JDK-8238687 : Investigate memory uncommit during young collections in G1
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: 15
  • Priority: P4
  • Status: Open
  • Resolution: Unresolved
  • Submitted: 2020-02-07
  • Updated: 2021-07-26
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Other
tbdUnresolved
Related Reports
Blocks :  
JDK-8247843 - Reconsider G1 default GCTimeRatio value
Blocks :  
JDK-8243672 - Short term pause time ratio calculation in G1 off
Blocks :  
JDK-8248324 - G1: Remove resizing during Remark
Blocks :  
JDK-8244603 - G1 incorrectly limiting young gen size when using the reserve can result in repeated full gcs
Relates :  
JDK-8238686 - G1 may waste lots of space or fail to uncommit when observing MinHeapFreeRatio during sizing after full gc
Description
Currently G1 uncommits memory in the Remark pause only. It may be better to uncommit at other places (see JDK-8238686) when heap usage is actually low.

Similar to expansion, G1 could regularly check whether the GCTimeRatio is low enough (e.g. < 1/2 GCTimeRatio) to warrant heap shrinking.

Note that the amount of uncommit should be so that heap resizing activity during further operation is minimal.
Comments
Before we can fix this, gc time ratio tracking must be fairly correct, i.e. properly track actual gc time ratio (JDK-8243672) and try to avoid unnecessary gc time ratio spikes (JDK-8244603)
15-05-2020