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

Answer 1

I thought I'd add another perspective here. The answer depends on whether the question is assuming weak scaling or strong scaling.

From Wikipedia:

Weak scaling: how the solution time varies with the number of processors for a fixed problem size per processor.

Strong scaling: how the solution time varies with the number of processors for a fixed total problem size.

If the question is assuming weak scaling then @Gonzalo's answer suffices. However if the question is assuming strong scaling, there's something more to add. In strong scaling you're assuming a fixed workload size so if you increase the number of threads, the size of the data that each thread needs to work on decreases. On modern CPUs memory accesses are expensive and would be preferable to maintain locality by keeping the data in caches. Therefore, the likely optimal number of threads can be found when the dataset of each thread fits in each core's cache (I'm not going into the details of discussing whether it's L1/L2/L3 cache(s) of the system).

This holds true even when the number of threads exceeds the number of cores. For example assume there's 8 arbitrary unit (or AU) of work in the program which will be executed on a 4 core machine.

Case 1: run with four threads where each thread needs to complete 2AU. Each thread takes 10s to complete (with a lot of cache misses). With four cores the total amount of time will be 10s (10s * 4 threads / 4 cores).

Case 2: run with eight threads where each thread needs to complete 1AU. Each thread takes only 2s (instead of 5s because of the reduced amount of cache misses). With four cores the total amount of time will be 4s (2s * 8 threads / 4 cores).

I've simplified the problem and ignored overheads mentioned in other answers (e.g., context switches) but hope you get the point that it might be beneficial to have more number of threads than the available number of cores, depending on the data size you're dealing with.