8191471: Elastic TLABs for G1

Reviewed-by: tschatzl, pliden

src/hotspot/share/gc/g1/g1AllocRegion.cpp

@@ -95,33 +95,40 @@ size_t G1AllocRegion::fill_up_remaining_space(HeapRegion* alloc_region) {
   return result;
 }
 
+size_t G1AllocRegion::retire_internal(HeapRegion* alloc_region, bool fill_up) {
+  // 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.
+  size_t waste = 0;
+  assert_alloc_region(!alloc_region->is_empty(),
+      "the alloc region should never be empty");
+
+  if (fill_up) {
+    waste = fill_up_remaining_space(alloc_region);
+  }
+
+  assert_alloc_region(alloc_region->used() >= _used_bytes_before, "invariant");
+  size_t allocated_bytes = alloc_region->used() - _used_bytes_before;
+  retire_region(alloc_region, allocated_bytes);
+  _used_bytes_before = 0;
+
+  return waste;
+}
+
 size_t G1AllocRegion::retire(bool fill_up) {
   assert_alloc_region(_alloc_region != NULL, "not initialized properly");
 
-  size_t result = 0;
+  size_t waste = 0;
 
   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(!alloc_region->is_empty(),
-                           "the alloc region should never be empty");
-
-    if (fill_up) {
-      result = fill_up_remaining_space(alloc_region);
-    }
-
-    assert_alloc_region(alloc_region->used() >= _used_bytes_before, "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;
+    waste = retire_internal(alloc_region, fill_up);
+    reset_alloc_region();
   }
   trace("retired");
 
-  return result;
+  return waste;
 }
 
 HeapWord* G1AllocRegion::new_alloc_region_and_allocate(size_t word_size,
@@ -245,7 +252,8 @@ void G1AllocRegion::trace(const char* str, size_t min_word_size, size_t desired_
 G1AllocRegion::G1AllocRegion(const char* name,
                              bool bot_updates)
   : _name(name), _bot_updates(bot_updates),
-    _alloc_region(NULL), _count(0), _used_bytes_before(0) { }
+    _alloc_region(NULL), _count(0),
+    _used_bytes_before(0) { }
 
 
 HeapRegion* MutatorAllocRegion::allocate_new_region(size_t word_size,
@@ -258,6 +266,82 @@ void MutatorAllocRegion::retire_region(HeapRegion* alloc_region,
   _g1h->retire_mutator_alloc_region(alloc_region, allocated_bytes);
 }
 
+void MutatorAllocRegion::init() {
+  assert(_retained_alloc_region == NULL, "Pre-condition");
+  G1AllocRegion::init();
+  _wasted_bytes = 0;
+}
+
+bool MutatorAllocRegion::should_retain(HeapRegion* region) {
+  size_t free_bytes = region->free();
+  if (free_bytes < MinTLABSize) {
+    return false;
+  }
+
+  if (_retained_alloc_region != NULL &&
+      free_bytes < _retained_alloc_region->free()) {
+    return false;
+  }
+
+  return true;
+}
+
+size_t MutatorAllocRegion::retire(bool fill_up) {
+  size_t waste = 0;
+  trace("retiring");
+  HeapRegion* current_region = get();
+  if (current_region != NULL) {
+    // Retain the current region if it fits a TLAB and has more
+    // free than the currently retained region.
+    if (should_retain(current_region)) {
+      trace("mutator retained");
+      if (_retained_alloc_region != NULL) {
+        waste = retire_internal(_retained_alloc_region, true);
+      }
+      _retained_alloc_region = current_region;
+    } else {
+      waste = retire_internal(current_region, fill_up);
+    }
+    reset_alloc_region();
+  }
+
+  _wasted_bytes += waste;
+  trace("retired");
+  return waste;
+}
+
+size_t MutatorAllocRegion::used_in_alloc_regions() {
+  size_t used = 0;
+  HeapRegion* hr = get();
+  if (hr != NULL) {
+    used += hr->used();
+  }
+
+  hr = _retained_alloc_region;
+  if (hr != NULL) {
+    used += hr->used();
+  }
+  return used;
+}
+
+HeapRegion* MutatorAllocRegion::release() {
+  HeapRegion* ret = G1AllocRegion::release();
+
+  // The retained alloc region must be retired and this must be
+  // done after the above call to release the mutator alloc region,
+  // since it might update the _retained_alloc_region member.
+  if (_retained_alloc_region != NULL) {
+    _wasted_bytes += retire_internal(_retained_alloc_region, false);
+    _retained_alloc_region = NULL;
+  }
+  log_debug(gc, alloc, region)("Mutator Allocation stats, regions: %u, wasted size: " SIZE_FORMAT "%s (%4.1f%%)",
+                               count(),
+                               byte_size_in_proper_unit(_wasted_bytes),
+                               proper_unit_for_byte_size(_wasted_bytes),
+                               percent_of(_wasted_bytes, count() * HeapRegion::GrainBytes));
+  return ret;
+}
+
 HeapRegion* G1GCAllocRegion::allocate_new_region(size_t word_size,
                                                  bool force) {
   assert(!force, "not supported for GC alloc regions");

