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

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Date:	Fri, 24 Aug 2007 13:24:57 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	linux-kernel@...r.kernel.org
Subject:  Re: RFC: issues concerning the next NAPI interface

On Fri, 24 Aug 2007 21:04:56 +0200
Bodo Eggert <7eggert@....de> wrote:

> 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.-)

I expect the overhead of OS timers and resolution makes this unsuitable for fast
networks. You need the hardware to do it. On slow networks, it doesn't matter.

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>

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