linux-kernel - Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay factor

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Message-ID: <CANN689HeTk0jNJb4sF_0t714MvFnU0vLw0=JeXYRUwMx8GDnXQ@mail.gmail.com>
Date:	Wed, 26 Dec 2012 22:07:50 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, aquini@...hat.com,
	lwoodman@...hat.com, jeremy@...p.org,
	Jan Beulich <JBeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay factor

On Wed, Dec 26, 2012 at 11:51 AM, Rik van Riel <riel@...hat.com> wrote:
> On 12/26/2012 02:10 PM, Eric Dumazet wrote:
>> We might try to use a hash on lock address, and an array of 16 different
>> delays so that different spinlocks have a chance of not sharing the same
>> delay.
>>
>> With following patch, I get 982 Mbits/s with same bench, so an increase
>> of 45 % instead of a 13 % regression.

Awesome :)

> I will probably keep it as a separate patch 4/4, with
> your report and performance numbers in it, to preserve
> the reason why we keep multiple hashed values, etc...
>
> There is enough stuff in this code that will be
> indishinguishable from magic if we do not document it
> properly...

If we go with per-spinlock tunings, I feel we'll most likely want to
add an associative cache in order to avoid the 1/16 chance (~6%) of
getting 595Mbit/s instead of 982Mbit/s when there is a hash collision.

I would still prefer if we could make up something that didn't require
per-spinlock tunings, but it's not clear if that'll work. At least we
now know of a simple enough workload to figure it out :)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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