Class unloading in jdk15

[Roman Kennke]

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.
>