ParallelGC issue: collector does only Full GC by default and above NewSize=1800m
Jon Masamitsu
jon.masamitsu at oracle.com
Mon Oct 21 15:29:10 PDT 2013
Andreas,
There was a bug fixed in jdk8 that had similar symptoms. If you
can try a jdk8 build that might tell us something.
If jdk8 doesn't help it's likely that the prediction model thinks that
there is not enough
free space in the old gen to support a young collection. We've been
working on 7098155 to
fix that.
Jon
On 10/21/2013 10:09 AM, Andreas Müller wrote:
> Hi all,
>
> while experimenting a bit with different Garbage Collectors and applying them to my homegrown micro benchmarks I stumbled into the
> following problem:
> I run the below sample with the following command line (using Java 1.7.0_40 on Windows and probably others):
> java -Xms6g -Xmx6g -XX:+UseParallelGC - de.am.gc.benchmarks.MixedRandomList 100 8 12500000
>
> The Default and proven ParallelGC collector does mostly Full GCs and shows only poor out-of-the-box performance, more than a factor 10 lower than the ParNew collector.
> More tests adding the -XX:NewSize=<xyz>m and -XX:MaxNewSize=<xyz>m reveal that the problem occurs as soon as the NewSize rises beyond 1800m which it obviously does by default.
> Below that threshold ParallelGC performance is similar to ParNewGC (in the range of 7500 MB/s on my i7-2500MHz notebook), but at NewSize=2000m is as low as 600 MB/s.
>
> Any ideas why this might happen?
>
> Note that the sample is constructed such that the live heap is always around 3GB. If any I would expect a problem only at around NewSize=3GB, when Old Gen shrinks to less than the live heap size. As a matter of fact, ParNewGC can do >7000 MB/s from NewSize=400m to NewSize=3500m with little variation around a maximum of 7600 MB/s at NewSize=2000m.
>
> I also provide source, gc.log and a plot of the NewSize dependency to anyone interested in that problem.
>
> Regards
> Andreas
>
> -------------------------------------------------------MixedRandomList.java------------------------------------------------------------------------------------------------------------------------
> package de.am.gc.benchmarks;
>
> import java.util.ArrayList;
> import java.util.List;
>
> /**
> * GC benchmark producing a mix of lifetime=0 and lifetime>0 objects which are kept in randomly updated lists.
> *
> * @author Andreas Mueller
> */
> public class MixedRandomList {
> private static final int DEFAULT_NUMBEROFTHREADS=1;
> // object size in bytes
> private static final int DEFAULT_OBJECTSIZE=100;
>
> private static int numberOfThreads=DEFAULT_NUMBEROFTHREADS;
> private static int objectSize=DEFAULT_OBJECTSIZE;
> // number of objects to fill half of the available memory with (permanent) live objects
> private static long numLive = (Runtime.getRuntime().maxMemory()/objectSize/5);
>
> /**
> * @param args the command line arguments
> */
> public static void main(String[] args) {
> if( args.length>0 ) {
> // first, optional argument is the size of the objects
> objectSize = Integer.parseInt(args[0]);
> // second, optional argument is the number of live objects
> if( args.length>1 ) {
> numberOfThreads = Integer.parseInt(args[1]);
> // third, optional argument is the number of live objects
> if( args.length>2 ) {
> numLive = Long.parseLong(args[2]);
> }
> }
> }
> for( int i=0; i<numberOfThreads; i++ ) {
> // run several GarbageProducer threads, each with its own mix of lifetime=0 and higher lifetime objects
> new Thread(new GarbageProducer((int)Math.pow(50.0,(double)(i+1)), numLive/numberOfThreads)).start();
> }
> try {
> Thread.sleep(1200000);
> } catch( InterruptedException iexc) {
> iexc.printStackTrace();
> }
> System.exit(0);
> }
>
> private static char[] getCharArray(int length) {
> char[] retVal = new char[length];
> for(int i=0; i<length; i++ ) {
> retVal[i] = 'a';
> }
> return retVal;
> }
>
> public static class GarbageProducer implements Runnable {
>
> // the fraction of newly created objects that do not become garbage immediately but are stored in the liveList
> int fractionLive;
> // the size of the liveList
> long myNumLive;
>
> /**
> * Each GarbageProducer creates objects that become garbage immediately (lifetime=0) and
> * objects that become garbage only after a lifetime>0 which is distributed about an average lifetime.
> * This average lifetime is a function of fractionLive and numLive
> *
> * @param fractionLive
> * @param numLive
> */
> public GarbageProducer(int fractionLive, long numLive) {
> this.fractionLive = fractionLive;
> this.myNumLive = numLive;
> }
>
> @Override
> public void run() {
> int osize = objectSize;
> char[] chars = getCharArray(objectSize);
> List<String> liveList = new ArrayList<String>((int)myNumLive);
> // initially, the lifeList is filled
> for(int i=0; i<myNumLive; i++) {
> liveList.add(new String(chars));
> }
> while(true) {
> // create the majority of objects as garbage
> for(int i=0; i<fractionLive; i++) {
> String garbageObject = new String(chars);
> }
> // keep the fraction of objects live by placing them in the list (at a random index)
> int index = (int)(Math.random()*myNumLive);
> liveList.set(index, new String(chars));
> }
> }
> }
> }
> -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Andreas Müller
>
> mgm technology partners GmbH
> Frankfurter Ring 105a
> 80807 München
> Tel. +49 (89) 35 86 80-633
> Fax +49 (89) 35 86 80-288
> E-Mail Andreas.Mueller at mgm-tp.com<mailto:Andreas.Mueller at mgm-tp.com>
> Innovation Implemented.
> Sitz der Gesellschaft: München
> Geschäftsführer: Hamarz Mehmanesh
> Handelsregister: AG München HRB 105068
>
>
>
>
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