6604422: G1: re-use half-promoted regions

6728271: G1: Cleanup G1CollectedHeap::get_gc_alloc_regions()
Summary: It allows the last half-full region to be allocated to during a GC to be reused during the next GC.
Reviewed-by: apetrusenko, jcoomes

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

@@ -786,6 +786,12 @@ void G1CollectedHeap::abandon_cur_alloc_region() {
   }
 }
 
+void G1CollectedHeap::abandon_gc_alloc_regions() {
+  // first, make sure that the GC alloc region list is empty (it should!)
+  assert(_gc_alloc_region_list == NULL, "invariant");
+  release_gc_alloc_regions(true /* totally */);
+}
+
 class PostMCRemSetClearClosure: public HeapRegionClosure {
   ModRefBarrierSet* _mr_bs;
 public:
@@ -914,6 +920,7 @@ void G1CollectedHeap::do_collection(bool full, bool clear_all_soft_refs,
 
     // Make sure we'll choose a new allocation region afterwards.
     abandon_cur_alloc_region();
+    abandon_gc_alloc_regions();
     assert(_cur_alloc_region == NULL, "Invariant.");
     g1_rem_set()->as_HRInto_G1RemSet()->cleanupHRRS();
     tear_down_region_lists();
@@ -1306,7 +1313,7 @@ void G1CollectedHeap::shrink_helper(size_t shrink_bytes)
 }
 
 void G1CollectedHeap::shrink(size_t shrink_bytes) {
-  release_gc_alloc_regions();
+  release_gc_alloc_regions(true /* totally */);
   tear_down_region_lists();  // We will rebuild them in a moment.
   shrink_helper(shrink_bytes);
   rebuild_region_lists();
@@ -1345,8 +1352,7 @@ G1CollectedHeap::G1CollectedHeap(G1CollectorPolicy* policy_) :
   _gc_time_stamp(0),
   _surviving_young_words(NULL),
   _in_cset_fast_test(NULL),
-  _in_cset_fast_test_base(NULL)
-{
+  _in_cset_fast_test_base(NULL) {
   _g1h = this; // To catch bugs.
   if (_process_strong_tasks == NULL || !_process_strong_tasks->valid()) {
     vm_exit_during_initialization("Failed necessary allocation.");
@@ -1371,9 +1377,19 @@ G1CollectedHeap::G1CollectedHeap(G1CollectorPolicy* policy_) :
   }
 
   for (int ap = 0; ap < GCAllocPurposeCount; ++ap) {
-    _gc_alloc_regions[ap]       = NULL;
-    _gc_alloc_region_counts[ap] = 0;
+    _gc_alloc_regions[ap]          = NULL;
+    _gc_alloc_region_counts[ap]    = 0;
+    _retained_gc_alloc_regions[ap] = NULL;
+    // by default, we do not retain a GC alloc region for each ap;
+    // we'll override this, when appropriate, below
+    _retain_gc_alloc_region[ap]    = false;
   }
+
+  // We will try to remember the last half-full tenured region we
+  // allocated to at the end of a collection so that we can re-use it
+  // during the next collection.
+  _retain_gc_alloc_region[GCAllocForTenured]  = true;
+
   guarantee(_task_queues != NULL, "task_queues allocation failure.");
 }
 
@@ -2644,7 +2660,7 @@ G1CollectedHeap::do_collection_pause_at_safepoint(HeapRegion* popular_region) {
         popular_region->set_popular_pending(false);
       }
 
-      release_gc_alloc_regions();
+      release_gc_alloc_regions(false /* totally */);
 
       cleanup_surviving_young_words();
 
