On Tue, Jan 24, 2006 at 03:15:27AM -0500, Lee Revell wrote:
> On Tue, 2006-01-24 at 09:13 +0100, Ingo Molnar wrote:
> > * Lee Revell <[email protected]> wrote:
> >
> > > On Tue, 2006-01-24 at 09:01 +0100, Ingo Molnar wrote:
> > > > * Lee Revell <[email protected]> wrote:
> > > >
> > > > > On Tue, 2006-01-24 at 08:56 +0100, Ingo Molnar wrote:
> > > > > > * Lee Revell <[email protected]> wrote:
> > > > > >
> > > > > > > I ported the latency tracer to 2.6.16 and got this 13ms latency within
> > > > > > > a few hours. This is a regression from 2.6.15.
> > > > > > >
> > > > > > > It appears that RCU can invoke ipv4_dst_destroy thousands of times in
> > > > > > > a single batch.
> > > > > >
> > > > > > could you try the PREEMPT_RCU patch below?
> > > > >
> > > > > Sure. If it works do you see this making it in 2.6.16? Otherwise we
> > > > > still would have a regression...
> > > >
> > > > nope, that likely wont make v2.6.16, which is frozen already.
> > > >
> > >
> > > How about just lowering maxbatch to 1000?
> >
> > does that fix the latency for you? I think "maxbatch=1000" should work
> > as a boot parameter too.
> >
>
> Have not tested yet but it appears that will reduce it substantially:
>
> $ grep "dst_destroy (dst_rcu_free)" /proc/latency_trace | wc -l
> 3027
>
> This implies the latency would be reduced to ~4ms, still not great but
> it will be overshadowed by rt_run_flush/rt_garbage_collect.
The other patch to try would be Dipankar Sarma's patch at:
http://marc.theaimsgroup.com/?l=linux-kernel&m=113657112726596&w=2
This patch was primarily designed to reduce memory overhead, but given
that it tends to reduce batch size, it should also reduce latency.
Thanx, Paul
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