Re: Dropping Packets in 2.6.17

Danial Thom
2006-06-22T12:47:03Z

--- Arjan van de Ven <arjan@infradead.org> wrote:

> On Thu, 2006-06-22 at 04:31 -0700, Danial Thom
> wrote:
> > I'm trying to make a case for using linux as
> a
> > network appliance, but I can't find any
> > combination of settings that will keep it
> from
> > dropping packets at an unacceptably high
> rate.
> > The test system is a 1.8Ghz Opteron with
> intel
> > gigE cards running 2.6.17. I'm passing about
> 70K
> > pps through the box, which is a light load,
> but
> > userland activities (such as building a
> kernel)
> > cause it to lose packets, even with backlog
> set
> > to 20000. I had the same problem with 2.6.12
> and
> > abandoned the effort. Has anything been done
> > since to give priority to networking? You
> can't
> > have a network appliance drop packets when
> some
> > application is gathering stats or a user is
> > looking at a graph. What tunings are
> available?
>
> Hi Danial,
>
> the most likely tunable that will help you is
>
> /proc/sys/vm/min_free_kbytes
>
> For the router kind of device that one usually
> needs bumping a bit;
> without the bumping the VM doesn't see enough
> "normal" activity to tune
> it's emergency/interrupt handling buffers (and
> most networking
> allocations happen in interrupt context), and
> then ends up failing
> allocations in interrupt context, which leads
> to dropped packets.

I don't think thats the problem, as I've tracked
the problem to packets being dropped because of
excessive backlog (ie, they are being dropped
gracefully). However with a backlog of 20000, and
a traffic level of about 75,000pps, that means
almost 1/3 of a second that the system doesn't
process packets, which is just unacceptable.

I'll try changing the setting, but running out of
memory doesn't seem to be the issue. I think what
I need is some mechanism to make interrupts a
priority, like it was back in the days when
networking was more important then mp3 playback.

Danial

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