-XX:+ExplicitGCInvokesConcurrent Disables Heap Shrinking at G1

[Ruslan Synytsky]

On Sat, 19 Sep 2020 at 12:26, Kim Barrett <kim.barrett at oracle.com> wrote:

> > On Sep 19, 2020, at 2:39 AM, Ruslan Synytsky <rs at jelastic.com> wrote:
> >
> > Hi, after analysis of one of the end user environments (JIRA), it looks
> > like specifying +ExplicitGCInvokesConcurrent disables memory uncommitment
> > even if you explicitly execute System.gc() or jcmd $pid GC.run.
> >
> > java -version
> >
> >
> >
> > *openjdk version "1.8.0_262"OpenJDK Runtime Environment
> > (AdoptOpenJDK)(build 1.8.0_262-b10)OpenJDK 64-Bit Server VM
> > (AdoptOpenJDK)(build 25.262-b10, mixed mode)*
> >
> > jcmd $pid VM.flags
> >
> > *-XX:CICompilerCount=3 -XX:ConcGCThreads=1
> -XX:+ExplicitGCInvokesConcurrent
> > -XX:G1HeapRegionSize=1048576 -XX:GCLogFileSize=20971520
> > -XX:InitialCodeCacheSize=33554432 -XX:InitialHeapSize=402653184
> > -XX:MarkStackSize=4194304 -XX:MaxHeapFreeRatio=30
> > -XX:MaxHeapSize=3007315968 -XX:MaxNewSize=1803550720
> > -XX:MinHeapDeltaBytes=1048576 -XX:MinHeapFreeRatio=10
> > -XX:NumberOfGCLogFiles=5 -XX:-OmitStackTraceInFastThrow -XX:+PrintGC
> > -XX:+PrintGCCause -XX:+PrintGCDateStamps -XX:+PrintGCDetails
> > -XX:+PrintGCTimeStamps -XX:ReservedCodeCacheSize=536870912
> > -XX:+UnlockExperimentalVMOptions -XX:+UseCompressedClassPointers
> > -XX:+UseCompressedOops -XX:+UseFastUnorderedTimeStamps -XX:+UseG1GC
> > -XX:+UseGCLogFileRotation *
> >
> > Min=384M
> > Used=654M
> > *Committed=1188M*
> > Max=2868M
> >
> > The same application running without -XX:+ExplicitGCInvokesConcurrent
> > option has twice smaller Committed heap.
> >
> > Min=384M
> > Used=431M
> > *Committed=598M*
> > Max=2868M
> >
> > Is it an expected behavior?
>
> Yes, this is expected.
>
> Reducing committed memory after a concurrent collection was
> RFE 6490394: G1: Allow heap shrinking / memory unmapping after reclaiming
> regions during Remark.
> https://bugs.openjdk.java.net/browse/JDK-6490394
> It was fixed in JDK 12, and has not been backported.
>
> Before that change, G1 will only uncommit in a STW Full GC, and System.GC
> doesn't do those when +ExplicitGCInvokesConcurrent.
>
Thank you, Kim. Your clarification helps.


>
> > Thanks
> > --
> > Ruslan Synytsky
> > CEO @ Jelastic Multi-Cloud PaaS
>
>