOCREGION_INLINE_HPP
+
+#include "gc_implementation/g1/g1AllocRegion.hpp"
+
+inline HeapWord* G1AllocRegion::allocate(HeapRegion* alloc_region,
+                                         size_t word_size,
+                                         bool bot_updates) {
+  assert(alloc_region != NULL, err_msg("pre-condition"));
+
+  if (!bot_updates) {
+    return alloc_region->allocate_no_bot_updates(word_size);
+  } else {
+    return alloc_region->allocate(word_size);
+  }
+}
+
+inline HeapWord* G1AllocRegion::par_allocate(HeapRegion* alloc_region,
+                                             size_t word_size,
+                                             bool bot_updates) {
+  assert(alloc_region != NULL, err_msg("pre-condition"));
+  assert(!alloc_region->is_empty(), err_msg("pre-condition"));
+
+  if (!bot_updates) {
+    return alloc_region->par_allocate_no_bot_updates(word_size);
+  } else {
+    return alloc_region->par_allocate(word_size);
+  }
+}
+
+inline HeapWord* G1AllocRegion::attempt_allocation(size_t word_size,
+                                                   bool bot_updates) {
+  assert(bot_updates == _bot_updates, ar_ext_msg(this, "pre-condition"));
+
+  HeapRegion* alloc_region = _alloc_region;
+  assert(alloc_region != NULL, ar_ext_msg(this, "not initialized properly"));
+
+  HeapWord* result = par_allocate(alloc_region, word_size, bot_updates);
+  if (result != NULL) {
+    trace("alloc", word_size, result);
+    return result;
+  }
+  trace("alloc failed", word_size);
+  return NULL;
+}
+
+inline HeapWord* G1AllocRegion::attempt_allocation_locked(size_t word_size,
+                                                          bool bot_updates) {
+  // First we have to tedo the allocation, assuming we're holding the
+  // appropriate lock, in case another thread changed the region while
+  // we were waiting to get the lock.
+  HeapWord* result = attempt_allocation(word_size, bot_updates);
+  if (result != NULL) {
+    return result;
+  }
+
+  retire(true /* fill_up */);
+  result = new_alloc_region_and_allocate(word_size, false /* force */);
+  if (result != NULL) {
+    trace("alloc locked (second attempt)", word_size, result);
+    return result;
+  }
+  trace("alloc locked failed", word_size);
+  return NULL;
+}
+
+inline HeapWord* G1AllocRegion::attempt_allocation_force(size_t word_size,
+                                                         bool bot_updates) {
+  assert(bot_updates == _bot_updates, ar_ext_msg(this, "pre-condition"));
+  assert(_alloc_region != NULL, ar_ext_msg(this, "not initialized properly"));
+
+  trace("forcing alloc");
+  HeapWord* result = new_alloc_region_and_allocate(word_size, true /* force */);
+  if (result != NULL) {
+    trace("alloc forced", word_size, result);
+    return result;
+  }
+  trace("alloc forced failed", word_size);
+  return NULL;
+}
+
+#endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1ALLOCREGION_INLINE_HPP

src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp

@@ -28,6 +28,7 @@
 #include "gc_implementation/g1/concurrentG1Refine.hpp"
 #include "gc_implementation/g1/concurrentG1RefineThread.hpp"
 #include "gc_implementation/g1/concurrentMarkThread.inline.hpp"
+#include "gc_implementation/g1/g1AllocRegion.inline.hpp"
 #include "gc_implementation/g1/g1CollectedHeap.inline.hpp"
 #include "gc_implementation/g1/g1CollectorPolicy.hpp"
 #include "gc_implementation/g1/g1MarkSweep.hpp"
@@ -517,8 +518,7 @@ G1CollectedHeap::new_region_try_secondary_free_list() {
   return NULL;
 }
 
-HeapRegion* G1CollectedHeap::new_region_work(size_t word_size,
-                                             bool do_expand) {
+HeapRegion* G1CollectedHeap::new_region(size_t word_size, bool do_expand) {
   assert(!isHumongous(word_size) ||
                                   word_size <= (size_t) HeapRegion::GrainWords,
          "the only time we use this to allocate a humongous region is "
@@ -566,7 +566,7 @@ HeapRegion* G1CollectedHeap::new_gc_alloc_region(int purpose,
                                                  size_t word_size) {
   HeapRegion* alloc_region = NULL;
   if (_gc_alloc_region_counts[purpose] < g1_policy()->max_regions(purpose)) {
-    alloc_region = new_region_work(word_size, true /* do_expand */);
+    alloc_region = new_region(word_size, true /* do_expand */);
     if (purpose == GCAllocForSurvived && alloc_region != NULL) {
       alloc_region->set_survivor();
     }
@@ -587,7 +587,7 @@ int G1CollectedHeap::humongous_obj_allocate_find_first(size_t num_regions,
     // Only one region to allocate, no need to go through the slower
     // path. The caller will attempt the expasion if this fails, so
     // let's not try to expand here too.
-    HeapRegion* hr = new_region_work(word_size, false /* do_expand */);
+    HeapRegion* hr = new_region(word_size, false /* do_expand */);
     if (hr != NULL) {
       first = hr->hrs_index();
     } else {
@@ -788,508 +788,268 @@ HeapWord* G1CollectedHeap::humongous_obj_allocate(size_t word_size) {
   return result;
 }
 
-void
-G1CollectedHeap::retire_cur_alloc_region(HeapRegion* cur_alloc_region) {
-  // Other threads might still be trying to allocate using CASes out
-  // of the region we are retiring, as they can do so without holding
-  // the Heap_lock. So we first have to make sure that noone else can
-  // allocate in it by doing a maximal allocation. Even if our CAS
-  // attempt fails a few times, we'll succeed sooner or later given
-  // that a failed CAS attempt mean that the region is getting closed
-  // to being full (someone else succeeded in allocating into it).
-  size_t free_word_size = cur_alloc_region->free() / HeapWordSize;
-
-  // This is the minimum free chunk we can turn into a dummy
-  // object. If the free space falls below this, then noone can
-  // allocate in this region anyway (all allocation requests will be
-  // of a size larger than this) so we won't have to perform the dummy
-  // allocation.
-  size_t min_word_size_to_fill = CollectedHeap::min_fill_size();
-
-  while (free_word_size >= min_word_size_to_fill) {
-    HeapWord* dummy =
-      cur_alloc_region->par_allocate_no_bot_updates(free_word_size);
-    if (dummy != NULL) {
-      // If the allocation was successful we should fill in the space.
-      CollectedHeap::fill_with_object(dummy, free_word_size);
-      break;
-    }
-
-    free_word_size = cur_alloc_region->free() / HeapWordSize;
-    // It's also possible that someone else beats us to the
-    // allocation and they fill up the region. In that case, we can
-    // just get out of the loop
-  }
-  assert(cur_alloc_region->free() / HeapWordSize < min_word_size_to_fill,
-         "sanity");
+HeapWord* G1CollectedHeap::allocate_new_tlab(size_t word_size) {
+  assert_heap_not_locked_and_not_at_safepoint();
+  assert(!isHumongous(word_size), "we do not allow humongous TLABs");
 
-  retire_cur_alloc_region_common(cur_alloc_region);
-  assert(_cur_alloc_region == NULL, "post-condition");
+  unsigned int dummy_gc_count_before;
+  return attempt_allocation(word_size, &dummy_gc_count_before);
 }
 
