RFR(M) 8186834:Expanding old area without full GC in parallel GC

[Thomas Schatzl]

Hi,

On Tue, 2017-08-29 at 00:20 +0900, Michihiro Horie wrote:
> Dear all,
> 
> Would you please review the following change?
> bug: https://bugs.openjdk.java.net/browse/JDK-8186834
> webrev: http://cr.openjdk.java.net/~mhorie/8186834/webrev.00/
> 
> In parallel GC, old area is expanded only after a full GC occurs.
> I am wondering if we could give an option to expand old area without
> full GC. So, I added an option
> UseAdaptiveGenerationSizePolicyBeforeMajorCollection

Sorry for the late (and probably stupid) question, but what is the
difference (in performance) to simply set -Xms==-Xmx here?

And why not make the (first) full gc expand the heap more aggressively?
(I think there is at least one way to do that, something like
Min/MaxFreeHeapRatio or so, I can look it up if needed).

Thanks,
  Thomas

> Following is a simple micro benchmark I used to see the benefit of
> this change.
> As a result, pause time of full GC reduced by 30%. Full GC count
> reduced by 54%.
> Elapsed time reduced by 7%.
> 
> import java.util.HashMap;
> import java.util.Map;
> public class HeapExpandTest {
>   static Map<Integer, byte[]> map = new HashMap<>();
>   public static void main(String[] args) throws Exception {
>     long start = System.currentTimeMillis();
>     for (int i = 0; i < 2200; ++i) {
>       map.put(i, new byte[1024*1024]); // 1MB
>     }
>     System.out.println("elapsed= " + (System.currentTimeMillis() -
> start));
>   }
> }
> 
> JVM options: -XX:+UseParallelGC -XX:+UseAdaptiveSizePolicy
> -XX:ParallelGCThreads=8 -Xms64m -Xmx3g
> -XX:+UseAdaptiveGenerationSizePolicyBeforeMajorCollection