Re: Concurrent calls to new Random() not random enough

[Xueming Shen]

6937857: Concurrent calls to new Random() not random enough

Martin Buchholz wrote:

[+fy, jeremymanson]

Here's a much better test case,
and a proposed fix:

http://cr.openjdk.java.net/~martin/webrevs/openjdk7/RandomSeedCollisions

This adds some initialization overhead, but also removes some
since
new Random()
no longer invokes a synchronized method.

----
import java.util.*;

public class RandomSeed {
    public static void main(String[] args) throws Throwable {
        class RandomCollector implements Runnable {
            long[] randoms = new long[1<<22];
            public void run() {
                for (int i = 0; i < randoms.length; i++)
                    randoms[i] = new Random().nextLong();
            }};
        final int threadCount = 2;
        List<RandomCollector> collectors = new ArrayList<RandomCollector>();
        List<Thread> threads = new ArrayList<Thread>();
        for (int i = 0; i < threadCount; i++) {
            RandomCollector r = new RandomCollector();
            collectors.add(r);
            threads.add(new Thread(r));
        }
        for (Thread thread : threads)
            thread.start();
        for (Thread thread : threads)
            thread.join();
        int collisions = 0;
        HashSet<Long> s = new HashSet<Long>();
        for (RandomCollector r : collectors) {
            for (long x : r.randoms) {
                if (s.contains(x))
                    collisions++;
                s.add(x);
            }
        }
        System.out.printf("collisions=%d%n", collisions);
    }
}


On Tue, Mar 23, 2010 at 15:50, Martin Buchholz <marti...@google.com> wrote:

Hi Sherman,

This is a bug report (sorry, no fix this time)

Synopsis: Concurrent calls to new Random() not random enough
Description:
new Random() promises this:
   /**
    * Creates a new random number generator. This constructor sets
    * the seed of the random number generator to a value very likely
    * to be distinct from any other invocation of this constructor.
    */

but if there are concurrent calls to new Random(), it does not
do very well at fulfilling its contract.

The following program should print out a number much closer to 0.

import java.util.*;

public class RandomSeed {
   public static void main(String[] args) throws Throwable {
       class RandomCollector implements Runnable {
           int[] randoms = new int[1<<21];
           public void run() {
               for (int i = 0; i < randoms.length; i++)
                   randoms[i] = new Random().nextInt();
           }};
       final int threadCount = 2;
       List<RandomCollector> collectors = new ArrayList<RandomCollector>();
       List<Thread> threads = new ArrayList<Thread>();
       for (int i = 0; i < threadCount; i++) {
           RandomCollector r = new RandomCollector();
           collectors.add(r);
           threads.add(new Thread(r));
       }
       for (Thread thread : threads)
           thread.start();
       for (Thread thread : threads)
           thread.join();
       int collisions = 0;
       HashSet<Integer> s = new HashSet<Integer>();
       for (RandomCollector r : collectors) {
           for (int x : r.randoms) {
               if (s.contains(x))
                   collisions++;
               s.add(x);
           }
       }
       System.out.println(collisions);
   }
}
---
==> javac -source 1.6 -Xlint:all RandomSeed.java
==> java -esa -ea RandomSeed
876