8348595: GenShen: Fix generational free-memory no-progress check

Reviewed-by: phh, xpeng

Author: kdnilsen

src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp

@@ -303,7 +303,7 @@ void ShenandoahDegenGC::op_degenerated() {
 
  // Check for futility and fail. There is no reason to do several back-to-back Degenerated cycles,
  // because that probably means the heap is overloaded and/or fragmented.
- if (!metrics.is_good_progress()) {
+ if (!metrics.is_good_progress(_generation)) {
  heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
  op_degenerated_futile();
  } else {

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp

@@ -435,6 +435,9 @@ class ShenandoahFreeSet : public CHeapObj<mtGC> {
  // Acquire heap lock and log status, assuming heap lock is not acquired by the caller.
  void log_status_under_lock();
 
+ // Note that capacity is the number of regions that had available memory at most recent rebuild. It is not the
+ // entire size of the young or global generation. (Regions within the generation that were fully utilized at time of
+ // rebuild are not counted as part of capacity.)
  inline size_t capacity() const { return _partitions.capacity_of(ShenandoahFreeSetPartitionId::Mutator); }
  inline size_t used() const { return _partitions.used_by(ShenandoahFreeSetPartitionId::Mutator); }
 

src/hotspot/share/gc/shenandoah/shenandoahFullGC.cpp

@@ -119,7 +119,7 @@ void ShenandoahFullGC::op_full(GCCause::Cause cause) {
 
  metrics.snap_after();
 
- if (metrics.is_good_progress()) {
+ if (metrics.is_good_progress(heap->global_generation())) {
  heap->notify_gc_progress();
  } else {
  // Nothing to do. Tell the allocation path that we have failed to make

src/hotspot/share/gc/shenandoah/shenandoahMetrics.cpp

@@ -43,10 +43,28 @@ void ShenandoahMetricsSnapshot::snap_after() {
  _ef_after = _heap->free_set()->external_fragmentation();
 }
 
-bool ShenandoahMetricsSnapshot::is_good_progress() {
+// For degenerated GC, generation is Young in generational mode, Global in non-generational mode.
+// For full GC, generation is always Global.
+//
+// Note that the size of the chosen collection set is proportional to the relevant generation's collection set.
+// Note also that the generation size may change following selection of the collection set, as a side effect
+// of evacuation. Evacuation may promote objects, causing old to grow and young to shrink. Or this may be a
+// mixed evacuation. When old regions are evacuated, this typically allows young to expand. In all of these
+// various scenarios, the purpose of asking is_good_progress() is to determine if there is enough memory available
+// within young generation to justify making an attempt to perform a concurrent collection. For this reason, we'll
+// use the current size of the generation (which may not be different than when the collection set was chosen) to
+// assess how much free memory we require in order to consider the most recent GC to have had good progress.
+
+bool ShenandoahMetricsSnapshot::is_good_progress(ShenandoahGeneration* generation) {
  // Under the critical threshold?
- size_t free_actual = _heap->free_set()->available();
- size_t free_expected = _heap->max_capacity() / 100 * ShenandoahCriticalFreeThreshold;
+ ShenandoahFreeSet* free_set = _heap->free_set();
+ size_t free_actual = free_set->available();
+
+ // ShenandoahCriticalFreeThreshold is expressed as a percentage. We multiple this percentage by 1/100th
+ // of the generation capacity to determine whether the available memory within the generation exceeds the
+ // critical threshold.
+ size_t free_expected = (generation->max_capacity() / 100) * ShenandoahCriticalFreeThreshold;
+
  bool prog_free = free_actual >= free_expected;
  log_info(gc, ergo)("%s progress for free space: %zu%s, need %zu%s",
  prog_free ? "Good" : "Bad",

src/hotspot/share/gc/shenandoah/shenandoahMetrics.hpp

@@ -40,7 +40,7 @@ class ShenandoahMetricsSnapshot : public StackObj {
  void snap_before();
  void snap_after();
 
- bool is_good_progress();
+ bool is_good_progress(ShenandoahGeneration *generation);
 };
 
 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHMETRICS_HPP