From f3ec83e45b3c6f7f3183c2726701dccd11a00550 Mon Sep 17 00:00:00 2001
Date: Wed, 3 Jun 2020 09:39:34 +0000
Subject: [PATCH] ZGC: correct free heap size excluding waste in
 rule_allocation_rate

Summary: <gc>: <need to consider waste for free heap size >
LLT:
Bug url:
---
 src/hotspot/share/gc/z/zDirector.cpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/hotspot/share/gc/z/zDirector.cpp b/src/hotspot/share/gc/z/zDirector.cpp
index 209bf0a..6f894c3 100644
--- a/src/hotspot/share/gc/z/zDirector.cpp
+++ b/src/hotspot/share/gc/z/zDirector.cpp
@@ -120,7 +120,9 @@ bool ZDirector::rule_allocation_rate() const {
   // the allocation rate variance, which means the probability is 1 in 1000
   // that a sample is outside of the confidence interval.
   const double max_alloc_rate = (ZStatAllocRate::avg() * ZAllocationSpikeTolerance) + (ZStatAllocRate::avg_sd() * one_in_1000);
-  const double time_until_oom = free / (max_alloc_rate + 1.0); // Plus 1.0B/s to avoid division by zero
+
+  // Plus 1.0B/s to avoid division by zero. Small or medium page allow 12.5% waste, so 87.5% used.
+  const double time_until_oom = (0.875 * free) / (max_alloc_rate + 1.0);
 
   // Calculate max duration of a GC cycle. The duration of GC is a moving
   // average, we add ~3.3 sigma to account for the GC duration variance.
-- 
1.8.3.1