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Message-ID: <4E946DBF.5050105@genband.com>
Date: Tue, 11 Oct 2011 10:24:31 -0600
From: Chris Friesen <chris.friesen@genband.com>
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To: Eric Dumazet <eric.dumazet@gmail.com>
CC: starlight@binnacle.cx, linux-kernel@vger.kernel.org,
        netdev <netdev@vger.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Christoph Lameter <cl@gentwo.org>, Willy Tarreau <w@1wt.eu>,
        Ingo Molnar <mingo@elte.hu>,
        Stephen Hemminger <stephen.hemminger@vyatta.com>,
        Benjamin LaHaise <bcrl@kvack.org>, Joe Perches <joe@perches.com>,
        Chetan Loke <Chetan.Loke@netscout.com>,
        Con Kolivas <conman@kolivas.org>,
        Serge Belyshev <belyshev@depni.sinp.msu.ru>
Subject: Re: big picture UDP/IP performance question re 2.6.18  -> 2.6.32
References: <6.2.5.6.2.20111006231958.039bb570@binnacle.cx> <1317966007.3457.47.camel@edumazet-laptop>
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On 10/06/2011 11:40 PM, Eric Dumazet wrote:
> Le jeudi 06 octobre 2011 à 23:27 -0400, starlight@binnacle.cx a écrit :

>> If the older kernels are switching to NAPI
>> for much of surge and the switching out
>> once the pulse falls off, it might
>> conceivably result in much better latency
>> and overall performance.

> Thats exactly the opposite : Your old kernel is not fast enough to
> enter/exit NAPI on every incoming frame.
>
> Instead of one IRQ per incoming frame, you have less interrupts :
> A napi run processes more than 1 frame.
>
> Now increase your incoming rate, and you'll discover a new kernel will
> be able to process more frames without losses.

I wonder if it would make sense to adjust the interrupt mitigation 
parameters in the NIC to allow it to accumulate a few packets before 
interrupting the CPU.  We had good luck using this to reduce interrupt 
rate on a quasi-pathological case where we were bouncing in and out of 
NAPI because we were *just* fast enough to keep up with incoming packets.

Chris



-- 
Chris Friesen
Software Developer
GENBAND
chris.friesen@genband.com
www.genband.com