src/hotspot/share/gc/g1/g1AllocRegion.hpp

@@ -80,6 +80,20 @@ private:
   // whether the _alloc_region is NULL or not.
   static HeapRegion* _dummy_region;
 
+  // After a region is allocated by alloc_new_region, this
+  // method is used to set it as the active alloc_region
+  void update_alloc_region(HeapRegion* alloc_region);
+
+  // 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);
+
+protected:
+  // Reset the alloc region to point a the dummy region.
+  void reset_alloc_region();
+
   // Perform a non-MT-safe allocation out of the given region.
   inline HeapWord* allocate(HeapRegion* alloc_region,
                             size_t word_size);
@@ -102,23 +116,14 @@ private:
   // the space.
   size_t fill_up_remaining_space(HeapRegion* alloc_region);
 
-  // After a region is allocated by alloc_new_region, this
-  // method is used to set it as the active alloc_region
-  void update_alloc_region(HeapRegion* alloc_region);
-
-  // 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);
-
-protected:
   // Retire the active allocating region. If fill_up is true then make
   // sure that the region is full before we retire it so that no one
   // else can allocate out of it.
   // Returns the number of bytes that have been filled up during retire.
   virtual size_t retire(bool fill_up);
 
+  size_t retire_internal(HeapRegion* alloc_region, bool fill_up);
+
   // For convenience as subclasses use it.
   static G1CollectedHeap* _g1h;
 
@@ -177,7 +182,7 @@ public:
   inline HeapWord* attempt_allocation_force(size_t word_size);
 
   // Should be called before we start using this object.
-  void init();
+  virtual void init();
 
   // This can be used to set the active region to a specific
   // region. (Use Example: we try to retain the last old GC alloc
@@ -197,14 +202,49 @@ public:
 };
 
 class MutatorAllocRegion : public G1AllocRegion {
+private:
+  // Keeps track of the total waste generated during the current
+  // mutator phase.
+  size_t _wasted_bytes;
+
+  // Retained allocation region. Used to lower the waste generated
+  // during mutation by having two active regions if the free space
+  // in a region about to be retired still could fit a TLAB.
+  HeapRegion* volatile _retained_alloc_region;
+
+  // Decide if the region should be retained, based on the free size
+  // in it and the free size in the currently retained region, if any.
+  bool should_retain(HeapRegion* region);
 protected:
   virtual HeapRegion* allocate_new_region(size_t word_size, bool force);
   virtual void retire_region(HeapRegion* alloc_region, size_t allocated_bytes);
+  virtual size_t retire(bool fill_up);
 public:
   MutatorAllocRegion()
-    : G1AllocRegion("Mutator Alloc Region", false /* bot_updates */) { }
-};
+    : G1AllocRegion("Mutator Alloc Region", false /* bot_updates */),
+      _wasted_bytes(0),
+      _retained_alloc_region(NULL) { }
 
+  // Returns the combined used memory in the current alloc region and
+  // the retained alloc region.
+  size_t used_in_alloc_regions();
+
+  // Perform an allocation out of the retained allocation region, with the given
+  // minimum and desired size. Returns the actual size allocated (between
+  // minimum and desired size) in actual_word_size if the allocation has been
+  // successful.
+  // Should be called without holding a lock. It will try to allocate lock-free
+  // out of the retained region, or return NULL if it was unable to.
+  inline HeapWord* attempt_retained_allocation(size_t min_word_size,
+                                               size_t desired_word_size,
+                                               size_t* actual_word_size);
+
+  // This specialization of release() makes sure that the retained alloc
+  // region is retired and set to NULL.
+  virtual HeapRegion* release();
+
+  virtual void init();
+};
 // Common base class for allocation regions used during GC.
 class G1GCAllocRegion : public G1AllocRegion {
 protected:

src/hotspot/share/gc/g1/g1AllocRegion.inline.hpp

@@ -36,6 +36,10 @@
   } while (0)
 
