Re: [RFC PATCH] mutex: Apply adaptive spinning on mutex_trylock()

[Andrey Kuzmin]

On Wed, Mar 23, 2011 at 6:48 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Mar 23, 2011 at 8:37 AM, Tejun Heo <tj@xxxxxxxxxx> wrote:
>>
>> Currently, mutex_trylock() doesn't use adaptive spinning. ÂIt tries
>> just once. ÂI got curious whether using adaptive spinning on
>> mutex_trylock() would be beneficial and it seems so, at least for
>> btrfs anyway.
>
> Hmm. Seems reasonable to me.

TAS/spin with exponential back-off has been preferred locking approach
in Postgres (and I believe other DBMSes) for years, at least since '04
when I had last touched Postgres code. Even with 'false negative' cost
in user-space being much higher than in the kernel, it's still just a
question of scale (no wonder measurable improvement here is reported
from dbench on SSD capable of few dozen thousand IOPS).

Regards,
Andrey

> The patch looks clean, although part of that is just the mutex_spin()
> cleanup that is independent of actually using it in trylock.
>
> So no objections from me.
>
> Â Â Â Â Â Â Â Â Â ÂLinus
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