linux-kernel - Re: RFC: issues concerning the next NAPI interface

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Message-Id: <200708241747.16592.ossthema@de.ibm.com>
Date:	Fri, 24 Aug 2007 17:47:15 +0200
From:	Jan-Bernd Themann <ossthema@...ibm.com>
To:	akepner@....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

Hi,

On Friday 24 August 2007 17:37, akepner@....com wrote:
> 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.
> 

I don't see how this should work. Our latest machines are fast enough that they
simply empty the queue during the first poll iteration (in most cases).
Even if you wait until X packets have been received, it does not help for
the next poll cycle. The average number of packets we process per poll queue
is low. So a timer would be preferable that periodically polls the 
queue, without the need of generating a HW interrupt. This would allow us
to wait until a reasonable amount of packets have been received in the meantime
to keep the poll overhead low. This would also be useful in combination
with LRO.

Regards,
Jan-Bernd
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