-// See the comment in the .hpp file about the locking protocol and
-// assumptions of this method (and other related ones).
 HeapWord*
-G1CollectedHeap::replace_cur_alloc_region_and_allocate(size_t word_size,
-                                                       bool at_safepoint,
-                                                       bool do_dirtying,
-                                                       bool can_expand) {
-  assert_heap_locked_or_at_safepoint(true /* should_be_vm_thread */);
-  assert(_cur_alloc_region == NULL,
-         "replace_cur_alloc_region_and_allocate() should only be called "
-         "after retiring the previous current alloc region");
-  assert(SafepointSynchronize::is_at_safepoint() == at_safepoint,
-         "at_safepoint and is_at_safepoint() should be a tautology");
-  assert(!can_expand || g1_policy()->can_expand_young_list(),
-         "we should not call this method with can_expand == true if "
-         "we are not allowed to expand the young gen");
-
-  if (can_expand || !g1_policy()->is_young_list_full()) {
-    HeapRegion* new_cur_alloc_region = new_alloc_region(word_size);
-    if (new_cur_alloc_region != NULL) {
-      assert(new_cur_alloc_region->is_empty(),
-             "the newly-allocated region should be empty, "
-             "as right now we only allocate new regions out of the free list");
-      g1_policy()->update_region_num(true /* next_is_young */);
-      set_region_short_lived_locked(new_cur_alloc_region);
-
-      assert(!new_cur_alloc_region->isHumongous(),
-             "Catch a regression of this bug.");
-
-      // We need to ensure that the stores to _cur_alloc_region and,
-      // subsequently, to top do not float above the setting of the
-      // young type.
-      OrderAccess::storestore();
-
-      // Now, perform the allocation out of the region we just
-      // allocated. Note that noone else can access that region at
-      // this point (as _cur_alloc_region has not been updated yet),
-      // so we can just go ahead and do the allocation without any
-      // atomics (and we expect this allocation attempt to
-      // suceeded). Given that other threads can attempt an allocation
-      // with a CAS and without needing the Heap_lock, if we assigned
-      // the new region to _cur_alloc_region before first allocating
-      // into it other threads might have filled up the new region
-      // before we got a chance to do the allocation ourselves. In
-      // that case, we would have needed to retire the region, grab a
-      // new one, and go through all this again. Allocating out of the
-      // new region before assigning it to _cur_alloc_region avoids
-      // all this.
-      HeapWord* result =
-                     new_cur_alloc_region->allocate_no_bot_updates(word_size);
-      assert(result != NULL, "we just allocate out of an empty region "
-             "so allocation should have been successful");
-      assert(is_in(result), "result should be in the heap");
-
-      // Now make sure that the store to _cur_alloc_region does not
-      // float above the store to top.
-      OrderAccess::storestore();
-      _cur_alloc_region = new_cur_alloc_region;
+G1CollectedHeap::mem_allocate(size_t word_size,
+                              bool   is_noref,
+                              bool   is_tlab,
+                              bool*  gc_overhead_limit_was_exceeded) {
+  assert_heap_not_locked_and_not_at_safepoint();
+  assert(!is_tlab, "mem_allocate() this should not be called directly "
+         "to allocate TLABs");
 
-      if (!at_safepoint) {
-        Heap_lock->unlock();
-      }
+  // Loop until the allocation is satisified, or unsatisfied after GC.
+  for (int try_count = 1; /* we'll return */; try_count += 1) {
+    unsigned int gc_count_before;
 
-      // do the dirtying, if necessary, after we release the Heap_lock
-      if (do_dirtying) {
-        dirty_young_block(result, word_size);
-      }
-      return result;
-    }
+    HeapWord* result = NULL;
+    if (!isHumongous(word_size)) {
+      result = attempt_allocation(word_size, &gc_count_before);
+    } else {
+      result = attempt_allocation_humongous(word_size, &gc_count_before);
     }
-
-  assert(_cur_alloc_region == NULL, "we failed to allocate a new current "
-         "alloc region, it should still be NULL");
-  assert_heap_locked_or_at_safepoint(true /* should_be_vm_thread */);
-  return NULL;
-}
-
-// See the comment in the .hpp file about the locking protocol and
-// assumptions of this method (and other related ones).
-HeapWord*
-G1CollectedHeap::attempt_allocation_slow(size_t word_size) {
-  assert_heap_locked_and_not_at_safepoint();
-  assert(!isHumongous(word_size), "attempt_allocation_slow() should not be "
-         "used for humongous allocations");
-
-  // We should only reach here when we were unable to allocate
-  // otherwise. So, we should have not active current alloc region.
-  assert(_cur_alloc_region == NULL, "current alloc region should be NULL");
-
-  // We will loop while succeeded is false, which means that we tried
-  // to do a collection, but the VM op did not succeed. So, when we
-  // exit the loop, either one of the allocation attempts was
-  // successful, or we succeeded in doing the VM op but which was
-  // unable to allocate after the collection.
-  for (int try_count = 1; /* we'll return or break */; try_count += 1) {
-    bool succeeded = true;
-
-    // Every time we go round the loop we should be holding the Heap_lock.
-    assert_heap_locked();
-
-    if (GC_locker::is_active_and_needs_gc()) {
-      // We are locked out of GC because of the GC locker. We can
-      // allocate a new region only if we can expand the young gen.
-
-      if (g1_policy()->can_expand_young_list()) {
-        // Yes, we are allowed to expand the young gen. Let's try to
-        // allocate a new current alloc region.
-        HeapWord* result =
-          replace_cur_alloc_region_and_allocate(word_size,
-                                                false, /* at_safepoint */
-                                                true,  /* do_dirtying */
-                                                true   /* can_expand */);
     if (result != NULL) {
-          assert_heap_not_locked();
       return result;
     }
-      }
-      // We could not expand the young gen further (or we could but we
-      // failed to allocate a new region). We'll stall until the GC
-      // locker forces a GC.
-
-      // If this thread is not in a jni critical section, we stall
-      // the requestor until the critical section has cleared and
-      // GC allowed. When the critical section clears, a GC is
-      // initiated by the last thread exiting the critical section; so
-      // we retry the allocation sequence from the beginning of the loop,
-      // rather than causing more, now probably unnecessary, GC attempts.
-      JavaThread* jthr = JavaThread::current();
-      assert(jthr != NULL, "sanity");
-      if (jthr->in_critical()) {
-        if (CheckJNICalls) {
-          fatal("Possible deadlock due to allocating while"
-                " in jni critical section");
-        }
-        // We are returning NULL so the protocol is that we're still
-        // holding the Heap_lock.
-        assert_heap_locked();
-        return NULL;
-      }
-
-      Heap_lock->unlock();
-      GC_locker::stall_until_clear();
 
-      // No need to relock the Heap_lock. We'll fall off to the code
-      // below the else-statement which assumes that we are not
-      // holding the Heap_lock.
-    } else {
-      // We are not locked out. So, let's try to do a GC. The VM op
-      // will retry the allocation before it completes.
-
-      // Read the GC count while holding the Heap_lock
-      unsigned int gc_count_before = SharedHeap::heap()->total_collections();
-
-      Heap_lock->unlock();
-
-      HeapWord* result =
-        do_collection_pause(word_size, gc_count_before, &succeeded);
-      assert_heap_not_locked();
-      if (result != NULL) {
-        assert(succeeded, "the VM op should have succeeded");
+    // Create the garbage collection operation...
+    VM_G1CollectForAllocation op(gc_count_before, word_size);
+    // ...and get the VM thread to execute it.
+    VMThread::execute(&op);
 
+    if (op.prologue_succeeded() && op.pause_succeeded()) {
+      // If the operation was successful we'll return the result even
+      // if it is NULL. If the allocation attempt failed immediately
+      // after a Full GC, it's unlikely we'll be able to allocate now.
+      HeapWord* result = op.result();
+      if (result != NULL && !isHumongous(word_size)) {
         // Allocations that take place on VM operations do not do any
-        // card dirtying and we have to do it here.
+        // card dirtying and we have to do it here. We only have to do
+        // this for non-humongous allocations, though.
         dirty_young_block(result, word_size);
-        return result;
       }
-    }
-
-    // Both paths that get us here from above unlock the Heap_lock.
-    assert_heap_not_locked();
-
-    // We can reach here when we were unsuccessful in doing a GC,
-    // because another thread beat us to it, or because we were locked
-    // out of GC due to the GC locker. In either case a new alloc
-    // region might be available so we will retry the allocation.
-    HeapWord* result = attempt_allocation(word_size);
-    if (result != NULL) {
-      assert_heap_not_locked();
       return result;
-    }
-
-    // So far our attempts to allocate failed. The only time we'll go
-    // around the loop and try again is if we tried to do a GC and the
-    // VM op that we tried to schedule was not successful because
-    // another thread beat us to it. If that happened it's possible
-    // that by the time we grabbed the Heap_lock again and tried to
-    // allocate other threads filled up the young generation, which
-    // means that the allocation attempt after the GC also failed. So,
-    // it's worth trying to schedule another GC pause.
-    if (succeeded) {
-      break;
+    } else {
+      assert(op.result() == NULL,
+             "the result should be NULL if the VM op did not succeed");
     }
 
     // Give a warning if we seem to be looping forever.
     if ((QueuedAllocationWarningCount > 0) &&
         (try_count % QueuedAllocationWarningCount == 0)) {
-      warning("G1CollectedHeap::attempt_allocation_slow() "
-              "retries %d times", try_count);
+      warning("G1CollectedHeap::mem_allocate retries %d times", try_count);
     }
   }
 
-  assert_heap_locked();
+  ShouldNotReachHere();
   return NULL;
 }
 
-// See the comment in the .hpp file about the locking protocol and
-// assumptions of this method (and other related ones).
-HeapWord*
-G1CollectedHeap::attempt_allocation_humongous(size_t word_size,
-                                              bool at_safepoint) {
-  // This is the method that will allocate a humongous object. All
-  // allocation paths that attempt to allocate a humongous object
-  // should eventually reach here. Currently, the only paths are from
-  // mem_allocate() and attempt_allocation_at_safepoint().
-  assert_heap_locked_or_at_safepoint(true /* should_be_vm_thread */);
-  assert(isHumongous(word_size), "attempt_allocation_humongous() "
-         "should only be used for humongous allocations");
-  assert(SafepointSynchronize::is_at_safepoint() == at_safepoint,
-         "at_safepoint and is_at_safepoint() should be a tautology");
+HeapWord* G1CollectedHeap::attempt_allocation_slow(size_t word_size,
+                                           unsigned int *gc_count_before_ret) {
+  // Make sure you read the note in attempt_allocation_humongous().
 
