is it possible to force a go routine to be run on a specific CPU?

Question

I am reading about the go package "runtime" and see that i can among other (func GOMAXPROCS(n int)) set the number of CPU units that can be used to run my program. Can I force a goroutine to be run on a specific CPU of my choice?

Answer 1

In modern Go, I wouldn't lock goroutines to threads for efficiency. Go 1.5 added goroutine scheduling affinity, to minimize how often goroutines switch between OS threads. And any cost of the remaining migrations between CPUs has to be weighed against the benefit of the user-mode scheduler avoiding context switches into kernel mode. Finally, when switching costs are a real problem, sometimes a better focus is changing your program logic so it needs to switch less, like by communicating batches of work instead of individual work items.

But even considering all that, sometimes you simply have to lock a goroutine, like when a C API requires it, and I'll assume that's the case below.

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If the whole program runs with GOMAXPROCS=1, then it's relatively simple to set a CPU affinity by calling out to the taskset utility from the schedutils package.

I had thought you were out of luck if GOMAXPROCS > 1 because then goroutines are migrated between OS threads at runtime. In fact, James Henstridge points out you can use runtime.LockOSThread() to keep your goroutine from migrating. I don't know of any Go stdlib function to set the current thread's CPU affinity once you've locked a goroutine to it, though. You might be able to use cgo and call pthread_setaffinity_np, since apparently Go uses pthreads in cgo mode. Since we're talking about system calls, details will vary by OS.

(If your whole program is pure Go (no C linked in), it may work to call sched_setaffinity with a zero pid parameter via the syscall module. But that's going to be tricky.)

Answer 2

Depends on your workload, but sometimes it's beneficial to start a go process per CPU, set gomaxprocs to 1 and pin the process to the CPU with taskset. Here is an excerpt on that topic from the awesome fasthttp library:

• Use reuseport listener.
• Run a separate server instance per CPU core with GOMAXPROCS=1.
• Pin each server instance to a separate CPU core using taskset.
• Ensure the interrupts of multiqueue network card are evenly distributed between CPU cores. See this article for details.
• Use Go 1.6 as it provides some considerable performance improvements.

Source: https://github.com/valyala/fasthttp#performance-optimization-tips-for-multi-core-systems