ParallelGC issue: collector does only Full GC by default and above NewSize=1800m

[Andreas Müller]

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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