Introduction to Golang Preemption Mechanisms (opens in new tab)

On Linux, go uses sched_getaffinity to know how many cpu core it is allowed to run on:

https://cs.opensource.google/go/go/+/master:src/runtime/os_l...

I am guessing the API isn't stable enough for letting the runtime set maxprocs. I use https://pkg.go.dev/go.uber.org/automaxprocs and have had to update it periodically because Redhat and Debian have different defaults. (Should one even run k8s on Redhat? I say no, but Redhat says yes. That's how I know about this.)

This, I think, is cgroups 1 vs. cgroups 2 and everyone should have cgroups 2 now, but ... it would feel weird for the Go runtime to decide on one. To me, anyway.

If you’re on Kubernetes, you can solve this/work around this by enabling the static CPU manager policy:

https://kubernetes.io/docs/tasks/administer-cluster/cpu-mana...

I don't know, but I remember hearing in a talk that the compiler had to be modified to insert them into the generated code.

Here's `isAsyncSafePoint`: https://github.com/golang/go/blob/d36353499f673c89a267a489be...

edit: The comments at the top of that file say:

    // 3. Asynchronous safe-points occur at any instruction in user code
    //    where the goroutine can be safely paused and a conservative
    //    stack and register scan can find stack roots. The runtime can
    //    stop a goroutine at an async safe-point using a signal.

No it’s an interesting comment. This is not really about load, but about control flow: if goroutine is just spinning wild without going through any function prologue, it won’t even be aware of the synchronous preemption request. Asynchronous preemption (signal-based) is mainly (I say “mainly” because I am not sure I can say “only”) for this kind of situation.

I don’t have the link ready, but twitch had this kind of issue with base64 decoding in some kind of servers. The GC would try to STW, but there would always be one or a few goroutines decoding base64 in a tight loop for the time STW was attempted, delaying it again and again.

Asynchronous preemption is a solution to this kind of issue. Load is not the issue here, as long as you go through the runtime often enough.

You'll actually see that's a general concurrency pattern, and I mean, far beyond Go. It is certainly ideal to trigger off of some signal (in a very, very generic sense of the term, not OS signal) in order to trigger some other process (again in a very generic sense), but in general if you're programming concurrent code you should always have some sort of time-based fallback for any wait you are doing, because you will hit it out in the field. It's all kinds of no fun to have processes that will wait forever. (Unless you're really sure you need it.)

The runtime never determines whether the goroutine is at a safe point. It "poisons" the stack guard so that the next time the goroutine reaches a function prologue, which is a safe point, it examines the stack guard and knows that it has been preempted.

Then there's the async case for tight loops that I remember reading about back in 2020 (it uses unix signals), but don't yet fully grok the specifics.