Java - multithreaded code does not run faster on more cores

Question

I was just running some multithreaded code on a 4-core machine in the hopes that it would be faster than on a single-core machine. Here\'s the idea: I got a fixed number of thre

Answer 1

You're sleeping nano-seconds instead of milli-seconds.

I changed from

Thread.sleep(0, 100000 / numberOfThreads); // sleep 0.025 ms for 4 threads

to

Thread.sleep(100000 / numberOfThreads);

and got a speed-up proportional to the number of threads started just as expected.

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I invented a CPU-intensive "countPrimes". Full test code available here.

I get the following speed-up on my quad-core machine:

4 threads: 1625
1 thread: 3747

(the CPU-load monitor indeed shows that 4 course are busy in the former case, and that 1 core is busy in the latter case.)

Conclusion: You're doing comparatively small portions of work in each thread between synchronization. The synchronization takes much much more time than the actual CPU-intensive computation work.

(Also, if you have memory intensive code, such as tons of array-accesses in the threads, the CPU won't be the bottle-neck anyway, and you won't see any speed-up by splitting it on multiple CPUs.)