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

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Message-Id: <E1IOeSm-0000bm-Jo@be1.lrz>
Date:	Fri, 24 Aug 2007 21:04:56 +0200
From:	Bodo Eggert <7eggert@....de>
To:	Linas Vepstas <linas@...tin.ibm.com>,
	Jan-Bernd Themann <ossthema@...ibm.com>,
	netdev <netdev@...r.kernel.org>,
	Thomas Klein <tklein@...ibm.com>,
	Jan-Bernd Themann <themann@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-ppc <linuxppc-dev@...abs.org>,
	Christoph Raisch <raisch@...ibm.com>,
	Marcus Eder <meder@...ibm.com>,
	Stefan Roscher <stefan.roscher@...ibm.com>
Subject: Re: RFC: issues concerning the next NAPI interface

Linas Vepstas <linas@...tin.ibm.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.
> 
> I saw this too, on a system that is "modern" but not terribly fast, and
> only slightly (2-way) smp. (the spidernet)
> 
> I experimented wih various solutions, none were terribly exciting.  The
> thing that killed all of them was a crazy test case that someone sprung on
> me:  They had written a worst-case network ping-pong app: send one
> packet, wait for reply, send one packet, etc.
> 
> If I waited (indefinitely) for a second packet to show up, the test case
> completely stalled (since no second packet would ever arrive).  And if I
> introduced a timer to wait for a second packet, then I just increased
> the latency in the response to the first packet, and this was noticed,
> and folks complained.

Possible solution / possible brainfart:

Introduce a timer, but don't start to use it to combine packets unless you
receive n packets within the timeframe. If you receive less than m packets
within one timeframe, stop using the timer. The system should now have a
decent response time when the network is idle, and when the network is
busy, nobody will complain about the latency.-)
-- 
Funny quotes:
22. When everything's going your way, you're in the wrong lane and and going
    the wrong way.
Friß, Spammer: rsRxhvmk@....7eggert.dyndns.org m@....7eggert.dyndns.org
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