netdev - Re: big picture UDP/IP performance question re 2.6.18 -> 2.6.32

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Date:	Tue, 11 Oct 2011 10:24:31 -0600
From:	Chris Friesen <chris.friesen@...band.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	starlight@...nacle.cx, linux-kernel@...r.kernel.org,
	netdev <netdev@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <cl@...two.org>, Willy Tarreau <w@....eu>,
	Ingo Molnar <mingo@...e.hu>,
	Stephen Hemminger <stephen.hemminger@...tta.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Joe Perches <joe@...ches.com>,
	Chetan Loke <Chetan.Loke@...scout.com>,
	Con Kolivas <conman@...ivas.org>,
	Serge Belyshev <belyshev@...ni.sinp.msu.ru>
Subject: Re: big picture UDP/IP performance question re 2.6.18  -> 2.6.32

On 10/06/2011 11:40 PM, Eric Dumazet wrote:
> Le jeudi 06 octobre 2011 à 23:27 -0400, starlight@...nacle.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@...band.com
www.genband.com
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