[PATCH] microoptimize by trying to avoid locking a locked mutex

[Hans Petter Selasky]

Hi,

Did you test this patch works like expected with non x86 platforms?

--HPS

On 11/05/15 00:32, Mateusz Guzik wrote:
> mtx_lock will unconditionally try to grab the lock and if that fails,
> will call __mtx_lock_sleep which will immediately try to do the same
> atomic op again.
>
> So, the obvious microoptimization is to check the state in
> __mtx_lock_sleep and avoid the operation if the lock is not free.
>
> This gives me ~40% speedup in a microbenchmark of 40 find processes
> traversing tmpfs and contending on mount mtx (only used as an easy
> benchmark, I have WIP patches to get rid of it).
>
> Second part of the patch is optional and just checks the state of the
> lock prior to doing any atomic operations, but it gives a very modest
> speed up when applied on top of the __mtx_lock_sleep change. As such,
> I'm not going to defend this part.