8257774: G1: Trigger collect when free region count drops below threshold to prevent evacuation failures

Reviewed-by: sjohanss, tschatzl

Author: Aditya Mandaleeka

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

@@ -164,21 +164,22 @@ class G1AllocRegion : public CHeapObj<mtGC> {
                                       size_t desired_word_size,
                                       size_t* actual_word_size);
 
-  // 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);
-  // Same as attempt_allocation_locked(size_t, bool), but allowing specification
-  // of minimum word size of the block in min_word_size, and the maximum word
-  // size of the allocation in desired_word_size. The actual size of the block is
-  // returned in actual_word_size.
+  // Second-level allocation: Should be called while holding a
+  // lock. We require that the caller takes the appropriate lock
+  // before calling this so that it is easier to make it conform
+  // to the locking protocol. The min and desired word size allow
+  // specifying a minimum and maximum size of the allocation. The
+  // actual size of allocation is returned in actual_word_size.
   inline HeapWord* attempt_allocation_locked(size_t min_word_size,
                                              size_t desired_word_size,
                                              size_t* actual_word_size);
 
+  // Perform an allocation out of a new allocation region, retiring the current one.
+  inline HeapWord* attempt_allocation_using_new_region(size_t min_word_size,
+                                                       size_t desired_word_size,
+                                                       size_t* actual_word_size);
+
   // 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

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

@@ -99,16 +99,19 @@ inline HeapWord* G1AllocRegion::attempt_allocation_locked(size_t word_size) {
 inline HeapWord* G1AllocRegion::attempt_allocation_locked(size_t min_word_size,
                                                           size_t desired_word_size,
                                                           size_t* actual_word_size) {
-  // First we have to redo 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(min_word_size, desired_word_size, actual_word_size);
   if (result != NULL) {
     return result;
   }
 
+  return attempt_allocation_using_new_region(min_word_size, desired_word_size, actual_word_size);
+}
+
+inline HeapWord* G1AllocRegion::attempt_allocation_using_new_region(size_t min_word_size,
+                                                                    size_t desired_word_size,
+                                                                    size_t* actual_word_size) {
   retire(true /* fill_up */);
-  result = new_alloc_region_and_allocate(desired_word_size, false /* force */);
+  HeapWord* result = new_alloc_region_and_allocate(desired_word_size, false /* force */);
   if (result != NULL) {
     *actual_word_size = desired_word_size;
     trace("alloc locked (second attempt)", min_word_size, desired_word_size, *actual_word_size, result);

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

@@ -112,10 +112,19 @@ class G1Allocator : public CHeapObj<mtGC> {
 
   // Allocate blocks of memory during mutator time.
 
+  // Attempt allocation in the current alloc region.
   inline HeapWord* attempt_allocation(size_t min_word_size,
                                       size_t desired_word_size,
                                       size_t* actual_word_size);
+
+  // Attempt allocation, retiring the current region and allocating a new one. It is
+  // assumed that attempt_allocation() has been tried and failed already first.
+  inline HeapWord* attempt_allocation_using_new_region(size_t word_size);
+
+  // This is to be called when holding an appropriate lock. It first tries in the
+  // current allocation region, and then attempts an allocation using a new region.
   inline HeapWord* attempt_allocation_locked(size_t word_size);
+
   inline HeapWord* attempt_allocation_force(size_t word_size);
 
   size_t unsafe_max_tlab_alloc();

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

@@ -53,16 +53,29 @@ inline HeapWord* G1Allocator::attempt_allocation(size_t min_word_size,
                                                  size_t desired_word_size,
                                                  size_t* actual_word_size) {
   uint node_index = current_node_index();
+
   HeapWord* result = mutator_alloc_region(node_index)->attempt_retained_allocation(min_word_size, desired_word_size, actual_word_size);
   if (result != NULL) {
     return result;
   }
+
   return mutator_alloc_region(node_index)->attempt_allocation(min_word_size, desired_word_size, actual_word_size);
 }
 
+inline HeapWord* G1Allocator::attempt_allocation_using_new_region(size_t word_size) {
+  uint node_index = current_node_index();
+  size_t temp;
+  HeapWord* result = mutator_alloc_region(node_index)->attempt_allocation_using_new_region(word_size, word_size, &temp);
+  assert(result != NULL || mutator_alloc_region(node_index)->get() == NULL,
+         "Must not have a mutator alloc region if there is no memory, but is " PTR_FORMAT,
+         p2i(mutator_alloc_region(node_index)->get()));
+  return result;
+}
+
 inline HeapWord* G1Allocator::attempt_allocation_locked(size_t word_size) {
   uint node_index = current_node_index();
   HeapWord* result = mutator_alloc_region(node_index)->attempt_allocation_locked(word_size);
+
   assert(result != NULL || mutator_alloc_region(node_index)->get() == NULL,
          "Must not have a mutator alloc region if there is no memory, but is " PTR_FORMAT, p2i(mutator_alloc_region(node_index)->get()));
   return result;

