Class unloading in jdk15

[Roman Kennke]

Also, I just learned that we made some changes (I have been away for a
week and something). In particular, we re-ordered weak-roots and
strong-roots processing and can reclaim memory a little earlier, and we
have separate phases for weak and strong roots, which will report
separate timings. Would it be possible that you test again with a recent
jdk15 build, and send us the logs?

https://builds.shipilev.net/openjdk-shenandoah-jdk/

Thanks,
Roman


> Hello Vicente,
> 
> Sometimes, the concurrent roots processing phase appears to become quite
> long, e.g.:
> 
> GC(6) Concurrent roots processing 661.442ms
> 
> During that time, the application keeps allocating while the GC cannot
> yet reclaim any memory, and eventually runs out of allocation space. At
> roughly 1GB/s allocation rate, it'll take about 1/3rd of the total heap
> during that time.
> 
> The concurrent root time seems very much out of proportion. For example,
> the corresponding marking phase only took:
> 
> GC(6) Concurrent marking (unload classes) 75.039ms
> 
> As a band-aid I suggest to run with more enforced headroom, e.g.
> -XX:ShenandoahMinFreeThreshold=20 (instead of the default value of 10).
> You may want to play with even higher values.
> 
> We (the Shenandoah devs) shall look into what's going on there. I guess
> that either the concurrent roots work is not parallelized well (and hogs
> a single thread instead, thus taking longer to complete). We should also
> make the heuristics take concurrent-roots-time into account when
> computing trigger thresholds.
> 
> Roman
> 
> 
>> Hi,
>>
>> I tried shenandoah with JDK15 (latest build from builds.shipilev.net) in
>> our build, it works better than with G1, without even tuning anything, so
>> that's great.
>>
>> I also tried enabling class unloading (we have it disabled with G1,
>> we don't really get a lot of memory back, but GC times do increase a little
>> bit), but it didn't work with shenandoah, GC times become very high.
>>
>> These are GC logs of our full build, with and without class unloading:
>> https://gist.github.com/cocorossello/2c7e774b89f7cb3e3c97ec5758d879ff
>>
>> Our application is a web application, using dependency injection and
>> dynamic proxies. I have tried to make a reproducer but I couldn't, it's not
>> an easy task... Maybe the problem comes from dependency injection that
>> creates proxies...
>>
>> I have tried reducing the build and enabling class load/unload logs
>> (-verbose:class), to see if that helps:
>>
>> https://gist.github.com/cocorossello/f77049148e694b91e785696b1808fb4e  ->
>> Reduced build, class unloading off, verbose class define
>>
>>
>> https://gist.github.com/cocorossello/52c1c88c782adc1a0537fd8aa92a6c82  ->
>> Reduced build, class unloading on, verbose class define
>>
>> Please let me know if I can do something else, unfortunately I can't share
>> the code.
>>
>> Thank you,
>> Vicente.
>>
>