JDK-8236073 : G1: Use SoftMaxHeapSize to guide GC heuristics
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: 15
  • Priority: P4
  • Status: Open
  • Resolution: Unresolved
  • Submitted: 2019-12-17
  • Updated: 2021-07-26
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Description
Use SoftMaxHeapSize as input for heap sizing and marking heuristics
Comments
FWIW, for ZGC you can set SoftMaxHeapSize below -Xms. The GC will try to keep the heap size below SoftMaxHeapSize, but it won't uncommit memory past -Xms. Some extra info about this that was thought about when SoftMaxHeapSize was introduced for ZGC: In HotSpot, -Xms denotes both the minimum heap size but also initial heap size. The minimum heap size is "the minimum committed memory for the heap", and the initial heap size is "the initially committed memory for the heap when the GC starts". There used to be a cumbersome way to set these two properties to different values. I "recently" created a new flag, MinHeapSize, so that the users could more easily set them. See JDK-8223837. We now flags with this relationship: MinHeapSize <= InitialHeapSize <= MaxHeapSize Now that SoftMaxHeapSize is manageable we can end up in a situation similar to what you are talking about: The user has set -Xmx == -Xms, but later decides that it might be good to shrink the heap. If SoftMaxHeapSize is set to a lower value, the GC will try to stay below that heap useage, but it won't uncommit the unused overhead. As a way to deal with that I created a prototype to make MinHeapSize manageable, see: JDK-8224879.
11-09-2020

Is it supported to set SoftMaxHeapSize below Xms? I hope it is supported, because it can help a process with Xms=Xmx to temporarily lower its footprint.
11-09-2020

Since a "hard" SoftMaxHeapSize may be followed by higher gc time, consider making the relevant options manageable so that a user can allow more gc usage during the time SoftMaxHeapSize is set lower to avoid unexpected expansions. I.e. giving the user the flexibility to opt for lower memory usage at higher GC activity.
07-02-2020