linux-kernel - Re: Network slowdown due to CFS

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Message-ID: <07E3AEH11@briare1.fullpliant.org>
Date:	Mon, 01 Oct 2007 22:27:52 GMT
From:	Hubert Tonneau <hubert.tonneau@...lpliant.org>
To:	linux-kernel@...r.kernel.org
Subject: Re: Network slowdown due to CFS

Ingo Molnar wrote:
>
> Really, i have never seen a _single_ mainstream app where the use of
> sched_yield() was the right choice.

Pliant 'FastSem' semaphore implementation (as oppsed to 'Sem') uses 'yield'
http://old.fullpliant.org/

Basically, if the ressource you are protecting with the semaphore will be held
for a significant time, then a full semaphore might be better, but if the
ressource will be held just a fiew cycles, then light aquiering might bring best
result because the most significant cost is in aquiering/releasing.

So the aquiering algorithm for fast semaphores might be:
try to aquire with a hardware atomic read and set instruction, then if it fails,
call yield then retry (at least on a single processor single core system).

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