+  assert_heap_not_locked_and_not_at_safepoint();
+  assert(!isHumongous(word_size), "attempt_allocation_slow() should not "
+         "be called for humongous allocation requests");
+
+  // We should only get here after the first-level allocation attempt
+  // (attempt_allocation()) failed to allocate.
+
+  // We will loop until a) we manage to successfully perform the
+  // allocation or b) we successfully schedule a collection which
+  // fails to perform the allocation. b) is the only case when we'll
+  // return NULL.
   HeapWord* result = NULL;
+  for (int try_count = 1; /* we'll return */; try_count += 1) {
+    bool should_try_gc;
+    unsigned int gc_count_before;
 
-  // We will loop while succeeded is false, which means that we tried
-  // to do a collection, but the VM op did not succeed. So, when we
-  // exit the loop, either one of the allocation attempts was
-  // successful, or we succeeded in doing the VM op but which was
-  // unable to allocate after the collection.
-  for (int try_count = 1; /* we'll return or break */; try_count += 1) {
-    bool succeeded = true;
+    {
+      MutexLockerEx x(Heap_lock);
 
-    // Given that humongous objects are not allocated in young
-    // regions, we'll first try to do the allocation without doing a
-    // collection hoping that there's enough space in the heap.
-    result = humongous_obj_allocate(word_size);
-    assert(_cur_alloc_region == NULL || !_cur_alloc_region->isHumongous(),
-           "catch a regression of this bug.");
+      result = _mutator_alloc_region.attempt_allocation_locked(word_size,
+                                                      false /* bot_updates */);
       if (result != NULL) {
-      if (!at_safepoint) {
-        // If we're not at a safepoint, unlock the Heap_lock.
-        Heap_lock->unlock();
-      }
         return result;
       }
 
-    // If we failed to allocate the humongous object, we should try to
-    // do a collection pause (if we're allowed) in case it reclaims
-    // enough space for the allocation to succeed after the pause.
-    if (!at_safepoint) {
-      // Read the GC count while holding the Heap_lock
-      unsigned int gc_count_before = SharedHeap::heap()->total_collections();
+      // If we reach here, attempt_allocation_locked() above failed to
+      // allocate a new region. So the mutator alloc region should be NULL.
+      assert(_mutator_alloc_region.get() == NULL, "only way to get here");
 
-      // If we're allowed to do a collection we're not at a
-      // safepoint, so it is safe to unlock the Heap_lock.
-      Heap_lock->unlock();
-
-      result = do_collection_pause(word_size, gc_count_before, &succeeded);
-      assert_heap_not_locked();
+      if (GC_locker::is_active_and_needs_gc()) {
+        if (g1_policy()->can_expand_young_list()) {
+          result = _mutator_alloc_region.attempt_allocation_force(word_size,
+                                                      false /* bot_updates */);
           if (result != NULL) {
-        assert(succeeded, "the VM op should have succeeded");
             return result;
           }
-
-      // If we get here, the VM operation either did not succeed
-      // (i.e., another thread beat us to it) or it succeeded but
-      // failed to allocate the object.
-
-      // If we're allowed to do a collection we're not at a
-      // safepoint, so it is safe to lock the Heap_lock.
-      Heap_lock->lock();
         }
-
-    assert(result == NULL, "otherwise we should have exited the loop earlier");
-
-    // So far our attempts to allocate failed. The only time we'll go
-    // around the loop and try again is if we tried to do a GC and the
-    // VM op that we tried to schedule was not successful because
-    // another thread beat us to it. That way it's possible that some
-    // space was freed up by the thread that successfully scheduled a
-    // GC. So it's worth trying to allocate again.
-    if (succeeded) {
-      break;
-    }
-
-    // Give a warning if we seem to be looping forever.
-    if ((QueuedAllocationWarningCount > 0) &&
-        (try_count % QueuedAllocationWarningCount == 0)) {
-      warning("G1CollectedHeap::attempt_allocation_humongous "
-              "retries %d times", try_count);
+        should_try_gc = false;
+      } else {
+        // Read the GC count while still holding the Heap_lock.
+        gc_count_before = SharedHeap::heap()->total_collections();
+        should_try_gc = true;
       }
     }
 
-  assert_heap_locked_or_at_safepoint(true /* should_be_vm_thread */);
-  return NULL;
-}
-
-HeapWord* G1CollectedHeap::attempt_allocation_at_safepoint(size_t word_size,
-                                           bool expect_null_cur_alloc_region) {
-  assert_at_safepoint(true /* should_be_vm_thread */);
-  assert(_cur_alloc_region == NULL || !expect_null_cur_alloc_region,
-         err_msg("the current alloc region was unexpectedly found "
-                 "to be non-NULL, cur alloc region: "PTR_FORMAT" "
-                 "expect_null_cur_alloc_region: %d word_size: "SIZE_FORMAT,
-                 _cur_alloc_region, expect_null_cur_alloc_region, word_size));
-
-  if (!isHumongous(word_size)) {
-    if (!expect_null_cur_alloc_region) {
-      HeapRegion* cur_alloc_region = _cur_alloc_region;
-      if (cur_alloc_region != NULL) {
-        // We are at a safepoint so no reason to use the MT-safe version.
-        HeapWord* result = cur_alloc_region->allocate_no_bot_updates(word_size);
+    if (should_try_gc) {
+      bool succeeded;
+      result = do_collection_pause(word_size, gc_count_before, &succeeded);
       if (result != NULL) {
-          assert(is_in(result), "result should be in the heap");
-
-          // We will not do any dirtying here. This is guaranteed to be
-          // called during a safepoint and the thread that scheduled the
-          // pause will do the dirtying if we return a non-NULL result.
+        assert(succeeded, "only way to get back a non-NULL result");
         return result;
       }
 
-        retire_cur_alloc_region_common(cur_alloc_region);
-      }
+      if (succeeded) {
+        // If we get here we successfully scheduled a collection which
+        // failed to allocate. No point in trying to allocate
+        // further. We'll just return NULL.
+        MutexLockerEx x(Heap_lock);
+        *gc_count_before_ret = SharedHeap::heap()->total_collections();
+        return NULL;
       }
-
-    assert(_cur_alloc_region == NULL,
-           "at this point we should have no cur alloc region");
-    return replace_cur_alloc_region_and_allocate(word_size,
-                                                 true, /* at_safepoint */
-                                                 false /* do_dirtying */,
-                                                 false /* can_expand */);
     } else {
-    return attempt_allocation_humongous(word_size,
-                                        true /* at_safepoint */);
+      GC_locker::stall_until_clear();
     }
 
-  ShouldNotReachHere();
-}
-
-HeapWord* G1CollectedHeap::allocate_new_tlab(size_t word_size) {
-  assert_heap_not_locked_and_not_at_safepoint();
-  assert(!isHumongous(word_size), "we do not allow TLABs of humongous size");
-
-  // First attempt: Try allocating out of the current alloc region
-  // using a CAS. If that fails, take the Heap_lock and retry the
-  // allocation, potentially replacing the current alloc region.
-  HeapWord* result = attempt_allocation(word_size);
-  if (result != NULL) {
-    assert_heap_not_locked();
+    // We can reach here if we were unsuccessul in scheduling a
+    // collection (because another thread beat us to it) or if we were
+    // stalled due to the GC locker. In either can we should retry the
+    // allocation attempt in case another thread successfully
+    // performed a collection and reclaimed enough space. We do the
+    // first attempt (without holding the Heap_lock) here and the
+    // follow-on attempt will be at the start of the next loop
+    // iteration (after taking the Heap_lock).
+    result = _mutator_alloc_region.attempt_allocation(word_size,
+                                                      false /* bot_updates */);
+    if (result != NULL ){
       return result;
     }
 
-  // Second attempt: Go to the slower path where we might try to
-  // schedule a collection.
-  result = attempt_allocation_slow(word_size);
-  if (result != NULL) {
-    assert_heap_not_locked();
-    return result;
+    // Give a warning if we seem to be looping forever.
+    if ((QueuedAllocationWarningCount > 0) &&
+        (try_count % QueuedAllocationWarningCount == 0)) {
+      warning("G1CollectedHeap::attempt_allocation_slow() "
+              "retries %d times", try_count);
+    }
   }
 
-  assert_heap_locked();
-  // Need to unlock the Heap_lock before returning.
-  Heap_lock->unlock();
+  ShouldNotReachHere();
   return NULL;
 }
 
