JVM stalls around uncommitting

Zoltán Baranyi blazember at gmail.com
Mon Apr 6 13:23:06 UTC 2020

Hi Per,

Thanks for the link to the NUMA issue, it could be part of the difference
indeed. My benchmarks use the fresh OpenJdk 14+36-1461 GA build, so I don't
have this improvement yet.

Btw, it turned out that running my same benchmark multiple times with 4K
pages and exactly the same parameters, it produces throughput results with
low and high values, where low can be ~75% of the high. This can very well
be behind the unexpectedly high gain I saw with large pages since I had
results only from the low range for 4K pages when I replied. I need to
figure this out, but currently, I think this is unrelated to ZGC even if
NUMA can play some role in it.

Thanks for the useful hints and explanations, I will come back if I find
anything interesting related to ZGC.


On Fri, Apr 3, 2020 at 9:36 AM Per Liden <per.liden at oracle.com> wrote:

> Hi Zoltan,
> On 4/3/20 1:27 AM, Zoltán Baranyi wrote:
> > Hi Per,
> >
> > Thank you for confirming the issue and for recommending large pages. I
> > re-run my benchmarks with large pages and it gave me a 25-30% performance
> > boost, which is a bit more than what I expected. My benchmarks run on a
> > 600G heap with 1.5-2GB/s allocation rate on a 40 core machine, so ZGC is
> > busy. Since a significant part of the workload is ZGC itself, I assume -
> > besides the higher TLB hit rate - this gain is from managing the ZPages
> > more effectively on large pages.
> A 25-30% improvement is indeed more than I would have expected. ZGC's
> internal handling of ZPages is the same regardless of the underlying
> page size, but as you say, you'll get better TLB hit-rate and the
> mmap/fallocate syscalls become a lot less expensive.
> Another reason for the boost might be that ZGC's NUMA-awareness, until
> recently, worked much better when using large pages. But this has now
> been fixed, see https://bugs.openjdk.java.net/browse/JDK-8237649.
> Btw, which JDK version are you using?
> >
> > I have a good experience overall, nice to see ZGC getting more and more
> > mature.
> Good to hear. Thanks for the feedback!
> /Per
> >
> > Cheers,
> > Zoltan
> >
> > On Wed, Apr 1, 2020 at 9:15 AM Per Liden <per.liden at oracle.com> wrote:
> >
> >> Hi,
> >>
> >> On 3/31/20 9:59 PM, Zoltan Baranyi wrote:
> >>> Hi ZGC Team,
> >>>
> >>> I run benchmarks against our application using ZGC on heaps in few
> >>> hundreds GB scale. In the beginning everything goes smooth, but
> >>> eventually I experience very long JVM stalls, sometimes longer than one
> >>> minute. According to the JVM log, reaching safepoints occasionally
> takes
> >>> very long time, matching to the duration of the stalls I experience.
> >>>
> >>> After a few iterations, I started looking at uncommitting and learned
> >>> that the way ZGC performs uncommitting - flushing the pages, punching
> >>> holes, removing blocks from the backing file - can be expensive [1]
> when
> >>> uncommitting tens or more than a hundred GB of memory. The trace level
> >>> heap logs confirmed that uncommitting blocks in this size takes many
> >>> seconds. After disabled uncommitting my benchmark runs without the huge
> >>> stalls and the overall experience with ZGC is quite good.
> >>>
> >>> Since uncommitting is done asynchronously to the mutators, I expected
> it
> >>> not to interfere with them. My understanding is that flushing,
> >>> bookeeping and uncommitting is done under a mutex [2], and contention
> on
> >>> that can be the source of the stalls I see, such as when there is a
> >>> demand to commit memory while uncommitting is taking place. Can you
> >>> confirm if this above is an explanation that makes sense to you? If so,
> >>> is there a cure to this that I couldn't find? Like a time bound or a
> cap
> >>> on the amount of the memory that can be uncommitted in one go.
> >>
> >> Yes, uncommitting is relatively expensive. And it's also true that there
> >> is a potential for lock contention affecting mutators. That can be
> >> improved in various ways. Like you say, uncommitting in smaller chunks,
> >> or possibly by releasing the lock while doing the actual syscall.
> >>
> >> If you still want uncommit to happen, one thing to try is using large
> >> pages (-XX:+UseLargePages), since committing/uncommitting large pages is
> >> typically less expensive.
> >>
> >> This issue is on our radar, so we intend to improve this going forward.
> >>
> >> cheers,
> >> Per
> >>
> >>

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