netdev - Re: High contention on the sk_buff

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Message-Id: <20090318.143844.173112261.davem@davemloft.net>
Date:	Wed, 18 Mar 2009 14:38:44 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	vernux@...ibm.com
Cc:	andi@...stfloor.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: High contention on the sk_buff_head.lock

From: Vernon Mauery <vernux@...ibm.com>
Date: Wed, 18 Mar 2009 14:10:33 -0700

> David Miller wrote:
> > From: Andi Kleen <andi@...stfloor.org>
> > Date: Wed, 18 Mar 2009 21:54:37 +0100
> > 
> >> But then again I'm not sure it's  worth it if the problem only
> >> happens in out of tree RT.
> > The list of problems that only show up with the RT kernel seems to be
> > constantly increasing, but honestly is very surprising to me.
> > I don't understand why we even need to be concerned about this stuff
> > upstream, to be honest.
> > Please reproduce this in the vanilla kernel, then get back to us.
> 
> Huh? The numbers that I posted *were* from the vanilla kernel. I ran
> the 2.6.29-rc8 kernel with lock_stat enabled.  The lock contention
> happens on the same lock in both vanilla and -rt, it just happens
> to be more pronounced in the -rt kernel because of the double context
> switches that the sleeping spinlock/rt-mutexes introduce.

And the double context switches are probably also why less
natural batching and locality are achieved in the RT kernel.

Isn't that true?
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