 
+inline void G1AllocRegion::reset_alloc_region() {
+  _alloc_region = _dummy_region;
+}
+
 inline HeapWord* G1AllocRegion::allocate(HeapRegion* alloc_region,
                                          size_t word_size) {
   assert(alloc_region != NULL, "pre-condition");
@@ -126,4 +130,17 @@ inline HeapWord* G1AllocRegion::attempt_allocation_force(size_t word_size) {
   return NULL;
 }
 
+inline HeapWord* MutatorAllocRegion::attempt_retained_allocation(size_t min_word_size,
+                                                                 size_t desired_word_size,
+                                                                 size_t* actual_word_size) {
+  if (_retained_alloc_region != NULL) {
+    HeapWord* result = par_allocate(_retained_alloc_region, min_word_size, desired_word_size, actual_word_size);
+    if (result != NULL) {
+      trace("alloc retained", min_word_size, desired_word_size, *actual_word_size, result);
+      return result;
+    }
+  }
+  return NULL;
+}
+
 #endif // SHARE_VM_GC_G1_G1ALLOCREGION_INLINE_HPP

src/hotspot/share/gc/g1/g1Allocator.cpp

@@ -157,14 +157,7 @@ size_t G1Allocator::unsafe_max_tlab_alloc() {
 
 size_t G1Allocator::used_in_alloc_regions() {
   assert(Heap_lock->owner() != NULL, "Should be owned on this thread's behalf.");
-  size_t result = 0;
-
-  // Read only once in case it is set to NULL concurrently
-  HeapRegion* hr = mutator_alloc_region()->get();
-  if (hr != NULL) {
-    result += hr->used();
-  }
-  return result;
+  return mutator_alloc_region()->used_in_alloc_regions();
 }
 
 

src/hotspot/share/gc/g1/g1Allocator.hpp

@@ -99,7 +99,9 @@ public:
 
   // Allocate blocks of memory during mutator time.
 
-  inline HeapWord* attempt_allocation(size_t word_size);
+  inline HeapWord* attempt_allocation(size_t min_word_size,
+                                      size_t desired_word_size,
+                                      size_t* actual_word_size);
   inline HeapWord* attempt_allocation_locked(size_t word_size);
   inline HeapWord* attempt_allocation_force(size_t word_size);
 

src/hotspot/share/gc/g1/g1Allocator.inline.hpp

@@ -41,8 +41,14 @@ inline OldGCAllocRegion* G1Allocator::old_gc_alloc_region() {
   return &_old_gc_alloc_region;
 }
 
-inline HeapWord* G1Allocator::attempt_allocation(size_t word_size) {
-  return mutator_alloc_region()->attempt_allocation(word_size);
+inline HeapWord* G1Allocator::attempt_allocation(size_t min_word_size,
+                                                 size_t desired_word_size,
+                                                 size_t* actual_word_size) {
+  HeapWord* result = mutator_alloc_region()->attempt_retained_allocation(min_word_size, desired_word_size, actual_word_size);
+  if (result != NULL) {
+    return result;
+  }
+  return mutator_alloc_region()->attempt_allocation(min_word_size, desired_word_size, actual_word_size);
 }
 
 inline HeapWord* G1Allocator::attempt_allocation_locked(size_t word_size) {

src/hotspot/share/gc/g1/g1CollectedHeap.cpp

@@ -384,11 +384,13 @@ HeapWord* G1CollectedHeap::humongous_obj_allocate(size_t word_size) {
   return result;
 }
 
-HeapWord* G1CollectedHeap::allocate_new_tlab(size_t word_size) {
+HeapWord* G1CollectedHeap::allocate_new_tlab(size_t min_size,
+                                             size_t requested_size,
+                                             size_t* actual_size) {
   assert_heap_not_locked_and_not_at_safepoint();
-  assert(!is_humongous(word_size), "we do not allow humongous TLABs");
+  assert(!is_humongous(requested_size), "we do not allow humongous TLABs");
 
