netdev - Re: loaded router, excessive getnstimeofday in oprofile

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Message-ID: <20080827165635.GY26610@one.firstfloor.org>
Date:	Wed, 27 Aug 2008 18:56:35 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Rick Jones <rick.jones2@...com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Eric Dumazet <dada1@...mosbay.com>,
	Denys Fedoryshchenko <denys@...p.net.lb>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: loaded router, excessive getnstimeofday in oprofile

On Wed, Aug 27, 2008 at 09:49:10AM -0700, Rick Jones wrote:
> Andi Kleen wrote:
> >>Those banks really want to crank down on latency - to the point they 
> >>start disabling interrupt coalescing.  I bet they'd toss anything out 
> >>they could to shave another microsecond.
> >
> >
> >This change would actually likely lower their latency.
> 
> I'm guessing you mean increase their latency?  I agree, it could - 
> depends entirely on the PPS in production I suspect.

No, moving the time stamps into the socket decreases latency
for all packets that don't need time stamps. And they likely 
have some packets which don't need time stamps too.

As a secondary effect if they use a RT kernel it might
be also beneficial to do the (depending on the platform)
costly time stamp in the lower priority socket context
than in the high priority interrupt thread.

-Andi

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