@@ -2735,6 +2751,10 @@ G1CollectedHeap::do_collection_pause_at_safepoint(HeapRegion* popular_region) {
 
 void G1CollectedHeap::set_gc_alloc_region(int purpose, HeapRegion* r) {
   assert(purpose >= 0 && purpose < GCAllocPurposeCount, "invalid purpose");
+  // make sure we don't call set_gc_alloc_region() multiple times on
+  // the same region
+  assert(r == NULL || !r->is_gc_alloc_region(),
+         "shouldn't already be a GC alloc region");
   HeapWord* original_top = NULL;
   if (r != NULL)
     original_top = r->top();
@@ -2851,23 +2871,55 @@ bool G1CollectedHeap::check_gc_alloc_regions() {
 }
 
 void G1CollectedHeap::get_gc_alloc_regions() {
+  // First, let's check that the GC alloc region list is empty (it should)
+  assert(_gc_alloc_region_list == NULL, "invariant");
+
   for (int ap = 0; ap < GCAllocPurposeCount; ++ap) {
+    assert(_gc_alloc_regions[ap] == NULL, "invariant");
+
     // Create new GC alloc regions.
-    HeapRegion* alloc_region = _gc_alloc_regions[ap];
-    // Clear this alloc region, so that in case it turns out to be
-    // unacceptable, we end up with no allocation region, rather than a bad
-    // one.
-    _gc_alloc_regions[ap] = NULL;
-    if (alloc_region == NULL || alloc_region->in_collection_set()) {
-      // Can't re-use old one.  Allocate a new one.
+    HeapRegion* alloc_region = _retained_gc_alloc_regions[ap];
+    _retained_gc_alloc_regions[ap] = NULL;
+
+    if (alloc_region != NULL) {
+      assert(_retain_gc_alloc_region[ap], "only way to retain a GC region");
+
+      // let's make sure that the GC alloc region is not tagged as such
+      // outside a GC operation
+      assert(!alloc_region->is_gc_alloc_region(), "sanity");
+
+      if (alloc_region->in_collection_set() ||
+          alloc_region->top() == alloc_region->end() ||
+          alloc_region->top() == alloc_region->bottom()) {
+        // we will discard the current GC alloc region if it's in the
+        // collection set (it can happen!), if it's already full (no
+        // point in using it), or if it's empty (this means that it
+        // was emptied during a cleanup and it should be on the free
+        // list now).
+
+        alloc_region = NULL;
+      }
+    }
+
+    if (alloc_region == NULL) {
+      // we will get a new GC alloc region
       alloc_region = newAllocRegionWithExpansion(ap, 0);
     }
+
     if (alloc_region != NULL) {
+      assert(_gc_alloc_regions[ap] == NULL, "pre-condition");
       set_gc_alloc_region(ap, alloc_region);
     }
+
+    assert(_gc_alloc_regions[ap] == NULL ||
+           _gc_alloc_regions[ap]->is_gc_alloc_region(),
+           "the GC alloc region should be tagged as such");
+    assert(_gc_alloc_regions[ap] == NULL ||
+           _gc_alloc_regions[ap] == _gc_alloc_region_list,
+           "the GC alloc region should be the same as the GC alloc list head");
   }
   // Set alternative regions for allocation purposes that have reached
-  // thier limit.
+  // their limit.
   for (int ap = 0; ap < GCAllocPurposeCount; ++ap) {
     GCAllocPurpose alt_purpose = g1_policy()->alternative_purpose(ap);
     if (_gc_alloc_regions[ap] == NULL && alt_purpose != ap) {
@@ -2877,28 +2929,56 @@ void G1CollectedHeap::get_gc_alloc_regions() {
   assert(check_gc_alloc_regions(), "alloc regions messed up");
 }
 
-void G1CollectedHeap::release_gc_alloc_regions() {
+void G1CollectedHeap::release_gc_alloc_regions(bool totally) {
   // We keep a separate list of all regions that have been alloc regions in
-  // the current collection pause.  Forget that now.
+  // the current collection pause. Forget that now. This method will
+  // untag the GC alloc regions and tear down the GC alloc region
+  // list. It's desirable that no regions are tagged as GC alloc
+  // outside GCs.
   forget_alloc_region_list();
 
