On 03/24/10 13:42, Xueming Shen wrote:
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.
Sorry for the long delay in looking at this, but if
this is still in need of a reviewer OK, then I think
it is fine.
-Doug
----
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
