Increased safepoint cleanup tasks in Java 11 (vs 8)

[Daniel D. Daugherty]

On 6/5/19 12:04 PM, Vitaly Davidovich wrote:
>
>
> On Wed, Jun 5, 2019 at 11:44 AM Daniel D. Daugherty 
> <daniel.daugherty at oracle.com <mailto:daniel.daugherty at oracle.com>> wrote:
>
>     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 <mailto:vitalyd at gmail.com>> wrote:
>     >
>     >> Hey Aleksey,
>     >>
>     >> On Wed, Jun 5, 2019 at 8:52 AM Aleksey Shipilev
>     <shade at redhat.com <mailto: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.
>
> Hmm, ok - I see what you mean: 
> https://github.com/openjdk/jdk11u/blob/d46b56eb81b07b452141c7dcd078d33fc8e49d63/src/hotspot/share/runtime/safepoint.cpp#L604
>
> So there's either monitor usage that I'm not aware of (or forgot 
> about) or the ratio is > 90 but the absolute # of them is small (which 
> I'd consider "don't use in any meaningful capacity").  Is there a way 
> to print out monitor population so I can see?

I've been adding a lot of logging support to the Java Monitor subsystem,
but those changes are in JDK13 and not in JDK11. I'm trying to remember
what there is in JDK11, but ...

Dan


>
> Thanks Dan
>
>
>     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
>     >>>
>     >>>
>