-  return attempt_allocation(word_size);
+  return attempt_allocation(min_size, requested_size, actual_size);
 }
 
 HeapWord*
@@ -399,7 +401,8 @@ G1CollectedHeap::mem_allocate(size_t word_size,
   if (is_humongous(word_size)) {
     return attempt_allocation_humongous(word_size);
   }
-  return attempt_allocation(word_size);
+  size_t dummy = 0;
+  return attempt_allocation(word_size, word_size, &dummy);
 }
 
 HeapWord* G1CollectedHeap::attempt_allocation_slow(size_t word_size) {
@@ -492,8 +495,8 @@ HeapWord* G1CollectedHeap::attempt_allocation_slow(size_t word_size) {
     // 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 = _allocator->attempt_allocation(word_size);
+    size_t dummy = 0;
+    result = _allocator->attempt_allocation(word_size, word_size, &dummy);
     if (result != NULL) {
       return result;
     }
@@ -722,20 +725,28 @@ void G1CollectedHeap::fill_archive_regions(MemRegion* ranges, size_t count) {
   }
 }
 
-inline HeapWord* G1CollectedHeap::attempt_allocation(size_t word_size) {
+inline HeapWord* G1CollectedHeap::attempt_allocation(size_t min_word_size,
+                                                     size_t desired_word_size,
+                                                     size_t* actual_word_size) {
   assert_heap_not_locked_and_not_at_safepoint();
-  assert(!is_humongous(word_size), "attempt_allocation() should not "
+  assert(!is_humongous(desired_word_size), "attempt_allocation() should not "
          "be called for humongous allocation requests");
 
-  HeapWord* result = _allocator->attempt_allocation(word_size);
+  HeapWord* result = _allocator->attempt_allocation(min_word_size, desired_word_size, actual_word_size);
 
   if (result == NULL) {
-    result = attempt_allocation_slow(word_size);
+    *actual_word_size = desired_word_size;
+    result = attempt_allocation_slow(desired_word_size);
   }
+
   assert_heap_not_locked();
   if (result != NULL) {
-    dirty_young_block(result, word_size);
+    assert(*actual_word_size != 0, "Actual size must have been set here");
+    dirty_young_block(result, *actual_word_size);
+  } else {
+    *actual_word_size = 0;
   }
+
   return result;
 }
 

src/hotspot/share/gc/g1/g1CollectedHeap.hpp

@@ -420,7 +420,9 @@ private:
   //   humongous allocation requests should go to mem_allocate() which
   //   will satisfy them with a special path.
 
-  virtual HeapWord* allocate_new_tlab(size_t word_size);
+  virtual HeapWord* allocate_new_tlab(size_t min_size,
+                                      size_t requested_size,
+                                      size_t* actual_size);
 
   virtual HeapWord* mem_allocate(size_t word_size,
                                  bool*  gc_overhead_limit_was_exceeded);
@@ -428,7 +430,9 @@ private:
   // 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);
+  inline HeapWord* attempt_allocation(size_t min_word_size,
+                                      size_t desired_word_size,
+                                      size_t* actual_word_size);
 
   // Second-level mutator allocation attempt: take the Heap_lock and
   // retry the allocation attempt, potentially scheduling a GC

src/hotspot/share/gc/parallel/parallelScavengeHeap.cpp

@@ -478,8 +478,13 @@ size_t ParallelScavengeHeap::unsafe_max_tlab_alloc(Thread* thr) const {
   return young_gen()->eden_space()->unsafe_max_tlab_alloc(thr);
 }
 
