8034948: Back out JDK-6976350 since it does not fix any issue

Summary: Revert JDK-6976350 because it does not improve PLAB fragmentation. To the contrary, it tends to increase the amount of wasted space with many threads.
Reviewed-by: brutisso

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

@@ -606,6 +606,11 @@ protected:
   // may not be a humongous - it must fit into a single heap region.
   HeapWord* par_allocate_during_gc(GCAllocPurpose purpose, size_t word_size);
 
+  HeapWord* allocate_during_gc_slow(GCAllocPurpose purpose,
+                                    HeapRegion*    alloc_region,
+                                    bool           par,
+                                    size_t         word_size);
+
   // Ensure that no further allocations can happen in "r", bearing in mind
   // that parallel threads might be attempting allocations.
   void par_allocate_remaining_space(HeapRegion* r);
@@ -1783,95 +1788,6 @@ public:
     ParGCAllocBuffer::retire(end_of_gc, retain);
     _retired = true;
   }
-
-  bool is_retired() {
-    return _retired;
-  }
-};
-
-class G1ParGCAllocBufferContainer {
-protected:
-  static int const _priority_max = 2;
-  G1ParGCAllocBuffer* _priority_buffer[_priority_max];
-
-public:
-  G1ParGCAllocBufferContainer(size_t gclab_word_size) {
-    for (int pr = 0; pr < _priority_max; ++pr) {
-      _priority_buffer[pr] = new G1ParGCAllocBuffer(gclab_word_size);
-    }
-  }
-
-  ~G1ParGCAllocBufferContainer() {
-    for (int pr = 0; pr < _priority_max; ++pr) {
-      assert(_priority_buffer[pr]->is_retired(), "alloc buffers should all retire at this point.");
-      delete _priority_buffer[pr];
-    }
-  }
-
-  HeapWord* allocate(size_t word_sz) {
-    HeapWord* obj;
-    for (int pr = 0; pr < _priority_max; ++pr) {
-      obj = _priority_buffer[pr]->allocate(word_sz);
-      if (obj != NULL) return obj;
-    }
-    return obj;
-  }
-
-  bool contains(void* addr) {
-    for (int pr = 0; pr < _priority_max; ++pr) {
-      if (_priority_buffer[pr]->contains(addr)) return true;
-    }
-    return false;
-  }
-
-  void undo_allocation(HeapWord* obj, size_t word_sz) {
-    bool finish_undo;
-    for (int pr = 0; pr < _priority_max; ++pr) {
-      if (_priority_buffer[pr]->contains(obj)) {
-        _priority_buffer[pr]->undo_allocation(obj, word_sz);
-        finish_undo = true;
-      }
-    }
-    if (!finish_undo) ShouldNotReachHere();
-  }
-
-  size_t words_remaining() {
-    size_t result = 0;
-    for (int pr = 0; pr < _priority_max; ++pr) {
-      result += _priority_buffer[pr]->words_remaining();
-    }
-    return result;
-  }
-
-  size_t words_remaining_in_retired_buffer() {
-    G1ParGCAllocBuffer* retired = _priority_buffer[0];
-    return retired->words_remaining();
-  }
-
-  void flush_stats_and_retire(PLABStats* stats, bool end_of_gc, bool retain) {
-    for (int pr = 0; pr < _priority_max; ++pr) {
-      _priority_buffer[pr]->flush_stats_and_retire(stats, end_of_gc, retain);
-    }
-  }
-
-  void update(bool end_of_gc, bool retain, HeapWord* buf, size_t word_sz) {
-    G1ParGCAllocBuffer* retired_and_set = _priority_buffer[0];
-    retired_and_set->retire(end_of_gc, retain);
-    retired_and_set->set_buf(buf);
-    retired_and_set->set_word_size(word_sz);
-    adjust_priority_order();
-  }
-
-private:
-  void adjust_priority_order() {
-    G1ParGCAllocBuffer* retired_and_set = _priority_buffer[0];
-
-    int last = _priority_max - 1;
-    for (int pr = 0; pr < last; ++pr) {
-      _priority_buffer[pr] = _priority_buffer[pr + 1];
-    }
-    _priority_buffer[last] = retired_and_set;
-  }
 };
 
 class G1ParScanThreadState : public StackObj {
@@ -1882,9 +1798,9 @@ protected:
   G1SATBCardTableModRefBS* _ct_bs;
   G1RemSet* _g1_rem;
 
-  G1ParGCAllocBufferContainer  _surviving_alloc_buffer;
-  G1ParGCAllocBufferContainer  _tenured_alloc_buffer;
-  G1ParGCAllocBufferContainer* _alloc_buffers[GCAllocPurposeCount];
+  G1ParGCAllocBuffer  _surviving_alloc_buffer;
+  G1ParGCAllocBuffer  _tenured_alloc_buffer;
+  G1ParGCAllocBuffer* _alloc_buffers[GCAllocPurposeCount];
   ageTable            _age_table;
 
   G1ParScanClosure    _scanner;
@@ -1950,7 +1866,7 @@ public:
   RefToScanQueue*   refs()            { return _refs;             }
   ageTable*         age_table()       { return &_age_table;       }
 
-  G1ParGCAllocBufferContainer* alloc_buffer(GCAllocPurpose purpose) {
+  G1ParGCAllocBuffer* alloc_buffer(GCAllocPurpose purpose) {
     return _alloc_buffers[purpose];
   }
 
@@ -1980,13 +1896,15 @@ public:
     HeapWord* obj = NULL;
     size_t gclab_word_size = _g1h->desired_plab_sz(purpose);
     if (word_sz * 100 < gclab_word_size * ParallelGCBufferWastePct) {
-      G1ParGCAllocBufferContainer* alloc_buf = alloc_buffer(purpose);
+      G1ParGCAllocBuffer* alloc_buf = alloc_buffer(purpose);
+      add_to_alloc_buffer_waste(alloc_buf->words_remaining());
+      alloc_buf->retire(false /* end_of_gc */, false /* retain */);
 
       HeapWord* buf = _g1h->par_allocate_during_gc(purpose, gclab_word_size);
       if (buf == NULL) return NULL; // Let caller handle allocation failure.
-
-      add_to_alloc_buffer_waste(alloc_buf->words_remaining_in_retired_buffer());
-      alloc_buf->update(false /* end_of_gc */, false /* retain */, buf, gclab_word_size);
+      // Otherwise.
+      alloc_buf->set_word_size(gclab_word_size);
+      alloc_buf->set_buf(buf);
 
       obj = alloc_buf->allocate(word_sz);
       assert(obj != NULL, "buffer was definitely big enough...");
@@ -2100,6 +2018,7 @@ public:
     }
   }
 
+public:
   void trim_queue();
 };
 

src/share/vm/gc_implementation/shared/parGCAllocBuffer.hpp

@@ -158,7 +158,7 @@ public:
   // Fills in the unallocated portion of the buffer with a garbage object.
   // If "end_of_gc" is TRUE, is after the last use in the GC.  IF "retain"
   // is true, attempt to re-use the unused portion in the next GC.
-  virtual void retire(bool end_of_gc, bool retain);
+  void retire(bool end_of_gc, bool retain);
 
   void print() PRODUCT_RETURN;
 };