Tsuna's blog: How long does it take to make a context switch?

Tsuna's blog: How long does it take to make a context switch?

First idea: with syscalls (fail)

My first idea was to make a cheap system call many times in a row, time how long it took, and compute the average time spent per syscall. The cheapest system call on Linux these days seems to be gettid. Turns out, this was a naive approach since system calls don't actually cause a full context switch anymore nowadays, the kernel can get away with a "mode switch" (go from user mode to kernel mode, then back to user mode). That's why when I ran my first test program, vmstat wouldn't show a noticeable increase in number of context switches. But this test is interesting too, although it's not what I wanted originally.

Source code: timesyscall.c Results:

• Intel 5150: 105ns/syscall
• Intel E5440: 87ns/syscall
• Intel E5520: 58ns/syscall
• Intel X5550: 52ns/syscall
• Intel L5630: 58ns/syscall
• Intel E5-2620: 67ns/syscall

Now that's nice, more expensive CPUs perform noticeably better (note however the slight increase in cost on Sandy Bridge). But that's not really what we wanted to know. So to test the cost of a context switch, we need to force the kernel to de-schedule the current process and schedule another one instead. And to benchmark the CPU, we need to get the kernel to do nothing but this in a tight loop. How would you do this?

Second idea: with futex

The way I did it was to abuse futex (RTFM). futex is the low level Linux-specific primitive used by most threading libraries to implement blocking operations such as waiting on a contended mutexes, semaphores that run out of permits, condition variables and friends. If you would like to know more, go read Futexes Are Tricky by Ulrich Drepper. Anyways, with a futex, it's easy to suspend and resume processes. What my test does is that it forks off a child process, and the parent and the child take turn waiting on the futex. When the parent waits, the child wakes it up and goes on to wait on the futex, until the parent wakes it and goes on to wait again. Some kind of a ping-pong "I wake you up, you wake me up...".

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