Re: RFC: issues concerning the next NAPI interface

Stephen Hemminger
2007-08-24T20:25:30Z

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

> Linas Vepstas <linas@austin.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@linux-foundation.org>