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

[Linus Torvalds]

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/