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

@@ -394,26 +394,42 @@ HeapWord* G1CollectedHeap::attempt_allocation_slow(size_t word_size) {
   HeapWord* result = NULL;
   for (uint try_count = 1, gclocker_retry_count = 0; /* we'll return */; try_count += 1) {
     bool should_try_gc;
+    bool preventive_collection_required = false;
     uint gc_count_before;
 
     {
       MutexLocker x(Heap_lock);
-      result = _allocator->attempt_allocation_locked(word_size);
+
+      // Now that we have the lock, we first retry the allocation in case another
+      // thread changed the region while we were waiting to acquire the lock.
+      size_t actual_size;
+      result = _allocator->attempt_allocation(word_size, word_size, &actual_size);
       if (result != NULL) {
         return result;
       }
 
-      // If the GCLocker is active and we are bound for a GC, try expanding young gen.
-      // This is different to when only GCLocker::needs_gc() is set: try to avoid
-      // waiting because the GCLocker is active to not wait too long.
-      if (GCLocker::is_active_and_needs_gc() && policy()->can_expand_young_list()) {
-        // No need for an ergo message here, can_expand_young_list() does this when
-        // it returns true.
-        result = _allocator->attempt_allocation_force(word_size);
+      preventive_collection_required = policy()->preventive_collection_required(1);
+      if (!preventive_collection_required) {
+        // We've already attempted a lock-free allocation above, so we don't want to
+        // do it again. Let's jump straight to replacing the active region.
+        result = _allocator->attempt_allocation_using_new_region(word_size);
         if (result != NULL) {
           return result;
         }
+
+        // If the GCLocker is active and we are bound for a GC, try expanding young gen.
+        // This is different to when only GCLocker::needs_gc() is set: try to avoid
+        // waiting because the GCLocker is active to not wait too long.
+        if (GCLocker::is_active_and_needs_gc() && policy()->can_expand_young_list()) {
+          // No need for an ergo message here, can_expand_young_list() does this when
+          // it returns true.
+          result = _allocator->attempt_allocation_force(word_size);
+          if (result != NULL) {
+            return result;
+          }
+        }
       }
+
       // Only try a GC if the GCLocker does not signal the need for a GC. Wait until
       // the GCLocker initiated GC has been performed and then retry. This includes
       // the case when the GC Locker is not active but has not been performed.
@@ -423,9 +439,10 @@ HeapWord* G1CollectedHeap::attempt_allocation_slow(size_t word_size) {
     }
 
     if (should_try_gc) {
+      GCCause::Cause gc_cause = preventive_collection_required ? GCCause::_g1_preventive_collection
+                                                              : GCCause::_g1_inc_collection_pause;
       bool succeeded;
-      result = do_collection_pause(word_size, gc_count_before, &succeeded,
-                                   GCCause::_g1_inc_collection_pause);
+      result = do_collection_pause(word_size, gc_count_before, &succeeded, gc_cause);
       if (result != NULL) {
         assert(succeeded, "only way to get back a non-NULL result");
         log_trace(gc, alloc)("%s: Successfully scheduled collection returning " PTR_FORMAT,
@@ -840,21 +857,25 @@ HeapWord* G1CollectedHeap::attempt_allocation_humongous(size_t word_size) {
   HeapWord* result = NULL;
   for (uint try_count = 1, gclocker_retry_count = 0; /* we'll return */; try_count += 1) {
     bool should_try_gc;
+    bool preventive_collection_required = false;
     uint gc_count_before;
 
 
     {
       MutexLocker 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);
-      if (result != NULL) {
-        size_t size_in_regions = humongous_obj_size_in_regions(word_size);
-        policy()->old_gen_alloc_tracker()->
-          add_allocated_humongous_bytes_since_last_gc(size_in_regions * HeapRegion::GrainBytes);
-        return result;
+      size_t size_in_regions = humongous_obj_size_in_regions(word_size);
+      preventive_collection_required = policy()->preventive_collection_required((uint)size_in_regions);
+      if (!preventive_collection_required) {
+        // 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) {
+          policy()->old_gen_alloc_tracker()->
+            add_allocated_humongous_bytes_since_last_gc(size_in_regions * HeapRegion::GrainBytes);
+          return result;
+        }
       }
 