-HeapWord* ParallelScavengeHeap::allocate_new_tlab(size_t size) {
-  return young_gen()->allocate(size);
+HeapWord* ParallelScavengeHeap::allocate_new_tlab(size_t min_size, size_t requested_size, size_t* actual_size) {
+  HeapWord* result = young_gen()->allocate(requested_size);
+  if (result != NULL) {
+    *actual_size = requested_size;
+  }
+
+  return result;
 }
 
 void ParallelScavengeHeap::accumulate_statistics_all_tlabs() {

src/hotspot/share/gc/parallel/parallelScavengeHeap.hpp

@@ -85,7 +85,7 @@ class ParallelScavengeHeap : public CollectedHeap {
 
  protected:
   static inline size_t total_invocations();
-  HeapWord* allocate_new_tlab(size_t size);
+  HeapWord* allocate_new_tlab(size_t min_size, size_t requested_size, size_t* actual_size);
 
   inline bool should_alloc_in_eden(size_t size) const;
   inline void death_march_check(HeapWord* const result, size_t size);

src/hotspot/share/gc/shared/collectedHeap.cpp

@@ -384,17 +384,24 @@ HeapWord* CollectedHeap::allocate_from_tlab_slow(Klass* klass, Thread* thread, s
     return NULL;
   }
 
-  // Allocate a new TLAB...
-  HeapWord* obj = Universe::heap()->allocate_new_tlab(new_tlab_size);
+  // Allocate a new TLAB requesting new_tlab_size. Any size
+  // between minimal and new_tlab_size is accepted.
+  size_t actual_tlab_size = 0;
+  size_t min_tlab_size = ThreadLocalAllocBuffer::compute_min_size(size);
+  HeapWord* obj = Universe::heap()->allocate_new_tlab(min_tlab_size, new_tlab_size, &actual_tlab_size);
   if (obj == NULL) {
+    assert(actual_tlab_size == 0, "Allocation failed, but actual size was updated. min: " SIZE_FORMAT ", desired: " SIZE_FORMAT ", actual: " SIZE_FORMAT,
+           min_tlab_size, new_tlab_size, actual_tlab_size);
     return NULL;
   }
+  assert(actual_tlab_size != 0, "Allocation succeeded but actual size not updated. obj at: " PTR_FORMAT " min: " SIZE_FORMAT ", desired: " SIZE_FORMAT,
+         p2i(obj), min_tlab_size, new_tlab_size);
 
-  AllocTracer::send_allocation_in_new_tlab(klass, obj, new_tlab_size * HeapWordSize, size * HeapWordSize, thread);
+  AllocTracer::send_allocation_in_new_tlab(klass, obj, actual_tlab_size * HeapWordSize, size * HeapWordSize, thread);
 
   if (ZeroTLAB) {
     // ..and clear it.
-    Copy::zero_to_words(obj, new_tlab_size);
+    Copy::zero_to_words(obj, actual_tlab_size);
   } else {
     // ...and zap just allocated object.
 #ifdef ASSERT
@@ -402,10 +409,10 @@ HeapWord* CollectedHeap::allocate_from_tlab_slow(Klass* klass, Thread* thread, s
     // ensure that the returned space is not considered parsable by
     // any concurrent GC thread.
     size_t hdr_size = oopDesc::header_size();
-    Copy::fill_to_words(obj + hdr_size, new_tlab_size - hdr_size, badHeapWordVal);
+    Copy::fill_to_words(obj + hdr_size, actual_tlab_size - hdr_size, badHeapWordVal);
 #endif // ASSERT
   }
-  thread->tlab().fill(obj, obj + size, new_tlab_size);
+  thread->tlab().fill(obj, obj + size, actual_tlab_size);
   return obj;
 }
 
@@ -506,7 +513,9 @@ void CollectedHeap::fill_with_objects(HeapWord* start, size_t words, bool zap)
   fill_with_object_impl(start, words, zap);
 }
 
-HeapWord* CollectedHeap::allocate_new_tlab(size_t size) {
+HeapWord* CollectedHeap::allocate_new_tlab(size_t min_size,
+                                           size_t requested_size,
+                                           size_t* actual_size) {
   guarantee(false, "thread-local allocation buffers not supported");
   return NULL;
 }

src/hotspot/share/gc/shared/collectedHeap.hpp

@@ -126,7 +126,13 @@ class CollectedHeap : public CHeapObj<mtInternal> {
   CollectedHeap();
 
   // Create a new tlab. All TLAB allocations must go through this.
-  virtual HeapWord* allocate_new_tlab(size_t size);
+  // To allow more flexible TLAB allocations min_size specifies
+  // the minimum size needed, while requested_size is the requested
+  // size based on ergonomics. The actually allocated size will be
+  // returned in actual_size.
+  virtual HeapWord* allocate_new_tlab(size_t min_size,
+                                      size_t requested_size,
+                                      size_t* actual_size);
 
