ExecutorService slow multi thread performance

Question

I am trying to execute a simple calculation (it calls Math.random() 10000000 times). Surprisingly running it in simple method performs much faster than using Ex

Answer 1

When you use multiple threads, you need to be aware of the overhead of using additional threads. You also need to determine if your algorithm has work which can be preformed in parallel or not. So you need to have work which can be run concurrently which is large enough that it will exceed the overhead of using multiple threads.

In this case, the simplest workaround is to use a separate Random in each thread. The problem you have is that as a micro-benchmark, your loop doesn't actually do anything and the JIT is very good at discarding code which doesn't do anything. A workaround for this is to sum the random results and return it from the call() as this is usually enough to prevent the JIT from discarding the code.

Lastly if you want to sum lots of numbers, you don't need to save them and sum them later. You can sum them as you go.