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


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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