-HeapWord*
-G1CollectedHeap::mem_allocate(size_t word_size,
-                              bool   is_noref,
-                              bool   is_tlab,
-                              bool*  gc_overhead_limit_was_exceeded) {
+HeapWord* G1CollectedHeap::attempt_allocation_humongous(size_t word_size,
+                                          unsigned int * gc_count_before_ret) {
+  // The structure of this method has a lot of similarities to
+  // attempt_allocation_slow(). The reason these two were not merged
+  // into a single one is that such a method would require several "if
+  // allocation is not humongous do this, otherwise do that"
+  // conditional paths which would obscure its flow. In fact, an early
+  // version of this code did use a unified method which was harder to
+  // follow and, as a result, it had subtle bugs that were hard to
+  // track down. So keeping these two methods separate allows each to
+  // be more readable. It will be good to keep these two in sync as
+  // much as possible.
+
   assert_heap_not_locked_and_not_at_safepoint();
-  assert(!is_tlab, "mem_allocate() this should not be called directly "
-         "to allocate TLABs");
+  assert(isHumongous(word_size), "attempt_allocation_humongous() "
+         "should only be called for humongous allocations");
 
-  // Loop until the allocation is satisified,
-  // or unsatisfied after GC.
+  // We will loop until a) we manage to successfully perform the
+  // allocation or b) we successfully schedule a collection which
+  // fails to perform the allocation. b) is the only case when we'll
+  // return NULL.
+  HeapWord* result = NULL;
   for (int try_count = 1; /* we'll return */; try_count += 1) {
+    bool should_try_gc;
     unsigned int gc_count_before;
-    {
-      if (!isHumongous(word_size)) {
-        // First attempt: Try allocating out of the current alloc region
-        // using a CAS. If that fails, take the Heap_lock and retry the
-        // allocation, potentially replacing the current alloc region.
-        HeapWord* result = attempt_allocation(word_size);
-        if (result != NULL) {
-          assert_heap_not_locked();
-          return result;
-        }
 
-        assert_heap_locked();
+    {
+      MutexLockerEx x(Heap_lock);
 
-        // Second attempt: Go to the slower path where we might try to
-        // schedule a collection.
-        result = attempt_allocation_slow(word_size);
+      // Given that humongous objects are not allocated in young
+      // regions, we'll first try to do the allocation without doing a
+      // collection hoping that there's enough space in the heap.
+      result = humongous_obj_allocate(word_size);
       if (result != NULL) {
-          assert_heap_not_locked();
         return result;
       }
-      } else {
-        // attempt_allocation_humongous() requires the Heap_lock to be held.
-        Heap_lock->lock();
 
-        HeapWord* result = attempt_allocation_humongous(word_size,
-                                                     false /* at_safepoint */);
-        if (result != NULL) {
-          assert_heap_not_locked();
-          return result;
+      if (GC_locker::is_active_and_needs_gc()) {
+        should_try_gc = false;
+      } else {
+        // Read the GC count while still holding the Heap_lock.
+        gc_count_before = SharedHeap::heap()->total_collections();
+        should_try_gc = true;
       }
     }
 
-      assert_heap_locked();
-      // Read the gc count while the heap lock is held.
-      gc_count_before = SharedHeap::heap()->total_collections();
+    if (should_try_gc) {
+      // If we failed to allocate the humongous object, we should try to
+      // do a collection pause (if we're allowed) in case it reclaims
+      // enough space for the allocation to succeed after the pause.
 
-      // Release the Heap_lock before attempting the collection.
-      Heap_lock->unlock();
+      bool succeeded;
+      result = do_collection_pause(word_size, gc_count_before, &succeeded);
+      if (result != NULL) {
+        assert(succeeded, "only way to get back a non-NULL result");
+        return result;
       }
 
-    // Create the garbage collection operation...
-    VM_G1CollectForAllocation op(gc_count_before, word_size);
-    // ...and get the VM thread to execute it.
-    VMThread::execute(&op);
-
-    assert_heap_not_locked();
-    if (op.prologue_succeeded() && op.pause_succeeded()) {
-      // If the operation was successful we'll return the result even
-      // if it is NULL. If the allocation attempt failed immediately
-      // after a Full GC, it's unlikely we'll be able to allocate now.
-      HeapWord* result = op.result();
-      if (result != NULL && !isHumongous(word_size)) {
-        // Allocations that take place on VM operations do not do any
-        // card dirtying and we have to do it here. We only have to do
-        // this for non-humongous allocations, though.
-        dirty_young_block(result, word_size);
+      if (succeeded) {
+        // If we get here we successfully scheduled a collection which
+        // failed to allocate. No point in trying to allocate
+        // further. We'll just return NULL.
+        MutexLockerEx x(Heap_lock);
+        *gc_count_before_ret = SharedHeap::heap()->total_collections();
+        return NULL;
       }
-      return result;
     } else {
-      assert(op.result() == NULL,
-             "the result should be NULL if the VM op did not succeed");
+      GC_locker::stall_until_clear();
     }
 
-    // Give a warning if we seem to be looping forever.
+    // We can reach here if we were unsuccessul in scheduling a
+    // collection (because another thread beat us to it) or if we were
+    // stalled due to the GC locker. In either can we should retry the
+    // allocation attempt in case another thread successfully
+    // performed a collection and reclaimed enough space.  Give a
+    // warning if we seem to be looping forever.
+
     if ((QueuedAllocationWarningCount > 0) &&
         (try_count % QueuedAllocationWarningCount == 0)) {
-      warning("G1CollectedHeap::mem_allocate retries %d times", try_count);
+      warning("G1CollectedHeap::attempt_allocation_humongous() "
+              "retries %d times", try_count);
     }
   }
 
   ShouldNotReachHere();
+  return NULL;
 }
 
-void G1CollectedHeap::abandon_cur_alloc_region() {
+HeapWord* G1CollectedHeap::attempt_allocation_at_safepoint(size_t word_size,
+                                       bool expect_null_mutator_alloc_region) {
   assert_at_safepoint(true /* should_be_vm_thread */);
+  assert(_mutator_alloc_region.get() == NULL ||
+                                             !expect_null_mutator_alloc_region,
+         "the current alloc region was unexpectedly found to be non-NULL");
 
-  HeapRegion* cur_alloc_region = _cur_alloc_region;
-  if (cur_alloc_region != NULL) {
-    assert(!cur_alloc_region->is_empty(),
-           "the current alloc region can never be empty");
-    assert(cur_alloc_region->is_young(),
-           "the current alloc region should be young");
-
-    retire_cur_alloc_region_common(cur_alloc_region);
+  if (!isHumongous(word_size)) {
+    return _mutator_alloc_region.attempt_allocation_locked(word_size,
+                                                      false /* bot_updates */);
+  } else {
+    return humongous_obj_allocate(word_size);
   }
-  assert(_cur_alloc_region == NULL, "post-condition");
+
+  ShouldNotReachHere();
 }
 
 void G1CollectedHeap::abandon_gc_alloc_regions() {
@@ -1417,8 +1177,8 @@ bool G1CollectedHeap::do_collection(bool explicit_gc,
 
     if (VerifyBeforeGC && total_collections() >= VerifyGCStartAt) {
       HandleMark hm;  // Discard invalid handles created during verification
-      prepare_for_verify();
       gclog_or_tty->print(" VerifyBeforeGC:");
+      prepare_for_verify();
       Universe::verify(true);
     }
 
@@ -1439,9 +1199,8 @@ bool G1CollectedHeap::do_collection(bool explicit_gc,
     concurrent_mark()->abort();
 
     // Make sure we'll choose a new allocation region afterwards.
-    abandon_cur_alloc_region();
+    release_mutator_alloc_region();
     abandon_gc_alloc_regions();
-    assert(_cur_alloc_region == NULL, "Invariant.");
     g1_rem_set()->cleanupHRRS();
     tear_down_region_lists();
 
@@ -1547,6 +1306,8 @@ bool G1CollectedHeap::do_collection(bool explicit_gc,
     // evacuation pause.
     clear_cset_fast_test();
 
+    init_mutator_alloc_region();
+
     double end = os::elapsedTime();
     g1_policy()->record_full_collection_end();
 
@@ -1720,8 +1481,9 @@ G1CollectedHeap::satisfy_failed_allocation(size_t word_size,
 
   *succeeded = true;
   // Let's attempt the allocation first.
-  HeapWord* result = attempt_allocation_at_safepoint(word_size,
-                                     false /* expect_null_cur_alloc_region */);
+  HeapWord* result =
+    attempt_allocation_at_safepoint(word_size,
+                                 false /* expect_null_mutator_alloc_region */);
   if (result != NULL) {
     assert(*succeeded, "sanity");
     return result;
@@ -1748,7 +1510,7 @@ G1CollectedHeap::satisfy_failed_allocation(size_t word_size,
 
