linux-kernel - Re: Network slowdown due to CFS

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Message-ID: <20071001153529.7c669c7a@laptopd505.fenrus.org>
Date:	Mon, 1 Oct 2007 15:35:29 -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 15:17:52 -0700
"David Schwartz" <davids@...master.com> wrote:

> 
> Arjan van de Ven wrote:
> 
> > > 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.
> 
> There are two answers to this. One is that you sometimes are writing
> POSIX code and Linux-specific optimizations don't change the fact
> that you still need a portable implementation.
> 
> The other answer is that futexes don't change anything in this case.
> In fact, in the last time I hit this, the lock was a futex on Linux.
> Nevertheless, that doesn't change the basic issue. The lock is
> locked, you cannot afford to wait for it, but not getting the lock is
> expensive. The solution is to yield and check the lock again. If it's
> still held, you dispatch to another thread, but many times, yielding
> can avoid that.

yielding IS blocking. Just with indeterminate fuzzyness added to it....
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