Understanding High Object Copy Times

yu.zhang at oracle.com yu.zhang at oracle.com
Fri Apr 28 22:44:05 UTC 2017

Hi, Nezih,

I agree with your estimation. There is not much promotion going on.

Though I do not see high system cpu, if you are on a Linux system, can 
you verify that the OS Transparent Huge Page is off?

Remember set is the data structure that keep track of pointers from 
other regions to the region we are operating on. If there are pointers 
from old regions to the objects in the young regions, we can not collect 
that object.
After a gc pause, the live objects are evacuated to either the survivor 
or old gen regions. Then we need to update the Remember set. That is 
what I was referring to.
It is part of object copy.

Like Kirk mentioned, the parallelism of the gc threads seems fine. 
Charlie.Hunt suggested maybe the object graph is very deep so that the 
gc threads can not steal work and end up spinning.

Some things we can try:
-XX:MaxGCPauseMillis=<200> Maybe increase it to 400
Maybe try to reduce the -XX:ParallelGCThreads

|-XX:+G1SummarizeRSetStats -XX:G1SummarizeRSetStatsPeriod=10|

It is very expensive to print those statistics, for now we know there is 
a lot of coarsening. We can make G1SumarizeRSetStatsPeriod=100

The RSet footprint is so big,  increasing -XX:G1RSetRegionEntries will 
get rid of coarsening but make the memory footprint bigger. We can delay 
this for now.

What is your hardware/software configuration? cpu/memory/cores? There is 
no swapping, right?


On 04/27/2017 08:52 PM, nezih yigitbasi 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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