   // Retry the allocation
   result = attempt_allocation_at_safepoint(word_size,
-                                      true /* expect_null_cur_alloc_region */);
+                                  true /* expect_null_mutator_alloc_region */);
   if (result != NULL) {
     assert(*succeeded, "sanity");
     return result;
@@ -1765,7 +1527,7 @@ G1CollectedHeap::satisfy_failed_allocation(size_t word_size,
 
   // Retry the allocation once more
   result = attempt_allocation_at_safepoint(word_size,
-                                      true /* expect_null_cur_alloc_region */);
+                                  true /* expect_null_mutator_alloc_region */);
   if (result != NULL) {
     assert(*succeeded, "sanity");
     return result;
@@ -1796,7 +1558,7 @@ HeapWord* G1CollectedHeap::expand_and_allocate(size_t word_size) {
   if (expand(expand_bytes)) {
     verify_region_sets_optional();
     return attempt_allocation_at_safepoint(word_size,
-                                          false /* expect_null_cur_alloc_region */);
+                                 false /* expect_null_mutator_alloc_region */);
   }
   return NULL;
 }
@@ -1940,7 +1702,6 @@ G1CollectedHeap::G1CollectedHeap(G1CollectorPolicy* policy_) :
   _evac_failure_scan_stack(NULL) ,
   _mark_in_progress(false),
   _cg1r(NULL), _summary_bytes_used(0),
-  _cur_alloc_region(NULL),
   _refine_cte_cl(NULL),
   _full_collection(false),
   _free_list("Master Free List"),
@@ -2099,7 +1860,6 @@ jint G1CollectedHeap::initialize() {
   _g1_max_committed = _g1_committed;
   _hrs = new HeapRegionSeq(_expansion_regions);
   guarantee(_hrs != NULL, "Couldn't allocate HeapRegionSeq");
-  guarantee(_cur_alloc_region == NULL, "from constructor");
 
   // 6843694 - ensure that the maximum region index can fit
   // in the remembered set structures.
@@ -2195,6 +1955,22 @@ jint G1CollectedHeap::initialize() {
   // Do later initialization work for concurrent refinement.
   _cg1r->init();
 
+  // Here we allocate the dummy full region that is required by the
+  // G1AllocRegion class. If we don't pass an address in the reserved
+  // space here, lots of asserts fire.
+  MemRegion mr(_g1_reserved.start(), HeapRegion::GrainWords);
+  HeapRegion* dummy_region = new HeapRegion(_bot_shared, mr, true);
+  // We'll re-use the same region whether the alloc region will
+  // require BOT updates or not and, if it doesn't, then a non-young
+  // region will complain that it cannot support allocations without
+  // BOT updates. So we'll tag the dummy region as young to avoid that.
+  dummy_region->set_young();
+  // Make sure it's full.
+  dummy_region->set_top(dummy_region->end());
+  G1AllocRegion::setup(this, dummy_region);
+
+  init_mutator_alloc_region();
+
   return JNI_OK;
 }
 
@@ -2261,7 +2037,7 @@ size_t G1CollectedHeap::used() const {
          "Should be owned on this thread's behalf.");
   size_t result = _summary_bytes_used;
   // Read only once in case it is set to NULL concurrently
-  HeapRegion* hr = _cur_alloc_region;
+  HeapRegion* hr = _mutator_alloc_region.get();
   if (hr != NULL)
     result += hr->used();
   return result;
@@ -2324,13 +2100,11 @@ size_t G1CollectedHeap::unsafe_max_alloc() {
   // to free(), resulting in a SIGSEGV. Note that this doesn't appear
   // to be a problem in the optimized build, since the two loads of the
   // current allocation region field are optimized away.
-  HeapRegion* car = _cur_alloc_region;
-
-  // FIXME: should iterate over all regions?
-  if (car == NULL) {
+  HeapRegion* hr = _mutator_alloc_region.get();
+  if (hr == NULL) {
     return 0;
   }
-  return car->free();
+  return hr->free();
 }
 
 bool G1CollectedHeap::should_do_concurrent_full_gc(GCCause::Cause cause) {
@@ -2781,16 +2555,12 @@ size_t G1CollectedHeap::unsafe_max_tlab_alloc(Thread* ignored) const {
   // since we can't allow tlabs to grow big enough to accomodate
   // humongous objects.
 
-  // We need to store the cur alloc region locally, since it might change
-  // between when we test for NULL and when we use it later.
-  ContiguousSpace* cur_alloc_space = _cur_alloc_region;
+  HeapRegion* hr = _mutator_alloc_region.get();
   size_t max_tlab_size = _humongous_object_threshold_in_words * wordSize;
-
-  if (cur_alloc_space == NULL) {
+  if (hr == NULL) {
     return max_tlab_size;
   } else {
-    return MIN2(MAX2(cur_alloc_space->free(), (size_t)MinTLABSize),
-                max_tlab_size);
+    return MIN2(MAX2(hr->free(), (size_t) MinTLABSize), max_tlab_size);
   }
 }
 
@@ -3364,6 +3134,7 @@ G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
   }
 
   verify_region_sets_optional();
+  verify_dirty_young_regions();
 
   {
     // This call will decide whether this pause is an initial-mark
@@ -3425,8 +3196,8 @@ G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
 
       if (VerifyBeforeGC && total_collections() >= VerifyGCStartAt) {
         HandleMark hm;  // Discard invalid handles created during verification
-        prepare_for_verify();
         gclog_or_tty->print(" VerifyBeforeGC:");
+        prepare_for_verify();
         Universe::verify(false);
       }
 
@@ -3442,7 +3213,7 @@ G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
 
       // Forget the current alloc region (we might even choose it to be part
       // of the collection set!).
-      abandon_cur_alloc_region();
+      release_mutator_alloc_region();
 
       // The elapsed time induced by the start time below deliberately elides
       // the possible verification above.
@@ -3573,6 +3344,8 @@ G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
       g1_policy()->print_collection_set(g1_policy()->inc_cset_head(), gclog_or_tty);
 #endif // YOUNG_LIST_VERBOSE
 
+      init_mutator_alloc_region();
+
       double end_time_sec = os::elapsedTime();
       double pause_time_ms = (end_time_sec - start_time_sec) * MILLIUNITS;
       g1_policy()->record_pause_time_ms(pause_time_ms);
@@ -3655,6 +3428,15 @@ size_t G1CollectedHeap::desired_plab_sz(GCAllocPurpose purpose)
   return gclab_word_size;
 }
 
+void G1CollectedHeap::init_mutator_alloc_region() {
+  assert(_mutator_alloc_region.get() == NULL, "pre-condition");
+  _mutator_alloc_region.init();
+}
+
+void G1CollectedHeap::release_mutator_alloc_region() {
+  _mutator_alloc_region.release();
+  assert(_mutator_alloc_region.get() == NULL, "post-condition");
+}
 
 void G1CollectedHeap::set_gc_alloc_region(int purpose, HeapRegion* r) {
   assert(purpose >= 0 && purpose < GCAllocPurposeCount, "invalid purpose");
@@ -5140,10 +4922,8 @@ class G1VerifyCardTableCleanup: public HeapRegionClosure {
   CardTableModRefBS* _ct_bs;
 public:
   G1VerifyCardTableCleanup(CardTableModRefBS* ct_bs)
-    : _ct_bs(ct_bs)
-  { }
-  virtual bool doHeapRegion(HeapRegion* r)
-  {
+    : _ct_bs(ct_bs) { }
+  virtual bool doHeapRegion(HeapRegion* r) {
     MemRegion mr(r->bottom(), r->end());
     if (r->is_survivor()) {
       _ct_bs->verify_dirty_region(mr);
@@ -5153,6 +4933,29 @@ public:
     return false;
   }
 };
+
+void G1CollectedHeap::verify_dirty_young_list(HeapRegion* head) {
+  CardTableModRefBS* ct_bs = (CardTableModRefBS*) (barrier_set());
+  for (HeapRegion* hr = head; hr != NULL; hr = hr->get_next_young_region()) {
+    // We cannot guarantee that [bottom(),end()] is dirty.  Threads
+    // dirty allocated blocks as they allocate them. The thread that
+    // retires each region and replaces it with a new one will do a
+    // maximal allocation to fill in [pre_dummy_top(),end()] but will
+    // not dirty that area (one less thing to have to do while holding
+    // a lock). So we can only verify that [bottom(),pre_dummy_top()]
+    // is dirty. Also note that verify_dirty_region() requires
+    // mr.start() and mr.end() to be card aligned and pre_dummy_top()
+    // is not guaranteed to be.
+    MemRegion mr(hr->bottom(),
+                 ct_bs->align_to_card_boundary(hr->pre_dummy_top()));
+    ct_bs->verify_dirty_region(mr);
+  }
+}
+
+void G1CollectedHeap::verify_dirty_young_regions() {
+  verify_dirty_young_list(_young_list->first_region());
+  verify_dirty_young_list(_young_list->first_survivor_region());
+}
 #endif
 
 void G1CollectedHeap::cleanUpCardTable() {
@@ -5500,6 +5303,44 @@ bool G1CollectedHeap::is_in_closed_subset(const void* p) const {
   }
 }
 