       // Only try a GC if the GCLocker does not signal the need for a GC. Wait until
@@ -866,9 +887,10 @@ HeapWord* G1CollectedHeap::attempt_allocation_humongous(size_t word_size) {
     }
 
     if (should_try_gc) {
+      GCCause::Cause gc_cause = preventive_collection_required ? GCCause::_g1_preventive_collection
+                                                              : GCCause::_g1_humongous_allocation;
       bool succeeded;
-      result = do_collection_pause(word_size, gc_count_before, &succeeded,
-                                   GCCause::_g1_humongous_allocation);
+      result = do_collection_pause(word_size, gc_count_before, &succeeded, gc_cause);
       if (result != NULL) {
         assert(succeeded, "only way to get back a non-NULL result");
         log_trace(gc, alloc)("%s: Successfully scheduled collection returning " PTR_FORMAT,

src/hotspot/share/gc/g1/g1CollectionSet.cpp

@@ -111,6 +111,10 @@ void G1CollectionSet::clear_candidates() {
   _candidates = NULL;
 }
 
+bool G1CollectionSet::has_candidates() {
+  return _candidates != NULL && !_candidates->is_empty();
+}
+
 void G1CollectionSet::set_recorded_rs_length(size_t rs_length) {
   _recorded_rs_length = rs_length;
 }

src/hotspot/share/gc/g1/g1CollectionSet.hpp

@@ -272,6 +272,7 @@ class G1CollectionSet {
   void initialize(uint max_region_length);
 
   void clear_candidates();
+  bool has_candidates();
 
   void set_candidates(G1CollectionSetCandidates* candidates) {
     assert(_candidates == NULL, "Trying to replace collection set candidates.");

src/hotspot/share/gc/g1/g1Policy.cpp

@@ -68,6 +68,8 @@ G1Policy::G1Policy(STWGCTimer* gc_timer) :
   _reserve_regions(0),
   _young_gen_sizer(),
   _free_regions_at_end_of_collection(0),
+  _predicted_surviving_bytes_from_survivor(0),
+  _predicted_surviving_bytes_from_old(0),
   _rs_length(0),
   _rs_length_prediction(0),
   _pending_cards_at_gc_start(0),
@@ -450,6 +452,7 @@ void G1Policy::record_full_collection_end() {
   // also call this on any additional surv rate groups
 
   _free_regions_at_end_of_collection = _g1h->num_free_regions();
+  update_survival_estimates_for_next_collection();
   _survivor_surv_rate_group->reset();
   update_young_list_max_and_target_length();
   update_rs_length_prediction();
@@ -779,6 +782,7 @@ void G1Policy::record_collection_pause_end(double pause_time_ms, bool concurrent
   _free_regions_at_end_of_collection = _g1h->num_free_regions();
 
   update_rs_length_prediction();
+  update_survival_estimates_for_next_collection();
 
   // Do not update dynamic IHOP due to G1 periodic collection as it is highly likely
   // that in this case we are not running in a "normal" operating mode.
@@ -1400,6 +1404,86 @@ void G1Policy::calculate_optional_collection_set_regions(G1CollectionSetCandidat
                             num_optional_regions, max_optional_regions, prediction_ms);
 }
 
