The cost of context switches

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Andrew

Context switches are not free. But how expensive are they? I wrote a small program to find out, and I’m sharing the program and its results here.

I focused on purely context switches (no work is actually performed between context switches). So it’s not a real-world scenario, but it really brings out the hidden costs. Below are the results 500,000 context switches performing no work between each one.

Notice how with each kind, the order of magnitude of the overhead increases. The code below will help you understand what each each scenario name actually means. Then we add a bit of work (counting to 500) per context switch, which is closer to a possible real-world work load (although relatively lightweight) that might occur for a given context:

Suddenly no context switch and async methods all share an order of magnitude, while thread switches still takes significantly longer. In fact closely comparing shows that Async w/o yield is faster than no switch at all. This of course is ludicrous and can be written off as noise. But several runs produced the same result, so we can glean from this that when doing even a small amount of work per context switch, that the no-yield async method adds insignificant overhead.

Following is the application that produced the above results.

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Andrew Arnott

Principal Software Engineer, Visual Studio Platform

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