+HeapRegion* G1CollectedHeap::new_mutator_alloc_region(size_t word_size,
+                                                      bool force) {
+  assert_heap_locked_or_at_safepoint(true /* should_be_vm_thread */);
+  assert(!force || g1_policy()->can_expand_young_list(),
+         "if force is true we should be able to expand the young list");
+  if (force || !g1_policy()->is_young_list_full()) {
+    HeapRegion* new_alloc_region = new_region(word_size,
+                                              false /* do_expand */);
+    if (new_alloc_region != NULL) {
+      g1_policy()->update_region_num(true /* next_is_young */);
+      set_region_short_lived_locked(new_alloc_region);
+      return new_alloc_region;
+    }
+  }
+  return NULL;
+}
+
+void G1CollectedHeap::retire_mutator_alloc_region(HeapRegion* alloc_region,
+                                                  size_t allocated_bytes) {
+  assert_heap_locked_or_at_safepoint(true /* should_be_vm_thread */);
+  assert(alloc_region->is_young(), "all mutator alloc regions should be young");
+
+  g1_policy()->add_region_to_incremental_cset_lhs(alloc_region);
+  _summary_bytes_used += allocated_bytes;
+}
+
+HeapRegion* MutatorAllocRegion::allocate_new_region(size_t word_size,
+                                                    bool force) {
+  return _g1h->new_mutator_alloc_region(word_size, force);
+}
+
+void MutatorAllocRegion::retire_region(HeapRegion* alloc_region,
+                                       size_t allocated_bytes) {
+  _g1h->retire_mutator_alloc_region(alloc_region, allocated_bytes);
+}
+
+// Heap region set verification
+
 class VerifyRegionListsClosure : public HeapRegionClosure {
 private:
   HumongousRegionSet* _humongous_set;

src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp

@@ -26,6 +26,7 @@
 #define SHARE_VM_GC_IMPLEMENTATION_G1_G1COLLECTEDHEAP_HPP
 
 #include "gc_implementation/g1/concurrentMark.hpp"
+#include "gc_implementation/g1/g1AllocRegion.hpp"
 #include "gc_implementation/g1/g1RemSet.hpp"
 #include "gc_implementation/g1/heapRegionSets.hpp"
 #include "gc_implementation/parNew/parGCAllocBuffer.hpp"
@@ -128,6 +129,15 @@ public:
   void          print();
 };
 
+class MutatorAllocRegion : public G1AllocRegion {
+protected:
+  virtual HeapRegion* allocate_new_region(size_t word_size, bool force);
+  virtual void retire_region(HeapRegion* alloc_region, size_t allocated_bytes);
+public:
+  MutatorAllocRegion()
+    : G1AllocRegion("Mutator Alloc Region", false /* bot_updates */) { }
+};
+
 class RefineCardTableEntryClosure;
 class G1CollectedHeap : public SharedHeap {
   friend class VM_G1CollectForAllocation;
@@ -135,6 +145,7 @@ class G1CollectedHeap : public SharedHeap {
   friend class VM_G1CollectFull;
   friend class VM_G1IncCollectionPause;
   friend class VMStructs;
+  friend class MutatorAllocRegion;
 
   // Closures used in implementation.
   friend class G1ParCopyHelper;
@@ -197,12 +208,15 @@ private:
   // The sequence of all heap regions in the heap.
   HeapRegionSeq* _hrs;
 
-  // The region from which normal-sized objects are currently being
-  // allocated.  May be NULL.
-  HeapRegion* _cur_alloc_region;
+  // Alloc region used to satisfy mutator allocation requests.
+  MutatorAllocRegion _mutator_alloc_region;
+
+  // It resets the mutator alloc region before new allocations can take place.
+  void init_mutator_alloc_region();
+
+  // It releases the mutator alloc region.
+  void release_mutator_alloc_region();
 
-  // Postcondition: cur_alloc_region == NULL.
-  void abandon_cur_alloc_region();
   void abandon_gc_alloc_regions();
 
   // The to-space memory regions into which objects are being copied during
@@ -360,27 +374,21 @@ protected:
   G1CollectorPolicy* _g1_policy;
 
   // This is the second level of trying to allocate a new region. If
-  // new_region_work didn't find a region in the free_list, this call
-  // will check whether there's anything available in the
-  // secondary_free_list and/or wait for more regions to appear in that
-  // list, if _free_regions_coming is set.
+  // new_region() didn't find a region on the free_list, this call will
+  // check whether there's anything available on the
+  // secondary_free_list and/or wait for more regions to appear on
+  // that list, if _free_regions_coming is set.
   HeapRegion* new_region_try_secondary_free_list();
 
   // Try to allocate a single non-humongous HeapRegion sufficient for
   // an allocation of the given word_size. If do_expand is true,
   // attempt to expand the heap if necessary to satisfy the allocation
   // request.
-  HeapRegion* new_region_work(size_t word_size, bool do_expand);
+  HeapRegion* new_region(size_t word_size, bool do_expand);
 
-  // Try to allocate a new region to be used for allocation by a
-  // mutator thread. Attempt to expand the heap if no region is
+  // Try to allocate a new region to be used for allocation by
+  // a GC thread. It will try to expand the heap if no region is
   // available.
-  HeapRegion* new_alloc_region(size_t word_size) {
-    return new_region_work(word_size, false /* do_expand */);
-  }
-
-  // Try to allocate a new region to be used for allocation by a GC
-  // thread. Attempt to expand the heap if no region is available.
   HeapRegion* new_gc_alloc_region(int purpose, size_t word_size);
 
   // Attempt to satisfy a humongous allocation request of the given
@@ -415,10 +423,6 @@ protected:
   // * All non-TLAB allocation requests should go to mem_allocate()
   //   and mem_allocate() should never be called with is_tlab == true.
   //
-  // * If the GC locker is active we currently stall until we can
-  //   allocate a new young region. This will be changed in the
-  //   near future (see CR 6994056).
-  //
   // * If either call cannot satisfy the allocation request using the
   //   current allocating region, they will try to get a new one. If
   //   this fails, they will attempt to do an evacuation pause and
@@ -441,122 +445,38 @@ protected:
                                  bool   is_tlab, /* expected to be false */
                                  bool*  gc_overhead_limit_was_exceeded);
 
