Why mixed GC could not clear memory while Full DC did?

Question

Running G1GC (Java 8) with below

MaxGCPauseMillis=100
InitiatingHeapOccupancyPercent=30
G1MixedGCLiveThresholdPercent=85

and memory is 115 GB.

I notice that there was a Full GC and it brought down memory from 111GB to 65 GB. Prior to Full GC there were few mixed GC but that was unable to clear memory. Is there any other parameter I should set?

In general, decisions about garbage collection are the runtime's responsibility. You shouldn't ever have to think about GC, and if you do, there are patterns like Object Pools that are designed to mitigate GC problems. — Robert Harvey, Dec 11 '18 at 18:56
If mixed GC could always reclaim all memory, the feature of doing a full GC did not exist. And if there was a magic option that improves the performance of every application, that option was already the default. — Holger, Dec 12 '18 at 14:37

score 0 · Answer 1 · answered Jan 06 '19 at 12:26

0

Promptly returning unused memory to the operating system is an OpenJDK 12 feature for the G1 collector:

JEP 346: Promptly Return Unused Committed Memory from G1 (tracking issue)

You are observing the limitation described in the proposal: only full GCs return memory to the operating system, mixed/minor GCs do no ot. Also see this related question.

answered Jan 06 '19 at 12:26

Florian Weimer

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Thanks. I am trying to see whether below options will help. G1OldCSetRegionThresholdPercent=20 G1MixedGCCountTarget=12 – user3660575 Jan 07 '19 at 16:21
Only indirectly, if it happens to increase the frequency of full GCs (which will make G1 perform poorly). – Florian Weimer Jan 07 '19 at 17:25

Why mixed GC could not clear memory while Full DC did?

1 Answers1