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SubjectRe: 2.6.12 Performance problems


--- Jesper Juhl <jesper.juhl@gmail.com> wrote:
> On 8/21/05, Jesper Juhl <jesper.juhl@gmail.com>
> wrote:
> > On 8/21/05, Danial Thom
> <danial_thom@yahoo.com> wrote:
> > > >
> > > Ok, well you'll have to explain this one:
> > >
> > > "Low latency comes at the cost of decreased
> > > throughput - can't have both"
> > >
> > > Seems to be a bit backwards. Threading the
> kernel
> > > adds latency, so its the additional latency
> in
> > > the kernel that causes the drop in
> throughput. Do
> > > you mean that kernel performance has been
> > > sacrificed in order to be able to service
> other
> > > threads more quickly, even when there are
> no
> > > other threads to be serviced?
> > >
> >
> > Ok, let me start with the way HZ influences
> things.
> >
> [snip]
>
> A small followup.
>
> I'm not saying that the value of HZ or your
> preempt setting (whatever
> it may be) is definately the cause of your
> problem. All I'm saying is
> that it might be a contributing factor, so it's
> probably something
> that's worth a little bit of time testing.
>
> In many cases people running a server on
> resonably new hardware with
> HZ=1000 and full preempt won't even notice, but
> that's depending on
> the load on the server and what jobs it has.
> For some tasks it
> matters, for some the differences in
> performance is negligible.
>
> You problem could very well be something else
> entirely, but try a
> kernel build with PREEMPT_NONE and HZ=100 and
> see if it makes a big
> difference (or if that's your current config,
> then try the opposite,
> HZ=1000 and PREEMPT). If it does make a
> difference, then that's a
> valuable piece of information to report on the
> list. If it turns out
> it makes next to no difference at all, then
> that as well is relevant
> information as then people will know that HZ &
> preempt is not the
> cause and can focus on finding the problem
> elsewhere.
>
Yes. Hz isn't going to make much difference on a
2.0Ghz opteron, but I can see how premption can
cause packet loss. Shouldn't packet processing be
the highest priority process? It seems pointless
to "keep the audio buffers full" if you're
dropping packets as a result.

Also some clown typing on the keyboard shouldn't
cause packet loss. Trading network integrity for
snappy responsiveness is a bad trade.

Danial



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