-  // The following methods, allocate_from_cur_allocation_region(),
-  // attempt_allocation(), attempt_allocation_locked(),
-  // replace_cur_alloc_region_and_allocate(),
-  // attempt_allocation_slow(), and attempt_allocation_humongous()
-  // have very awkward pre- and post-conditions with respect to
-  // locking:
-  //
-  // If they are called outside a safepoint they assume the caller
-  // holds the Heap_lock when it calls them. However, on exit they
-  // will release the Heap_lock if they return a non-NULL result, but
-  // keep holding the Heap_lock if they return a NULL result. The
-  // reason for this is that we need to dirty the cards that span
-  // allocated blocks on young regions to avoid having to take the
-  // slow path of the write barrier (for performance reasons we don't
-  // update RSets for references whose source is a young region, so we
-  // don't need to look at dirty cards on young regions). But, doing
-  // this card dirtying while holding the Heap_lock can be a
-  // scalability bottleneck, especially given that some allocation
-  // requests might be of non-trivial size (and the larger the region
-  // size is, the fewer allocations requests will be considered
-  // humongous, as the humongous size limit is a fraction of the
-  // region size). So, when one of these calls succeeds in allocating
-  // a block it does the card dirtying after it releases the Heap_lock
-  // which is why it will return without holding it.
-  //
-  // The above assymetry is the reason why locking / unlocking is done
-  // explicitly (i.e., with Heap_lock->lock() and
-  // Heap_lock->unlocked()) instead of using MutexLocker and
-  // MutexUnlocker objects. The latter would ensure that the lock is
-  // unlocked / re-locked at every possible exit out of the basic
-  // block. However, we only want that action to happen in selected
-  // places.
-  //
-  // Further, if the above methods are called during a safepoint, then
-  // naturally there's no assumption about the Heap_lock being held or
-  // there's no attempt to unlock it. The parameter at_safepoint
-  // indicates whether the call is made during a safepoint or not (as
-  // an optimization, to avoid reading the global flag with
-  // SafepointSynchronize::is_at_safepoint()).
-  //
-  // The methods share these parameters:
-  //
-  // * word_size     : the size of the allocation request in words
-  // * at_safepoint  : whether the call is done at a safepoint; this
-  //                   also determines whether a GC is permitted
-  //                   (at_safepoint == false) or not (at_safepoint == true)
-  // * do_dirtying   : whether the method should dirty the allocated
-  //                   block before returning
-  //
-  // They all return either the address of the block, if they
-  // successfully manage to allocate it, or NULL.
-
-  // It tries to satisfy an allocation request out of the current
-  // alloc region, which is passed as a parameter. It assumes that the
-  // caller has checked that the current alloc region is not NULL.
-  // Given that the caller has to check the current alloc region for
-  // at least NULL, it might as well pass it as the first parameter so
-  // that the method doesn't have to read it from the
-  // _cur_alloc_region field again. It is called from both
-  // attempt_allocation() and attempt_allocation_locked() and the
-  // with_heap_lock parameter indicates whether the caller was holding
-  // the heap lock when it called it or not.
-  inline HeapWord* allocate_from_cur_alloc_region(HeapRegion* cur_alloc_region,
-                                                  size_t word_size,
-                                                  bool with_heap_lock);
-
-  // First-level of allocation slow path: it attempts to allocate out
-  // of the current alloc region in a lock-free manner using a CAS. If
-  // that fails it takes the Heap_lock and calls
-  // attempt_allocation_locked() for the second-level slow path.
-  inline HeapWord* attempt_allocation(size_t word_size);
-
-  // Second-level of allocation slow path: while holding the Heap_lock
-  // it tries to allocate out of the current alloc region and, if that
-  // fails, tries to allocate out of a new current alloc region.
-  inline HeapWord* attempt_allocation_locked(size_t word_size);
-
-  // It assumes that the current alloc region has been retired and
-  // tries to allocate a new one. If it's successful, it performs the
-  // allocation out of the new current alloc region and updates
-  // _cur_alloc_region. Normally, it would try to allocate a new
-  // region if the young gen is not full, unless can_expand is true in
-  // which case it would always try to allocate a new region.
-  HeapWord* replace_cur_alloc_region_and_allocate(size_t word_size,
-                                                  bool at_safepoint,
-                                                  bool do_dirtying,
-                                                  bool can_expand);
-
-  // Third-level of allocation slow path: when we are unable to
-  // allocate a new current alloc region to satisfy an allocation
-  // request (i.e., when attempt_allocation_locked() fails). It will
-  // try to do an evacuation pause, which might stall due to the GC
-  // locker, and retry the allocation attempt when appropriate.
-  HeapWord* attempt_allocation_slow(size_t word_size);
-
-  // The method that tries to satisfy a humongous allocation
-  // request. If it cannot satisfy it it will try to do an evacuation
-  // pause to perhaps reclaim enough space to be able to satisfy the
-  // allocation request afterwards.
+  // The following three methods take a gc_count_before_ret
+  // parameter which is used to return the GC count if the method
+  // returns NULL. Given that we are required to read the GC count
+  // while holding the Heap_lock, and these paths will take the
+  // Heap_lock at some point, it's easier to get them to read the GC
+  // count while holding the Heap_lock before they return NULL instead
+  // of the caller (namely: mem_allocate()) having to also take the
+  // Heap_lock just to read the GC count.
+
+  // First-level mutator allocation attempt: try to allocate out of
+  // the mutator alloc region without taking the Heap_lock. This
+  // should only be used for non-humongous allocations.
+  inline HeapWord* attempt_allocation(size_t word_size,
+                                      unsigned int* gc_count_before_ret);
+
+  // Second-level mutator allocation attempt: take the Heap_lock and
+  // retry the allocation attempt, potentially scheduling a GC
+  // pause. This should only be used for non-humongous allocations.
+  HeapWord* attempt_allocation_slow(size_t word_size,
+                                    unsigned int* gc_count_before_ret);
+
+  // Takes the Heap_lock and attempts a humongous allocation. It can
+  // potentially schedule a GC pause.
   HeapWord* attempt_allocation_humongous(size_t word_size,
-                                         bool at_safepoint);
+                                         unsigned int* gc_count_before_ret);
 
-  // It does the common work when we are retiring the current alloc region.
-  inline void retire_cur_alloc_region_common(HeapRegion* cur_alloc_region);
-
-  // It retires the current alloc region, which is passed as a
-  // parameter (since, typically, the caller is already holding on to
-  // it). It sets _cur_alloc_region to NULL.
-  void retire_cur_alloc_region(HeapRegion* cur_alloc_region);
-
-  // It attempts to do an allocation immediately before or after an
-  // evacuation pause and can only be called by the VM thread. It has
-  // slightly different assumptions that the ones before (i.e.,
-  // assumes that the current alloc region has been retired).
+  // Allocation attempt that should be called during safepoints (e.g.,
+  // at the end of a successful GC). expect_null_mutator_alloc_region
+  // specifies whether the mutator alloc region is expected to be NULL
+  // or not.
   HeapWord* attempt_allocation_at_safepoint(size_t word_size,
-                                            bool expect_null_cur_alloc_region);
+                                       bool expect_null_mutator_alloc_region);
 
   // It dirties the cards that cover the block so that so that the post
   // write barrier never queues anything when updating objects on this
@@ -583,6 +503,12 @@ protected:
   // GC pause.
   void  retire_alloc_region(HeapRegion* alloc_region, bool par);
 
+  // These two methods are the "callbacks" from the G1AllocRegion class.
+
+  HeapRegion* new_mutator_alloc_region(size_t word_size, bool force);
+  void retire_mutator_alloc_region(HeapRegion* alloc_region,
+                                   size_t allocated_bytes);
+
   // - if explicit_gc is true, the GC is for a System.gc() or a heap
   //   inspection request and should collect the entire heap
   // - if clear_all_soft_refs is true, all soft references should be
@@ -1027,6 +953,9 @@ public:
   // The number of regions available for "regular" expansion.
   size_t expansion_regions() { return _expansion_regions; }
 
+  void verify_dirty_young_list(HeapRegion* head) PRODUCT_RETURN;
+  void verify_dirty_young_regions() PRODUCT_RETURN;
+
   // verify_region_sets() performs verification over the region
   // lists. It will be compiled in the product code to be used when
   // necessary (i.e., during heap verification).

src/share/vm/gc_implementation/g1/g1CollectedHeap.inline.hpp

@@ -27,6 +27,7 @@
 
 #include "gc_implementation/g1/concurrentMark.hpp"
 #include "gc_implementation/g1/g1CollectedHeap.hpp"
+#include "gc_implementation/g1/g1AllocRegion.inline.hpp"
 #include "gc_implementation/g1/g1CollectorPolicy.hpp"
 #include "gc_implementation/g1/heapRegionSeq.inline.hpp"
 #include "utilities/taskqueue.hpp"
@@ -59,131 +60,23 @@ inline bool G1CollectedHeap::obj_in_cs(oop obj) {
   return r != NULL && r->in_collection_set();
 }
 
-// See the comment in the .hpp file about the locking protocol and
-// assumptions of this method (and other related ones).
 inline HeapWord*
-G1CollectedHeap::allocate_from_cur_alloc_region(HeapRegion* cur_alloc_region,
-                                                size_t word_size,
-                                                bool with_heap_lock) {
-  assert_not_at_safepoint();
-  assert(with_heap_lock == Heap_lock->owned_by_self(),
-         "with_heap_lock and Heap_lock->owned_by_self() should be a tautology");
-  assert(cur_alloc_region != NULL, "pre-condition of the method");
-  assert(cur_alloc_region->is_young(),
-         "we only support young current alloc regions");
-  assert(!isHumongous(word_size), "allocate_from_cur_alloc_region() "
-         "should not be used for humongous allocations");
-  assert(!cur_alloc_region->isHumongous(), "Catch a regression of this bug.");
-
-  assert(!cur_alloc_region->is_empty(),
-         err_msg("region ["PTR_FORMAT","PTR_FORMAT"] should not be empty",
-                 cur_alloc_region->bottom(), cur_alloc_region->end()));
-  HeapWord* result = cur_alloc_region->par_allocate_no_bot_updates(word_size);
-  if (result != NULL) {
-    assert(is_in(result), "result should be in the heap");
-
-    if (with_heap_lock) {
-      Heap_lock->unlock();
-    }
-    assert_heap_not_locked();
-    // Do the dirtying after we release the Heap_lock.
-    dirty_young_block(result, word_size);
-    return result;
-  }
-
-  if (with_heap_lock) {
-    assert_heap_locked();
-  } else {
-    assert_heap_not_locked();
-  }
-  return NULL;
-}
-
-// See the comment in the .hpp file about the locking protocol and
-// assumptions of this method (and other related ones).
-inline HeapWord*
-G1CollectedHeap::attempt_allocation(size_t word_size) {
+G1CollectedHeap::attempt_allocation(size_t word_size,
+                                    unsigned int* gc_count_before_ret) {
   assert_heap_not_locked_and_not_at_safepoint();
-  assert(!isHumongous(word_size), "attempt_allocation() should not be called "
-         "for humongous allocation requests");
-
-  HeapRegion* cur_alloc_region = _cur_alloc_region;
-  if (cur_alloc_region != NULL) {
-    HeapWord* result = allocate_from_cur_alloc_region(cur_alloc_region,
-                                                   word_size,
-                                                   false /* with_heap_lock */);
-    assert_heap_not_locked();
-    if (result != NULL) {
-      return result;
-    }
-  }
+  assert(!isHumongous(word_size), "attempt_allocation() should not "
+         "be called for humongous allocation requests");
 
