Understanding High Object Copy Times

Kirk Pepperdine kirk at kodewerk.com
Fri Apr 28 19:05:15 UTC 2017


Hi Nezih,

Can you share your GC log.

Kind regards,
Kirk Pepperdine

> On Apr 28, 2017, at 12:52 AM, nezih yigitbasi <nezihyigitbasi at gmail.com> wrote:
> 
> My first question is incorrect actually, so I am giving a better example to rephrase my question. At time "2017-04-26T03:12:24.259-0700"  there is a young GC that took 35.47 s, where object copy took 28983.4 ms. In that event I see the following log:
> 
>    [Eden: 7168.0M(7168.0M)->0.0B(7168.0M) Survivors: 1024.0M->1024.0M Heap: 153.4G(160.0G)->149.4G(160.0G)]
> 
> My interpretation for this is, ~4GB of garbage was collected from heap in total and we see that eden usage goes down by ~7GB, this means ~3GB of the eden was live objects. Is this interpretation correct? If it is, how come copying over 3GB takes ~29s? In your answer you said "most of the object copy time is dealing with the Remember Set", can you please give some details about what kind of operations on rsets are done during the object copy phase, and can we see that from these logs?
> 
> Thanks again,
> Nezih
> 
> 
> 2017-04-27 17:20 GMT-07:00 nezih yigitbasi <nezihyigitbasi at gmail.com <mailto:nezihyigitbasi at gmail.com>>:
> Thanks for the suggestions. We use the default pause time. And here is our entire set of JVM args: https://gist.github.com/nezihyigitbasi/04f5fdb9c32ac56097011819e20602d8 <https://gist.github.com/nezihyigitbasi/04f5fdb9c32ac56097011819e20602d8>
> 
> I have some followup questions:
> 
> - In some case the object copy took 39406.8 ms, even if the remembered set is ~30GB isn't this too slow (that's <1GB/s of data)?
> - Is there any way to reduce the rset overhead?
> - My initial thought when I saw the high object copy times was there may be some sort of contention to have such a low throughput during the copy. Although it may not be the case here, I just wonder whether there is a way to see the amount of contention from the gc logs?
> 
> Nezih
> 
> 
> 2017-04-27 16:58 GMT-07:00 Jenny Zhang <yu.zhang at oracle.com <mailto:yu.zhang at oracle.com>>:
> Hi, Hezih,
> 
> It seems this workload is very heavy on Remember Set. It has about 31G native memory for RSet (old gen) and still with coarsening.
> 
> What is you pause time goal? The default (200ms) might be too small for you.  Can you increase that so G1 can increase the young gen size? Since there is not much promotion, I guess most of the object copy time is dealing with the Remember Set.
> 
> There are other things you can try, like increase the G1RSetReginEntries, but the memory footprint will be bigger.
> 
> So if you can, I suggest increase the pause time goal first.
> 
> Thanks
> 
> Jenny
> 
> 
> 
> 
> On 4/27/2017 9:22 AM, nezih yigitbasi wrote:
> Hi,
> We see huge object copy times (and relatively high termination times) during young GCs in our production system running on Java 1.8.0_112-b15. You can find the GC logs here: https://gist.github.com/nezihyigitbasi/1f7a92da7860908a611cb1197bd8626b <https://gist.github.com/nezihyigitbasi/1f7a92da7860908a611cb1197bd8626b>
> 
> The young GC times start going high after the timestamp "2017-04-26T03:07:22.164-0700".
> 
> I will appreciate if you can give some details about:
> - what goes into the "Object Copy" phase during young GCs and how we can reduce it.
> - why we see high Termination times and what we can do about it
> 
> Thanks,
> Nezih
> 
> 
> 

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