Date: Wed, 27 Aug 2008 18:56:35 +0200
From: Andi Kleen <andi@firstfloor.org>
To: Rick Jones <rick.jones2@hp.com>
Cc: Andi Kleen <andi@firstfloor.org>, Evgeniy Polyakov <johnpol@2ka.mipt.ru>,
       Eric Dumazet <dada1@cosmosbay.com>,
       Denys Fedoryshchenko <denys@visp.net.lb>, netdev@vger.kernel.org,
       linux-kernel@vger.kernel.org
Subject: Re: loaded router, excessive getnstimeofday in oprofile
Message-ID: <20080827165635.GY26610@one.firstfloor.org>
References: <200808220457.40892.denys@visp.net.lb> <20080826201406.GA24827@2ka.mipt.ru> <48B46B48.7030609@cosmosbay.com> <20080826205158.GA15266@2ka.mipt.ru> <87vdxmr53f.fsf@basil.nowhere.org> <48B57BD3.5050206@hp.com> <20080827162735.GW26610@one.firstfloor.org> <48B58586.3080806@hp.com>
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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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