netdev - Re: RFC: issues concerning the next NAPI interface

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Date:	Fri, 24 Aug 2007 08:37:03 -0700
From:	akepner@....com
To:	Jan-Bernd Themann <ossthema@...ibm.com>
Cc:	netdev <netdev@...r.kernel.org>,
	Christoph Raisch <raisch@...ibm.com>,
	Jan-Bernd Themann <themann@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-ppc <linuxppc-dev@...abs.org>,
	Marcus Eder <meder@...ibm.com>,
	Thomas Klein <tklein@...ibm.com>,
	Stefan Roscher <stefan.roscher@...ibm.com>
Subject: Re: RFC: issues concerning the next NAPI interface

On Fri, Aug 24, 2007 at 03:59:16PM +0200, Jan-Bernd Themann wrote:
> .......
> 3) On modern systems the incoming packets are processed very fast. Especially
>    on SMP systems when we use multiple queues we process only a few packets
>    per napi poll cycle. So NAPI does not work very well here and the interrupt 
>    rate is still high. What we need would be some sort of timer polling mode 
>    which will schedule a device after a certain amount of time for high load 
>    situations. With high precision timers this could work well. Current
>    usual timers are too slow. A finer granularity would be needed to keep the
>    latency down (and queue length moderate).
> 

We found the same on ia64-sn systems with tg3 a couple of years 
ago. Using simple interrupt coalescing ("don't interrupt until 
you've received N packets or M usecs have elapsed") worked 
reasonably well in practice. If your h/w supports that (and I'd 
guess it does, since it's such a simple thing), you might try 
it.

-- 
Arthur

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