Strange STW pauses

[David Holmes]

On 23/08/2017 6:48 PM, Nick Chadwick wrote:
> Thanks both for your replies.
> 
> Yes, it is a recurring issue - it doesn't go away after the JVM has warmed
> up. I am able to trigger the pauses quite easily and regularly with minimal
> load.
> 
> I am already logging GC in addition, but there are no GCs close to the
> pauses that I am seeing. As I understand it, the log entries above are
> explicitly not garbage collections - they would show something other than
> "no vm operation" under vmop (e.g. CollectForMetadataAllocation,
> ParallelGCFailedAllocation, etc) - but guaranteed safepoints. I understand I
> can tweak the GuaranteedSafepointInterval, but that doesn't feel like the
> solution to me, and would probably just make the pauses longer, albeit less
> often.
> 
> If you do think it's still worth me posting some GC logs here as well, I
> can, but it definitely feels like it is not GC-related. (I am running with a
> large heap, and see only a few, short pauses for GC)
> 
> Although I agree it looks like the vmop itself is not taking up the time, I
> am seeing stop-the-world pauses that correlate very closely with the total
> time spent under sync/cleanup in these specific pauses.

I did not glean that from the log fragments you posted. The cleaning 
times seemed very short, as was the time to reach the safepoint. But the 
formatting was problematic so perhaps I misread something ??

If there are long delays in reaching a safepoint then it is likely a C2 
issue where safepoint checks have been elided from lengthy code 
fragments. Try running with -XX:+UseCountedLoopSafepoints

David

> Grateful for any further insight.
> 
> 
> 
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