   // The current alloc regions contain objs that have survived
   // collection. Make them no longer GC alloc regions.
   for (int ap = 0; ap < GCAllocPurposeCount; ++ap) {
     HeapRegion* r = _gc_alloc_regions[ap];
-    if (r != NULL && r->is_empty()) {
-      {
+    _retained_gc_alloc_regions[ap] = NULL;
+
+    if (r != NULL) {
+      // we retain nothing on _gc_alloc_regions between GCs
+      set_gc_alloc_region(ap, NULL);
+      _gc_alloc_region_counts[ap] = 0;
+
+      if (r->is_empty()) {
+        // we didn't actually allocate anything in it; let's just put
+        // it on the free list
         MutexLockerEx x(ZF_mon, Mutex::_no_safepoint_check_flag);
         r->set_zero_fill_complete();
         put_free_region_on_list_locked(r);
+      } else if (_retain_gc_alloc_region[ap] && !totally) {
+        // retain it so that we can use it at the beginning of the next GC
+        _retained_gc_alloc_regions[ap] = r;
       }
     }
-    // set_gc_alloc_region will also NULLify all aliases to the region
-    set_gc_alloc_region(ap, NULL);
-    _gc_alloc_region_counts[ap] = 0;
   }
 }
 
+#ifndef PRODUCT
+// Useful for debugging
+
+void G1CollectedHeap::print_gc_alloc_regions() {
+  gclog_or_tty->print_cr("GC alloc regions");
+  for (int ap = 0; ap < GCAllocPurposeCount; ++ap) {
+    HeapRegion* r = _gc_alloc_regions[ap];
+    if (r == NULL) {
+      gclog_or_tty->print_cr("  %2d : "PTR_FORMAT, ap, NULL);
+    } else {
+      gclog_or_tty->print_cr("  %2d : "PTR_FORMAT" "SIZE_FORMAT,
+                             ap, r->bottom(), r->used());
+    }
+  }
+}
+#endif // PRODUCT
+
 void G1CollectedHeap::init_for_evac_failure(OopsInHeapRegionClosure* cl) {
   _drain_in_progress = false;
   set_evac_failure_closure(cl);

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

@@ -172,7 +172,6 @@ private:
     NumAPIs = HeapRegion::MaxAge
   };
 
-
   // The one and only G1CollectedHeap, so static functions can find it.
   static G1CollectedHeap* _g1h;
 
@@ -217,11 +216,20 @@ private:
 
   // 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
   // a GC.
   HeapRegion* _gc_alloc_regions[GCAllocPurposeCount];
   size_t _gc_alloc_region_counts[GCAllocPurposeCount];
+  // These are the regions, one per GCAllocPurpose, that are half-full
+  // at the end of a collection and that we want to reuse during the
+  // next collection.
+  HeapRegion* _retained_gc_alloc_regions[GCAllocPurposeCount];
+  // This specifies whether we will keep the last half-full region at
+  // the end of a collection so that it can be reused during the next
+  // collection (this is specified per GCAllocPurpose)
+  bool _retain_gc_alloc_region[GCAllocPurposeCount];
 
   // A list of the regions that have been set to be alloc regions in the
   // current collection.
@@ -589,8 +597,21 @@ protected:
 
   // Ensure that the relevant gc_alloc regions are set.
   void get_gc_alloc_regions();
-  // We're done with GC alloc regions; release them, as appropriate.
-  void release_gc_alloc_regions();
+  // We're done with GC alloc regions. We are going to tear down the
+  // gc alloc list and remove the gc alloc tag from all the regions on
+  // that list. However, we will also retain the last (i.e., the one
+  // that is half-full) GC alloc region, per GCAllocPurpose, for
+  // possible reuse during the next collection, provided
+  // _retain_gc_alloc_region[] indicates that it should be the
+  // case. Said regions are kept in the _retained_gc_alloc_regions[]
+  // array. If the parameter totally is set, we will not retain any
+  // regions, irrespective of what _retain_gc_alloc_region[]
+  // indicates.
+  void release_gc_alloc_regions(bool totally);
+#ifndef PRODUCT
+  // Useful for debugging.
+  void print_gc_alloc_regions();
+#endif // !PRODUCT
 
   // ("Weak") Reference processing support
   ReferenceProcessor* _ref_processor;

src/share/vm/gc_implementation/g1/g1CollectorPolicy.cpp

@@ -1087,6 +1087,7 @@ void G1CollectorPolicy::record_collection_pause_start(double start_time_sec,
 
   assert(_g1->used_regions() == _g1->recalculate_used_regions(),
          "sanity");
+  assert(_g1->used() == _g1->recalculate_used(), "sanity");
 
   double s_w_t_ms = (start_time_sec - _stop_world_start) * 1000.0;
   _all_stop_world_times_ms->add(s_w_t_ms);