linux-kernel - Re: [RFC PATCH] mutex: Apply adaptive spinning on mutex

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Message-ID: <AANLkTiktmhP6-JGTKHf+wZhdJRd3Eb-PYzMRR=CyRsUP@mail.gmail.com>
Date:	Wed, 23 Mar 2011 22:46:56 +0300
From:	Andrey Kuzmin <andrey.v.kuzmin@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Tejun Heo <tj@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Mason <chris.mason@...cle.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org
Subject: Re: [RFC PATCH] mutex: Apply adaptive spinning on mutex_trylock()

On Wed, Mar 23, 2011 at 6:48 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Mar 23, 2011 at 8:37 AM, Tejun Heo <tj@...nel.org> wrote:
>>
>> Currently, mutex_trylock() doesn't use adaptive spinning.  It tries
>> just once.  I 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.
>
>                    Linus
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