RFR/RFC: 8196071: Change G1 Full GC heap and thread sizing ergonomics (original) (raw)

Thomas Schatzl thomas.schatzl at oracle.com
Mon Mar 26 08:58:27 UTC 2018

Hi,

On Thu, 2018-03-22 at 16:42 +0100, Stefan Johansson wrote:

Hi,

Please review or comment on this change to let the G1 Full GC calculate the number of worker threads. Links JBS: https://bugs.openjdk.java.net/browse/JDK-8196071 Webrev: http://cr.openjdk.java.net/~sjohanss/8196071/00/ Summary After making UseDynamicNumberOfGCThreads true by default, having a sane thread sizing model for the G1 Full GC is important. Before this change the Full GC would use the same number of threads as the previous collection, and if being the first just use 1 thread. The proposed change now have the Full GC consider two metrics to get a good number of worker threads. The first metric is to try to avoid to much wasted space. On average each worker used by the parallel full GC will cause half a region of unused memory. To make sure we don't use too many workers we use G1HeapWastePercent to limit the number of workers. The other metric is the one used in AdaptiveSizePolicy, it looks at the flag HeapSizePerGCWorker and limit the number by this. In most cases this is more restrictive than using G1HeapWastePercent, but for small heaps setting a large G1HeapRegionSize, G1HeapWastePercent will come into play. This change also changes the default value of HeapSizePerGCWorker to 32M instead of 64M, this can be reverted if anyone has a good argument to leave it as is. My testing shows that for DaCapo, SpecJVM98 and GCBasher (gc intensive stress test) with small heaps we get better pause times with the flag set to 32M. There might be other arguments, such as not using to much resources on small systems that might also be important so please let me know if you have reasons why the flag should be left unchanged. Testing Functional testing through Mach5 and manual performance testing with small heaps and different values for HeapSizePerGCThread.

looks good, although I would prefer to separate the HeapSizePerGCWorker change into a separate CR.

Please update the copyrights on push.

Thanks, Thomas

More information about the hotspot-gc-dev mailing list