--- old/src/hotspot/share/gc/g1/g1HeapSizingPolicy.hpp	2020-06-10 10:12:05.097048906 +0200
+++ new/src/hotspot/share/gc/g1/g1HeapSizingPolicy.hpp	2020-06-10 10:12:05.001045608 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2016, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -26,38 +26,82 @@
 #define SHARE_GC_G1_G1HEAPSIZINGPOLICY_HPP
 
 #include "memory/allocation.hpp"
+#include "utilities/numberSeq.hpp"
 
 class G1Analytics;
 class G1CollectedHeap;
 
+//
+// Contains heuristics for heap sizing, i.e. expansion or shrinking, during operation
+// based on gc time ratio.
+//
+// The heuristics tracks both short term and long term behavior to effect heap
+// size change.
+// 
+// Short term tracking is based on short-term gc time ratio behavior: for this we
+// record events for when actual gc time ratio is outside the range of
+// [GCTimeRatio * G1MinimumPercentOfGCTimeRatio, GCTimeRatio] or not in a counter.
+// If below that range, we decrement that counter, if above, we increment it.
+//
+// The intent of this mechanism is to filter short term events as heap sizing has
+// some overhead.
+//
+// If that counter reaches the MinOverThresholdForExpansion we consider expansion,
+// if that counter reaches -MinOverThresholdForShrink we consider heap shrinking.
+//
+// While doing so, we accumulate the difference to the midpoint of this range to
+// guide the expansion/shrinking amount.
+//
+// Further, if there is no short-term based resizing event for a "long" time, we
+// decay that counter, i.e. drop it towards zero again to avoid that previous
+// intermediate length short term behavior followed by a quiet time and a single
+// short term event causes unnecessary resizes.
+//
+// Long term behavior is solely managed by regularly comparing actual long term
+// gc time ratio with the boundaries of above range in  regular long term
+// intervals. If current long term gc time ratio is outside, expand or shrink
+// respectively.
+//
 class G1HeapSizingPolicy: public CHeapObj<mtGC> {
-  // MinOverThresholdForGrowth must be less than the number of recorded
-  // pause times in G1Analytics, representing the minimum number of pause
-  // time ratios that exceed GCTimeRatio before a heap expansion will be triggered.
-  const static uint MinOverThresholdForGrowth = 4;
+  // MinOverThresholdForExpansion/Shrink define the number of actual gc time
+  // ratios over the upper and lower thresholds respectively.
+  const static int MinOverThresholdForExpansion = 4;
+  const static int MinOverThresholdForShrink = 4;
 
   const G1CollectedHeap* _g1h;
   const G1Analytics* _analytics;
 
-  const uint _num_prev_pauses_for_heuristics;
-  // Ratio check data for determining if heap growth is necessary.
-  uint _ratio_over_threshold_count;
-  double _ratio_over_threshold_sum;
-  uint _pauses_since_start;
+  const uint _long_term_interval;
+  // Number of times actual gc time ratio crossed lower and upper threshold
+  // recently; every time the upper threshold is exceeded, it is incremented,
+  // and decremented if the lower threshold is undercut.
+  int _ratio_exceeds_threshold;
+  // Recent actual gc time ratios relative to the middle of lower and upper threshold.
+  TruncatedSeq _recent_pause_ratios;
+  uint _long_term_count;
+
+  // Clear ratio tracking data used by resize_amount().
+  void reset_ratio_tracking_data();
+  void decay_ratio_tracking_data();
 
-  // Scale "full" gc pause time threshold with heap size as we want to resize more
+  // Scale "full" gc time ratio threshold with heap size as we want to resize more
   // eagerly at small heap sizes.
   double scale_with_heap(double pause_time_threshold);
 
+  // Scale the ratio delta depending on how far it exceeds the actual target gc time
+  // ratio.
+  double scale_resize_ratio_delta(double ratio_delta);
+
   G1HeapSizingPolicy(const G1CollectedHeap* g1h, const G1Analytics* analytics);
 public:
 
-  // If an expansion would be appropriate, because recent GC overhead had
-  // exceeded the desired limit, return an amount to expand by.
-  size_t expansion_amount();
+  // Return by how many bytes the heap should be changed based on recent gc time
+  // ratio after young collection. Positive values mean expansion is desired, and
+  // negative values mean desired shrinking.
+  ssize_t resize_amount_after_young_gc();
 
-  // Clear ratio tracking data used by expansion_amount().
-  void clear_ratio_check_data();
+  // Calculate the target capacity based on used bytes and free ratio.
+  size_t target_heap_capacity(size_t used_bytes, uintx free_ratio) const;
 
   static G1HeapSizingPolicy* create(const G1CollectedHeap* g1h, const G1Analytics* analytics);
 };