Question regarding JEP 8204089 Timely Reducing Unused Committed Memory

Mon Jun 4 02:00:23 UTC 2018

Replied inline.

From: Ruslan Synytsky [mailto:synytskyy at jelastic.com]
Sent: Friday, June 1, 2018 9:22 PM
To: Sunny Chan, CLSA <sunny.chan at clsa.com>
Cc: rbruno at gsd.inesc-id.pt; hotspot-gc-dev at openjdk.java.net; synytskyy at jelastic.com
Subject: Re: Question regarding JEP 8204089 Timely Reducing Unused Committed Memory

Hi Sunny.

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

You say “enough for the beginning” –  do you want to change them to be the default at some point?

Please document this in the JEP and highlight that this is optional.

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.

I would like to see this as a goal in the JEP, and this should be clearly documented how people can track down GC caused by the flag.

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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