Dual-Core Hyperthreading: Should I use 4 threads or 3 or 2?

Question

If you're spawning multiple threads (or processes) concurrently, is it better to spawn as many as the number of physical processors or the number of logical processors, assuming the task is CPU-bound? Or is it better to do something in between (say, 3 threads)?

Does the performance depend on the kind of instructions that are getting executed (say, would non-local memory access be much different from cache hits)? If so, in which cases is it better to take advantage of hyperthreading?

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Update:

The reason I'm asking is, I remember reading somewhere that if you have as many tasks as the number of virtual processors, tasks on the same physical core can sometimes starve some CPU resources and prevent each other from getting as many resources as needed, possibly decreasing performance. That's why I'm wondering if having as many threads as virtual cores is a good idea.

Answer 1

The performance depends on a huge variety of factors. Most tasks are not strictly CPU bound, since even if all of the data is in memory it is usually not on-board in the processor cache. I have seen examples (like this one) where memory access patterns can dramatically change the performance profile of a given 'parallel' process.

In short, there is no perfect number for all situations.