Optimal number of threads per core

Question

Let's say I have a 4-core CPU, and I want to run some process in the minimum amount of time. The process is ideally parallelizable, so I can run chunks of it on an infinite number of threads and each thread takes the same amount of time.

Since I have 4 cores, I don't expect any speedup by running more threads than cores, since a single core is only capable of running a single thread at a given moment. I don't know much about hardware, so this is only a guess.

Is there a benefit to running a parallelizable process on more threads than cores? In other words, will my process finish faster, slower, or in about the same amount of time if I run it using 4000 threads rather than 4 threads?

Answer 1

Benchmark.

I'd start ramping up the number of threads for an application, starting at 1, and then go to something like 100, run three-five trials for each number of threads, and build yourself a graph of operation speed vs. number of threads.

You should that the four thread case is optimal, with slight rises in runtime after that, but maybe not. It may be that your application is bandwidth limited, ie, the dataset you're loading into memory is huge, you're getting lots of cache misses, etc, such that 2 threads are optimal.

You can't know until you test.