+// Number of regions required to store the given number of bytes, taking
+// into account the target amount of wasted space in PLABs.
+static size_t get_num_regions_adjust_for_plab_waste(size_t byte_count) {
+  size_t byte_count_adjusted = byte_count * (size_t)(100 + TargetPLABWastePct) / 100.0;
+
+  // Round up the region count
+  return (byte_count_adjusted + HeapRegion::GrainBytes - 1) / HeapRegion::GrainBytes;
+}
+
+bool G1Policy::preventive_collection_required(uint alloc_region_count) {
+  if (!G1AllowPreventiveGC || !Universe::is_fully_initialized()) {
+    // Don't attempt any preventive GC's if the feature is disabled,
+    // or before initialization is complete.
+    return false;
+  }
+
+  if (_g1h->young_regions_count() == 0 && !_collection_set->has_candidates()) {
+    return false;
+  }
+
+  uint eden_count = _g1h->eden_regions_count();
+  size_t const eden_surv_bytes_pred = _eden_surv_rate_group->accum_surv_rate_pred(eden_count) * HeapRegion::GrainBytes;
+  size_t const total_young_predicted_surviving_bytes = eden_surv_bytes_pred + _predicted_surviving_bytes_from_survivor;
+
+  uint required_regions = (uint)(get_num_regions_adjust_for_plab_waste(total_young_predicted_surviving_bytes) +
+                                get_num_regions_adjust_for_plab_waste(_predicted_surviving_bytes_from_old));
+
+  if (required_regions > _g1h->num_free_regions() - alloc_region_count) {
+    log_debug(gc, ergo, cset)("Preventive GC, insufficient free regions. Predicted need %u. Curr Eden %u (Pred %u). Curr Survivor %u (Pred %u). Curr Old %u (Pred %u) Free %u Alloc %u",
+            required_regions,
+            eden_count,
+            (uint)get_num_regions_adjust_for_plab_waste(eden_surv_bytes_pred),
+            _g1h->survivor_regions_count(),
+            (uint)get_num_regions_adjust_for_plab_waste(_predicted_surviving_bytes_from_survivor),
+            _g1h->old_regions_count(),
+            (uint)get_num_regions_adjust_for_plab_waste(_predicted_surviving_bytes_from_old),
+            _g1h->num_free_regions(),
+            alloc_region_count);
+
+    return true;
+  }
+
+  return false;
+}
+
+void G1Policy::update_survival_estimates_for_next_collection() {
+  // Predict the number of bytes of surviving objects from survivor and old
+  // regions and update the associated members.
+
+  // Survivor regions
+  size_t survivor_bytes = 0;
+  const GrowableArray<HeapRegion*>* survivor_regions = _g1h->survivor()->regions();
+  for (GrowableArrayIterator<HeapRegion*> it = survivor_regions->begin();
+       it != survivor_regions->end();
+       ++it) {
+    survivor_bytes += predict_bytes_to_copy(*it);
+  }
+
+  _predicted_surviving_bytes_from_survivor = survivor_bytes;
+
+  // Old regions
+  if (!_collection_set->has_candidates()) {
+    _predicted_surviving_bytes_from_old = 0;
+    return;
+  }
+
+  // Use the minimum old gen collection set as conservative estimate for the number
+  // of regions to take for this calculation.
+  G1CollectionSetCandidates *candidates = _collection_set->candidates();
+  uint iterate_count = MIN2(candidates->num_remaining(), calc_min_old_cset_length(candidates));
+  uint current_index = candidates->cur_idx();
+  size_t old_bytes = 0;
+  for (uint i = 0; i < iterate_count; i++) {
+    HeapRegion *region = candidates->at(current_index + i);
+    old_bytes += predict_bytes_to_copy(region);
+  }
+
+  _predicted_surviving_bytes_from_old = old_bytes;
+}
+
 void G1Policy::transfer_survivors_to_cset(const G1SurvivorRegions* survivors) {
   note_start_adding_survivor_regions();
 

src/hotspot/share/gc/g1/g1Policy.hpp

@@ -98,6 +98,11 @@ class G1Policy: public CHeapObj<mtGC> {
 
   uint _free_regions_at_end_of_collection;
 
+  // These values are predictions of how much we think will survive in each
+  // section of the heap.
+  size_t _predicted_surviving_bytes_from_survivor;
+  size_t _predicted_surviving_bytes_from_old;
+
   size_t _rs_length;
 
   size_t _rs_length_prediction;
@@ -345,7 +350,17 @@ class G1Policy: public CHeapObj<mtGC> {
                                                  double time_remaining_ms,
                                                  uint& num_optional_regions);
 
+  // Returns whether a collection should be done proactively, taking into
+  // account the current number of free regions and the expected survival
+  // rates in each section of the heap.
+  bool preventive_collection_required(uint region_count);
+
 private:
+
+  // Predict the number of bytes of surviving objects from survivor and old
+  // regions and update the associated members.
+  void update_survival_estimates_for_next_collection();
+
   // Set the state to start a concurrent marking cycle and clear
   // _initiate_conc_mark_if_possible because it has now been
   // acted on.

src/hotspot/share/gc/g1/g1VMOperations.cpp

@@ -119,10 +119,15 @@ VM_G1CollectForAllocation::VM_G1CollectForAllocation(size_t         word_size,
   _gc_cause = gc_cause;
 }
 
+bool VM_G1CollectForAllocation::should_try_allocation_before_gc() {
+  // Don't allocate before a preventive GC.
+  return _gc_cause != GCCause::_g1_preventive_collection;
+}
+
 void VM_G1CollectForAllocation::doit() {
   G1CollectedHeap* g1h = G1CollectedHeap::heap();
 
-  if (_word_size > 0) {
+  if (should_try_allocation_before_gc() && _word_size > 0) {
     // An allocation has been requested. So, try to do that first.
     _result = g1h->attempt_allocation_at_safepoint(_word_size,
                                                    false /* expect_null_cur_alloc_region */);