linux-kernel - Re: Network slowdown due to CFS

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Message-ID: <20071001125350.055ef9bb@laptopd505.fenrus.org>
Date:	Mon, 1 Oct 2007 12:53:50 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	davids@...master.com
Cc:	"Ingo Molnar" <mingo@...e.hu>, <linux-kernel@...r.kernel.org>
Subject: Re: Network slowdown due to CFS

On Mon, 1 Oct 2007 09:49:35 -0700
"David Schwartz" <davids@...master.com> wrote:

> 
> > * Jarek Poplawski <jarkao2@...pl> wrote:
> >
> > > BTW, it looks like risky to criticise sched_yield too much: some
> > > people can misinterpret such discussions and stop using this at
> > > all, even where it's right.
> 
> > Really, i have never seen a _single_ mainstream app where the use of
> > sched_yield() was the right choice.
> 
> It can occasionally be an optimization. You may have a case where you
> can do something very efficiently if a lock is not held, but you
> cannot afford to wait for the lock to be released. So you check the
> lock, if it's held, you yield and then check again. If that fails,
> you do it the less optimal way (for example, dispatching it to a
> thread that *can* afford to wait).


at this point it's "use a futex" instead; once you're doing system
calls you might as well use the right one for what you're trying to
achieve.
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