Question regarding JEP 8204089 Timely Reducing Unused Committed Memory

Mon Jun 4 11:13:38 UTC 2018

On 1 June 2018 at 16:21, Ruslan Synytsky <rs at jelastic.com> wrote:

> 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?
>
It's getting more attention. A new mentioning of the implementation
in OpenJ9 - DZone Java 2018: FEATURES, IMPROVEMENTS, & UPDATES
<https://dzone.com/storage/assets/9318874-dzone2018-researchguide-java.pdf>
(Page 31).

* You can also specify -XX:+IdleTuningGcOnIdle on the command line. When
set, OpenJ9 determines whether an application is idle based on CPU
utilization and other internal heuristics. When an idle state is
recognized, a GC cycle runs if there is significant garbage in the heap and
releases unused memory back to the operating system. *

>
> Thank you
>
>>
>>
>> Thanks.
>>
>>
>>
>> *Sunny Chan*
>>
>> *Senior Lead Engineer, Executive Services*
>>
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>
>
>
> --
> Ruslan
> CEO @ Jelastic <https://jelastic.com/>
>

-- 
Ruslan
CEO @ Jelastic <https://jelastic.com/>
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