   // Accumulate statistics on all tlabs.
   virtual void accumulate_statistics_all_tlabs();

src/hotspot/share/gc/shared/genCollectedHeap.cpp

@@ -1146,11 +1146,18 @@ size_t GenCollectedHeap::unsafe_max_tlab_alloc(Thread* thr) const {
   return 0;
 }
 
-HeapWord* GenCollectedHeap::allocate_new_tlab(size_t size) {
+HeapWord* GenCollectedHeap::allocate_new_tlab(size_t min_size,
+                                              size_t requested_size,
+                                              size_t* actual_size) {
   bool gc_overhead_limit_was_exceeded;
-  return mem_allocate_work(size /* size */,
-                           true /* is_tlab */,
-                           &gc_overhead_limit_was_exceeded);
+  HeapWord* result = mem_allocate_work(requested_size /* size */,
+                                       true /* is_tlab */,
+                                       &gc_overhead_limit_was_exceeded);
+  if (result != NULL) {
+    *actual_size = requested_size;
+  }
+
+  return result;
 }
 
 // Requires "*prev_ptr" to be non-NULL.  Deletes and a block of minimal size

src/hotspot/share/gc/shared/genCollectedHeap.hpp

@@ -297,7 +297,9 @@ public:
   virtual size_t tlab_capacity(Thread* thr) const;
   virtual size_t tlab_used(Thread* thr) const;
   virtual size_t unsafe_max_tlab_alloc(Thread* thr) const;
-  virtual HeapWord* allocate_new_tlab(size_t size);
+  virtual HeapWord* allocate_new_tlab(size_t min_size,
+                                      size_t requested_size,
+                                      size_t* actual_size);
 
   // The "requestor" generation is performing some garbage collection
   // action for which it would be useful to have scratch space.  The

src/hotspot/share/gc/shared/threadLocalAllocBuffer.hpp

@@ -141,6 +141,9 @@ public:
   // Otherwise return 0;
   inline size_t compute_size(size_t obj_size);
 
+  // Compute the minimal needed tlab size for the given object size.
+  static inline size_t compute_min_size(size_t obj_size);
+
   // Record slow allocation
   inline void record_slow_allocation(size_t obj_size);
 

src/hotspot/share/gc/shared/threadLocalAllocBuffer.inline.hpp

@@ -54,18 +54,15 @@ inline HeapWord* ThreadLocalAllocBuffer::allocate(size_t size) {
 }
 
 inline size_t ThreadLocalAllocBuffer::compute_size(size_t obj_size) {
-  const size_t aligned_obj_size = align_object_size(obj_size);
-
   // Compute the size for the new TLAB.
   // The "last" tlab may be smaller to reduce fragmentation.
   // unsafe_max_tlab_alloc is just a hint.
   const size_t available_size = Universe::heap()->unsafe_max_tlab_alloc(myThread()) /
                                                   HeapWordSize;
-  size_t new_tlab_size = MIN3(available_size, desired_size() + aligned_obj_size, max_size());
+  size_t new_tlab_size = MIN3(available_size, desired_size() + align_object_size(obj_size), max_size());
 
   // Make sure there's enough room for object and filler int[].
-  const size_t obj_plus_filler_size = aligned_obj_size + alignment_reserve();
-  if (new_tlab_size < obj_plus_filler_size) {
+  if (new_tlab_size < compute_min_size(obj_size)) {
     // If there isn't enough room for the allocation, return failure.
     log_trace(gc, tlab)("ThreadLocalAllocBuffer::compute_size(" SIZE_FORMAT ") returns failure",
                         obj_size);
@@ -76,6 +73,11 @@ inline size_t ThreadLocalAllocBuffer::compute_size(size_t obj_size) {
   return new_tlab_size;
 }
 
+inline size_t ThreadLocalAllocBuffer::compute_min_size(size_t obj_size) {
+  const size_t aligned_obj_size = align_object_size(obj_size);
+  const size_t size_with_reserve = aligned_obj_size + alignment_reserve();
+  return MAX2(size_with_reserve, MinTLABSize);
+}
 
 void ThreadLocalAllocBuffer::record_slow_allocation(size_t obj_size) {
   // Raise size required to bypass TLAB next time. Why? Else there's