-  // Our attempt to allocate lock-free failed as the current
-  // allocation region is either NULL or full. So, we'll now take the
-  // Heap_lock and retry.
-  Heap_lock->lock();
-
-  HeapWord* result = attempt_allocation_locked(word_size);
-  if (result != NULL) {
-    assert_heap_not_locked();
-    return result;
+  HeapWord* result = _mutator_alloc_region.attempt_allocation(word_size,
+                                                      false /* bot_updates */);
+  if (result == NULL) {
+    result = attempt_allocation_slow(word_size, gc_count_before_ret);
   }
-
-  assert_heap_locked();
-  return NULL;
-}
-
-inline void
-G1CollectedHeap::retire_cur_alloc_region_common(HeapRegion* cur_alloc_region) {
-  assert_heap_locked_or_at_safepoint(true /* should_be_vm_thread */);
-  assert(cur_alloc_region != NULL && cur_alloc_region == _cur_alloc_region,
-         "pre-condition of the call");
-  assert(cur_alloc_region->is_young(),
-         "we only support young current alloc regions");
-
-  // The region is guaranteed to be young
-  g1_policy()->add_region_to_incremental_cset_lhs(cur_alloc_region);
-  _summary_bytes_used += cur_alloc_region->used();
-  _cur_alloc_region = NULL;
-}
-
-inline HeapWord*
-G1CollectedHeap::attempt_allocation_locked(size_t word_size) {
-  assert_heap_locked_and_not_at_safepoint();
-  assert(!isHumongous(word_size), "attempt_allocation_locked() "
-         "should not be called for humongous allocation requests");
-
-  // First, reread the current alloc region and retry the allocation
-  // in case somebody replaced it while we were waiting to get the
-  // Heap_lock.
-  HeapRegion* cur_alloc_region = _cur_alloc_region;
-  if (cur_alloc_region != NULL) {
-    HeapWord* result = allocate_from_cur_alloc_region(
-                                                  cur_alloc_region, word_size,
-                                                  true /* with_heap_lock */);
-    if (result != NULL) {
   assert_heap_not_locked();
-      return result;
-    }
-
-    // We failed to allocate out of the current alloc region, so let's
-    // retire it before getting a new one.
-    retire_cur_alloc_region(cur_alloc_region);
-  }
-
-  assert_heap_locked();
-  // Try to get a new region and allocate out of it
-  HeapWord* result = replace_cur_alloc_region_and_allocate(word_size,
-                                                     false, /* at_safepoint */
-                                                     true,  /* do_dirtying */
-                                                     false  /* can_expand */);
   if (result != NULL) {
-    assert_heap_not_locked();
-    return result;
+    dirty_young_block(result, word_size);
   }
-
-  assert_heap_locked();
-  return NULL;
+  return result;
 }
 
 // It dirties the cards that cover the block so that so that the post

src/share/vm/gc_implementation/g1/heapRegion.cpp

@@ -360,6 +360,7 @@ void HeapRegion::hr_clear(bool par, bool clear_space) {
   set_young_index_in_cset(-1);
   uninstall_surv_rate_group();
   set_young_type(NotYoung);
+  reset_pre_dummy_top();
 
   if (!par) {
     // If this is parallel, this will be done later.
@@ -923,11 +924,11 @@ void G1OffsetTableContigSpace::set_saved_mark() {
     ContiguousSpace::set_saved_mark();
     OrderAccess::storestore();
     _gc_time_stamp = curr_gc_time_stamp;
-    // The following fence is to force a flush of the writes above, but
-    // is strictly not needed because when an allocating worker thread
-    // calls set_saved_mark() it does so under the ParGCRareEvent_lock;
-    // when the lock is released, the write will be flushed.
-    // OrderAccess::fence();
+    // No need to do another barrier to flush the writes above. If
+    // this is called in parallel with other threads trying to
+    // allocate into the region, the caller should call this while
+    // holding a lock and when the lock is released the writes will be
+    // flushed.
   }
 }
 

src/share/vm/gc_implementation/g1/heapRegion.hpp

@@ -149,6 +149,13 @@ class G1OffsetTableContigSpace: public ContiguousSpace {
   G1BlockOffsetArrayContigSpace _offsets;
   Mutex _par_alloc_lock;
   volatile unsigned _gc_time_stamp;
+  // When we need to retire an allocation region, while other threads
+  // are also concurrently trying to allocate into it, we typically
+  // allocate a dummy object at the end of the region to ensure that
+  // no more allocations can take place in it. However, sometimes we
+  // want to know where the end of the last "real" object we allocated
+  // into the region was and this is what this keeps track.
+  HeapWord* _pre_dummy_top;
 
  public:
   // Constructor.  If "is_zeroed" is true, the MemRegion "mr" may be
@@ -163,6 +170,17 @@ class G1OffsetTableContigSpace: public ContiguousSpace {
   virtual void set_saved_mark();
   void reset_gc_time_stamp() { _gc_time_stamp = 0; }
 
+  // See the comment above in the declaration of _pre_dummy_top for an
+  // explanation of what it is.
+  void set_pre_dummy_top(HeapWord* pre_dummy_top) {
+    assert(is_in(pre_dummy_top) && pre_dummy_top <= top(), "pre-condition");
+    _pre_dummy_top = pre_dummy_top;
+  }
+  HeapWord* pre_dummy_top() {
+    return (_pre_dummy_top == NULL) ? top() : _pre_dummy_top;
+  }
+  void reset_pre_dummy_top() { _pre_dummy_top = NULL; }
+
   virtual void initialize(MemRegion mr, bool clear_space, bool mangle_space);
   virtual void clear(bool mangle_space);
 

src/share/vm/gc_implementation/g1/heapRegion.inline.hpp

@@ -38,15 +38,8 @@ inline HeapWord* G1OffsetTableContigSpace::allocate(size_t size) {
 // this is used for larger LAB allocations only.
 inline HeapWord* G1OffsetTableContigSpace::par_allocate(size_t size) {
   MutexLocker x(&_par_alloc_lock);
-  // This ought to be just "allocate", because of the lock above, but that
-  // ContiguousSpace::allocate asserts that either the allocating thread
-  // holds the heap lock or it is the VM thread and we're at a safepoint.
-  // The best I (dld) could figure was to put a field in ContiguousSpace
-  // meaning "locking at safepoint taken care of", and set/reset that
-  // here.  But this will do for now, especially in light of the comment
-  // above.  Perhaps in the future some lock-free manner of keeping the
-  // coordination.
-  HeapWord* res = ContiguousSpace::par_allocate(size);
+  // Given that we take the lock no need to use par_allocate() here.
+  HeapWord* res = ContiguousSpace::allocate(size);
   if (res != NULL) {
     _offsets.alloc_block(res, size);
   }

src/share/vm/memory/cardTableModRefBS.hpp

@@ -382,6 +382,11 @@ public:
     return (addr_for(pcard) == p);
   }
 
+  HeapWord* align_to_card_boundary(HeapWord* p) {
+    jbyte* pcard = byte_for(p + card_size_in_words - 1);
+    return addr_for(pcard);
+  }
+
   // The kinds of precision a CardTableModRefBS may offer.
   enum PrecisionStyle {
     Precise,

src/share/vm/memory/space.cpp

@@ -818,9 +818,14 @@ size_t ContiguousSpace::block_size(const HeapWord* p) const {
 // This version requires locking.
 inline HeapWord* ContiguousSpace::allocate_impl(size_t size,
                                                 HeapWord* const end_value) {
+  // In G1 there are places where a GC worker can allocates into a
+  // region using this serial allocation code without being prone to a
+  // race with other GC workers (we ensure that no other GC worker can
+  // access the same region at the same time). So the assert below is
+  // too strong in the case of G1.
   assert(Heap_lock->owned_by_self() ||
          (SafepointSynchronize::is_at_safepoint() &&
-          Thread::current()->is_VM_thread()),
+                               (Thread::current()->is_VM_thread() || UseG1GC)),
          "not locked");
   HeapWord* obj = top();
   if (pointer_delta(end_value, obj) >= size) {