Number of processor core vs the size of a thread pool

Question

Many times I\'ve heard that it is better to maintain the number of threads in a thread pool below the number of cores in that system. Having twice or more threads than the numbe

Answer 1

Having fewer threads than cores generally means you can't take advantage of all available cores.

The usual question is how many more threads than cores you want. That, however, varies, depending on the amount of time (overall) that your threads spend doing things like I/O vs. the amount of time they spend doing computation. If they're all doing pure computation, then you'd normally want about the same number of threads as cores. If they're doing a fair amount of I/O, you'd typically want quite a few more threads than cores.

Looking at it from the other direction for a moment, you want enough threads running to ensure that whenever one thread blocks for some reason (typically waiting on I/O) you have another thread (that's not blocked) available to run on that core. The exact number that takes depends on how much of its time each thread spends blocked.