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

[Tao Mao]

Hi Andreas,

What's your exact VM options for ParNewGC? If possible, please attach 
ParNew gc log. I'd like to investigate and compare the two cases to see 
GC behavioral differences.

Thanks.
Tao

On 10/21/13 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>
>
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>
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>
>
>
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