--- old/src/share/vm/gc/g1/g1Allocator.cpp	2015-08-14 10:17:19.307239345 +0200
+++ new/src/share/vm/gc/g1/g1Allocator.cpp	2015-08-14 10:17:19.219236705 +0200
@@ -223,23 +223,35 @@
   }
 }
 
+bool G1PLABAllocator::may_throw_away_buffer(size_t const allocation_word_sz, size_t const buffer_size) const {
+  return (allocation_word_sz * 100 < buffer_size * ParallelGCBufferWastePct);
+}
+
 HeapWord* G1PLABAllocator::allocate_direct_or_new_plab(InCSetState dest,
                                                        size_t word_sz,
                                                        AllocationContext_t context,
                                                        bool* plab_refill_failed) {
-  size_t gclab_word_size = _g1h->desired_plab_sz(dest);
-  if (word_sz * 100 < gclab_word_size * ParallelGCBufferWastePct) {
+  size_t plab_word_size = G1CollectedHeap::heap()->desired_plab_sz(dest);
+  size_t required_in_plab = word_sz + PLAB::alignment_reserve();
+
+  // Only get a new PLAB if the allocation fits and it would not waste more than
+  // ParallelGCBufferWastePct in the existing buffer.
+  if ((required_in_plab <= plab_word_size) &&
+    may_throw_away_buffer(required_in_plab, plab_word_size)) {
+
     G1PLAB* alloc_buf = alloc_buffer(dest, context);
     alloc_buf->retire();
 
-    HeapWord* buf = _allocator->par_allocate_during_gc(dest, gclab_word_size, context);
+    HeapWord* buf = _allocator->par_allocate_during_gc(dest, plab_word_size, context);
     if (buf != NULL) {
       // Otherwise.
-      alloc_buf->set_word_size(gclab_word_size);
+      alloc_buf->set_word_size(plab_word_size);
       alloc_buf->set_buf(buf);
 
       HeapWord* const obj = alloc_buf->allocate(word_sz);
-      assert(obj != NULL, "buffer was definitely big enough...");
+      assert(obj != NULL, err_msg("PLAB should have been big enough, tried to allocate "
+                                  SIZE_FORMAT " with alignment reserve " SIZE_FORMAT " PLAB size " SIZE_FORMAT,
+                                  word_sz, PLAB::alignment_reserve(), plab_word_size));
       return obj;
     }
     // Otherwise.
--- old/src/share/vm/gc/g1/g1Allocator.hpp	2015-08-14 10:17:19.850255640 +0200
+++ new/src/share/vm/gc/g1/g1Allocator.hpp	2015-08-14 10:17:19.769253209 +0200
@@ -235,6 +235,7 @@
                               size_t word_sz,
                               AllocationContext_t context);
 
+  bool may_throw_away_buffer(size_t const allocation_word_sz, size_t const buffer_size) const;
 public:
   G1PLABAllocator(G1Allocator* allocator);
   virtual ~G1PLABAllocator() { }
--- old/src/share/vm/gc/shared/plab.hpp	2015-08-14 10:17:20.381271575 +0200
+++ new/src/share/vm/gc/shared/plab.hpp	2015-08-14 10:17:20.289268814 +0200
@@ -74,6 +74,8 @@
   PLAB(size_t word_sz);
   virtual ~PLAB() {}
 
+  static size_t alignment_reserve() { return AlignmentReserve; }
+
   // Minimum PLAB size.
   static size_t min_size();
   // Maximum PLAB size.