Question regarding JEP 8204089 Timely Reducing Unused Committed Memory

Fri Jun 1 13:21:48 UTC 2018

Hi Sunny.

On 1 June 2018 at 07:36, Sunny Chan, CLSA <sunny.chan at clsa.com> wrote:

> Hello,
>
>
>
> I have a number of question about the proposed changes for the JEP and I
> would like to make the following suggestions and comments
>
>
>
> 1)      Are we planning to make this change the default behavior for G1?
> Or this is an optional switch for people who runs in a container
> environment?
>
Optional. It will be enough for beginning.

> 2)      In a trading systems, sometimes there are period of quiescence
> and then suddenly a burst of activity towards market close or market event.
> The loadavg you suggest to “monitor” the activity in the system only
> reports up to 14mins and it might not necessary a good measure for this
> type of applications, especially you would trigger a Full GC
>
I'm sure certain number of workloads can't afford it. Such kind of projects
or other mission critical environments simply should not use this option.

> 3)      You haven’t fill in the details for “The GCFrequency value is
> ignored and therefore, i.e., no full collection is triggered, if:”
>
Thanks. It's a missed part. Here are the "if" rules:

   - GCFrequency is zero or below
   - the average load on the host system is above MaxLoadGC. The MaxLoadGC
   is a dynamically user-defined variable. This check is ignored if MaxLoadGC
   is zero or below
   - the committed memory is above MinCommitted bytes. MinCommitted is a
   dynamically user-defined variable. This check is ignored if MinCommitted is
   zero or below
   - the difference between the current heap capacity and the current heap
   usage is below MaxOverCommitted bytes. The MaxOverCommitted is a
   dynamically user-defined variable. This check is ignored if
   MaxOverCommitted is zero or below

The doc will be updated.

4)      If we are trigging full GC with this we should make sure the GC
reason is populated and log properly in the GC log so we can track it down.
Good point.

> 5)      I have not heard of J9 Gencon/Shenandoah providing similar
> functionality. Can you point me to further documentation on which feature
> you model upon?
>
What we know so far that OpenJ9 provides

   - -XX:+IdleTuningCompactOnIdle
   <https://www.ibm.com/support/knowledgecenter/en/SSYKE2_8.0.0/com.ibm.java.vm.80.doc/docs/xxidletuningcompactonidle.html>
- this
   option controls garbage collection processing with compaction when the
   status of the JVM is set to idle
   - and -Xsoftmx
   <https://www.ibm.com/support/knowledgecenter/en/SSYKE2_8.0.0/com.ibm.java.vm.80.doc/docs/xsoftmx.html>
-
   this option sets a "soft" maximum limit for the initial size of the Java™
   heap.

We have not tested it yet, but the idea looks similar. Do we have anyone in
the group involved in OpenJ9 to confirm or refute this statement?

Thank you

>
>
> Thanks.
>
>
>
> *Sunny Chan*
>
> *Senior Lead Engineer, Executive Services*
>
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Ruslan
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