Re: CPUs, thread pools, and wasted time.

Joshua Cranmer <>
Thu, 27 Sep 2007 21:03:22 GMT
Ken wrote:

I'm trying to build an application that will use multiple cores
efficiently, but I'm running into to some difficulties. I've written a
simple small example to demonstrate the problem and attached it below.
When I run this program on my dual core computer I expect that both cores
should be actively running wasteTime() and my CPU utilization should be
nearly 100% on both, but this isn't the result I'm getting. A single CPU
is maxed out and the other one is sitting idle. This isn't what I want
for obvious reasons.

Thread scheduling is handled at OS-level. It is impossible to bind the
Java threads to separate CPUs using only native code; I do not know
whether it is even possible to bind threads to CPUs at below a process

Can any explain to me why this is happening and what to do about it?

I know that one can fiddle with the amount of work done by each thread and
get it to behave correctly, but I don't know why it isn't behaving
correctly without this tweaking of parameters.


    public static double wasteTime() {
        double sum = 0.0;
        for (int index = 0; index < 100; index++) {
            sum += Math.cos(index / 1000.0);
        return sum;

This is hardly going to be wasting fair amounts of time. Assuming that
computing the cosine takes as many as 20 FLOPs, this code would be on
the order of ~2000 FLOPs. Try computing some eigenvalues or solving
simple ordinary differential equations to REALLY waste time.

I am merely hypothesizing here, but it could be that you wasteTime is
sufficiently computationally nonintensive that the OS is deciding that
it is worthwhile to use the one core.

Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

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