[concurrency-interest] Strange behaviour when sorting an uniform array

Kasper Nielsen kasper at kav.dk
Tue May 25 05:44:10 EDT 2010


Ahh, got it. You still want to remove that synchronized statement for 
serious usage though.

What you are seing is the worst case behavior for quicksort which 
ParallelArray is using under the hood. All equal elements will kill most 
quicksort implementations. Instead of the average running time of n*logn 
you are seing worst case behaviour of n*n.

Even large datasets with just a few different values will slow most 
implementations down significantly.
I tried replacing
   return 1L;//rand.nextLong();
with
   return rand.nextInt(20)
and PA was still around 70 times slower compared to using rand.nextLong().

We might want to switch to the dual pivot implementation introduced in 
OpenJDK 7. Don't know about licensing issues though.

- Kasper


On 25/5/2010 15:10, Lukas Krecan wrote:
> Hi,
>     I am quite sure that it has nothing to do the synchronization. The
> problem remains even if synchronized keyword is removed. Besides, the
> method was used only for generating values, which works well. Problem is
> with sorting. It works well on random values, but runs for ages on
> uniform array.
>
>
> On Tue, May 25, 2010 at 10:00 AM, Kasper Nielsen <kasper at kav.dk
> <mailto:kasper at kav.dk>> wrote:
>
>     Hi Lukas,
>
>     try removing the synchronized keyword in your op.
>
>     Taken from the extra166y.Ops javadoc:
>
>     In addition to stated signatures, implementations of these
>     interfaces must work safely in parallel. In general, this means
>     methods should operate only on their arguments, and should not rely
>     on ThreadLocals, unsafely published globals, or other unsafe
>     constructions. Additionally, they should not block waiting for
>     synchronization.
>
>     If you want to return random numbers you should use the
>     ThreadLocalRandom.
>
>
>     - Kasper
>
>
>     On 25/5/2010 07:9, Lukas Krecan wrote:
>
>         Hi,
>             I have been playing with extra166y library and found that
>         when I try
>         to sort an uniform array (full of ones) it takes ages. With older
>         version of the library it even threw StackOverflowError. After
>         update it
>         runs for minutes (I always kill the process after while so I do
>         not know
>         if it ends successfully).
>
>         Probably it's nothing new for you, maybe it's expected
>         behaviour. But
>         for me it's confusing and I just wanted to let you know about
>         the issue
>         in case you were not aware of it.
>                Best regards
>                  Lukas Krecan
>
>         -------------------------------------
>         package net.krecan.forkjoin;
>
>         import java.util.Random;
>
>         import jsr166y.ForkJoinPool;
>         import extra166y.Ops;
>         import extra166y.ParallelLongArray;
>
>
>         public class SortTest {
>              private static final int THREADS = 2;
>              private static final int SIZE = 40000000;
>
>              public static void testSort()
>              {
>                  ForkJoinPool fjPool = new ForkJoinPool(THREADS);
>                  ParallelLongArray pa = ParallelLongArray.createEmpty(SIZE,
>         fjPool);
>                  createData(pa);
>                  System.out.println("Sorting "+pa.summary());
>                  long start = System.currentTimeMillis();
>                  pa.sort();
>                  System.out.println("Done in
>         "+(System.currentTimeMillis()-start)+"ms.");
>                  System.out.println(pa.summary());
>
>
>              }
>              private static void createData(ParallelLongArray pa) {
>                  long start = System.currentTimeMillis();
>                  System.out.println("Creating data using "+THREADS+"
>         threads.");
>                  pa.setLimit(SIZE);
>                  final Random rand = new Random();
>                  pa.replaceWithGeneratedValue(new Ops.LongGenerator(){
>                      public synchronized long op() {
>                          return 1L;//rand.nextLong();
>                      }
>                  });
>                  System.out.println("Done in
>         "+(System.currentTimeMillis()-start)+"ms.");
>              }
>
>              public static void main(String[] args) {
>                  testSort();
>              }
>
>         }
>
>
>
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