Re: [PATCH][RFC]: mutex: adaptive spin

Linus Torvalds
2009-01-06T15:57:22Z

On Tue, 6 Jan 2009, Ingo Molnar wrote:
>
> The thing i like most about Peter's patch (compared to most other adaptive
> spinning approaches i've seen, which all sucked as they included various
> ugly heuristics complicating the whole thing) is that it solves the "how
> long should we spin" question elegantly: we spin until the owner runs on a
> CPU.

The other way around, you mean: we spin until the owner is no longer
holding a cpu.

I agree that it's better than the normal "spin for some random time"
model, but I can't say I like the "return 0" cases where it just retries
the whole loop if the semaphore was gotten by somebody else instead.
Sounds like an easyish live-lock to me.

I also still strongly suspect that whatever lock actually needs this,
should be seriously re-thought.

But apart from the "return 0" craziness I at least dont' _hate_ this
patch. Do we have numbers? Do we know which locks this matters on?

Linus