Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

[Linus Torvalds]

On Wed, 7 Jan 2009, Gregory Haskins wrote:
>
> Can I ask a simple question in light of all this discussion? 
> 
> "Is get_task_struct() really that bad?"

Yes. It's an atomic access (two, in fact, since you need to release it 
too), which is a huge deal if we're talking about a timing-critical 
section of code.

And this is timing-critical, or we wouldn't even care - even in the 
contention case. Admittedly btrfs apparently makes it more so that it 
_should_ be, but Peter had some timings that happened with just regular 
create/unlink that showed a big difference.

So the whole and only point of spinning mutexes is to get rid of the 
scheduler overhead, but to also not replace it with some other thing ;)

			Linus
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