Increased safepoint cleanup tasks in Java 11 (vs 8)

[Daniel D. Daugherty]

On 6/5/19 11:37 AM, Vitaly Davidovich wrote:
> On Wed, Jun 5, 2019 at 8:58 AM Vitaly Davidovich <vitalyd at gmail.com> wrote:
>
>> Hey Aleksey,
>>
>> On Wed, Jun 5, 2019 at 8:52 AM Aleksey Shipilev <shade at redhat.com> wrote:
>>
>>> On 6/5/19 2:33 PM, Vitaly Davidovich wrote:
>>>> Does anyone have ideas on pinpointing this further?
>>> I have the educated guess:
>>>    https://bugs.openjdk.java.net/browse/JDK-8181859
>>>
>>> Which means you can try if -XX:MonitorUsedDeflationThreshold=0 reduces
>>> the number of "guaranteed"
>>> safepoints.
>>>
>> Hmm, the application doesn't use JVM monitors (in any meaningful
>> capacity).  But interesting JBS entry - I will try it out, thanks! The
>> description of safepoint clean up in there is/was consistent with my
>> understanding as well.  On Java 8, we'd see the "no vm operation" safepoint
>> operations, and I always assumed it was IC buffer cleaning (even had a mail
>> thread on that topic a few years ago, either on this list or
>> hotspot-compiler).  That tended to stabilize (i.e. they went away) after
>> the application reached steady state, whereas I'm seeing essentially
>> non-stop safepoint cleanups on 11.
>>
> Ok, adding -XX:MonitorUsedDeflationThreshold=0 seems to alleviate the issue
> - # of entered safepoint cleanups looks on par with Java 8, although I'm
> still assessing a bit more.

This observation doesn't really match with your earlier comment:

> Hmm, the application doesn't use JVM monitors (in any meaningful
> capacity).

The default value for MonitorUsedDeflationThreshold is 90. So a cleanup
safepoint should only be triggered if monitor use exceeds 90% of the
monitor pool.

Dan


>
> Of course now the trouble is this is an experimental option, and enabling
> things like that in production raises eyebrows.  Is it safe to assume,
> however, that setting this to 0 essentially behaves like Java 8? It appears
> so, based on that JBS Aleksey linked above.  But just double checking.
>
> If that's the case, then perhaps we can overlook the experimental bit and
> wait until this is done outside safepoints, per Robbin's email.
>
> Thanks to everyone that responded on this thread! Very helpful.
>
>>> --
>>> Thanks,
>>> -Aleksey
>>>
>>>