CODEWITHSUNDEEP
c++multithreading
FrozenHeart
FrozenHeart

Reputation: 20746

Which threads exactly are CPU bound

I heard that the optimal amount of threads depends on whether they are CPU bound or not. But what exactly does it mean?

Suppose that the most time my threads will sleep via Sleep function from WinAPI. Should I considered such threads as non-CPU bound and increase their amount over the CPU cores count?

Upvotes: 0

Views: 301

Answers (1)

lodo
lodo

Reputation: 2383

A thread is bound by a resource if it spends most of its time using it, and thus its speed is bound by the speed of that resource.

Given the above definition, a thread is CPU bound if its most used resource is the computing power of the CPU, that is, it's a thread that does heavy computation. You gain nothing from putting more of these than there are available cores, because they will compete for CPU time.

You can (instead) put more threads than available cores when the threads are bound by other resources (most commonly files), because they will spend most time waiting for those to be ready, and thus leave the CPU available for other threads.

A thread that spends most time sleeping does not use the CPU very much, and thus it is not CPU bound.

EDIT: examples of non-CPU bound threads are threads that read files, wait for network connections, talk to PCI connected devices, spend most time waiting on condition variables and GUI threads that wait for user input.

Upvotes: 4

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