RFR(XS): 8198510: Limit GC threads for small Java heap

Thomas Schatzl thomas.schatzl at oracle.com
Wed Feb 21 13:55:53 UTC 2018


Hi,

On Wed, 2018-02-21 at 13:48 +0000, Doerr, Martin wrote:
> Hi,
>  
> the VM currently starts a large amount of GC threads on systems with
> many CPUs regardless of the Java heap size.
> This doesn't make sense for very small heap sizes.
>  
> The number of ParallelGCThreads can be observed by e.g.:
> jdk/bin/java -Xmx128m -XX:+PrintFlagsFinal -version|grep GCThreads
>  
> My proposal is to use up to 1 GC thread per 64 MB of heap:
> http://cr.openjdk.java.net/~mdoerr/8198510_gc_threads/webrev.00/
>  
> Please review.
> 

  did you try using -XX:+UseDynamicNumberOfGCThreads? It uses exactly
the same policy iirc.

I would rather discuss making -XX:+UseDynamicNumberOfGCThreads default
true instead of adding the same policy at a second place.

Also there is some work in that direction pending as part of JDK-
8172792 [1]. As I am the person working on it, this policy is kind of
too crude, giving some regressions particularly at startup when the
application is initializing. Actually trying to base the number of
threads on the amount of copied bytes improves this a lot...

I am talking about this a bit in a recent FOSDEM talk [2].

Thoughts?

Thanks,
  Thomas

[1] https://bugs.openjdk.java.net/browse/JDK-8172792
[2] https://fosdem.org/2018/schedule/event/g1/


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