JDK-8195115 : G1 Old Gen MemoryPool CollectionUsage.used values don't reflect mixed GC results
  • Type: Bug
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: 8u144,9.0.1,10
  • Priority: P4
  • Status: Resolved
  • Resolution: Fixed
  • OS: generic
  • CPU: generic
  • Submitted: 2018-01-15
  • Updated: 2019-05-16
  • Resolved: 2018-06-19
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JDK 11 Other
11 b19Fixed openjdk8u212Fixed
Related Reports
CSR :  
JDK-8196719 - G1 Old Gen MemoryPool CollectionUsage.used values don't reflect mixed GC results
Relates :  
JDK-8196989 - Revamp G1 JMX MemoryPool and GarbageCollector MXBean definitions
Relates :  
JDK-8210193 - [TESTBUG]gc/g1/mixedgc/TestOldGenCollectionUsage.java fails intermittently with OutOfMemoryError in CDS mode
Relates :  
JDK-8175375 - MemoryPoolMXBean.getCollectionUsage() does not works with -XX:+UseG1GC -XX:+ExplicitGCInvokesConcurrent
Relates :  
JDK-8224030 - Old Gen usage in MemoryUsage.getCollectionUsage() not updated for young collections
Relates :  
JDK-8205344 - TraceMemoryManagerStats changes in JDK-8195115 broke ZGC
Description
The specification for the MemoryPoolMXBean.CollectionUsage says that it "Returns the memory usage after the Java virtual machine most recently expended effort in recycling unused objects in this memory pool." For the two G1 MemoryPools (G1 Young Gen and G1 Old Gen), CollectionUsage.used does not reflect the effect of mixed GCs on G1 Old Gen. For G1 Old Gen, CollectionUsage.used reflects only the effect of a full collection. Reproducer attached.

A fix is to (1) add the G1 Old Gen MemoryPool to the G1 Young Gen GarbageCollector, and (2) inform the latter when a mixed GC occurs so it can save the state of the G1 Old Gen MemoryPool.

At Amazon, we find MemoryPool.CollectionUsage to be a more useful metric than MemoryPool.Usage because the former is a much better measure of long term heap occupancy than the latter.