Increased safepoint cleanup tasks in Java 11 (vs 8)

[Vitaly Davidovich]

On Wed, Jun 5, 2019 at 11:44 AM Daniel D. Daugherty <
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>
> 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.
>
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?

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
> >>>
> >>>
>
>