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SubjectRe: Slow pty's (was Re: libdivecomputer interfaces?)
On Thu, 10 Jun 2010, Brian Bloniarz wrote:

> On 06/10/2010 02:10 PM, Chris Wedgwood wrote:
> > (sorry if this reponse isn't on target, i was just pointed to this
> > thread a few minutes ago)
> >
> >
> > On Thu, Jun 10, 2010 at 10:25:36AM -0700, Linus Torvalds wrote:
> >
> >> I thought we long since (ie back last fall) fixed the latency
> >> problems with pty's, but there does seem to be something very fishy
> >> going on there still.
> >
> > this might not be related, but i have slow serial ports with NOHZ that
> > goes away when i revert 39c0cbe2150cbd848a25ba6cdb271d1ad46818ad.
>
> Unrelated or not, I think Chris is right about this. Somewhere before
> -rc1, the emulated serial console on my KVM instance became slow
> to echo input. I just tested with the commit reverted and it's
> back to normal.

So let's CC Mike then.

>
> > commit 39c0cbe2150cbd848a25ba6cdb271d1ad46818ad
> > Author: Mike Galbraith <efault@gmx.de>
> > Date: Thu Mar 11 17:17:13 2010 +0100
> >
> > sched: Rate-limit nohz
> >
> > Entering nohz code on every micro-idle is costing ~10% throughput for netperf
> > TCP_RR when scheduling cross-cpu. Rate limiting entry fixes this, but raises
> > ticks a bit. On my Q6600, an idle box goes from ~85 interrupts/sec to 128.
> >
> > The higher the context switch rate, the more nohz entry costs. With this patch
> > and some cycle recovery patches in my tree, max cross cpu context switch rate is
> > improved by ~16%, a large portion of which of which is this ratelimiting.
> >
> > and looking at the only two interesting hunks it's not clear why:
> >
> > +int nohz_ratelimit(int cpu)
> > +{
> > + struct rq *rq = cpu_rq(cpu);
> > + u64 diff = rq->clock - rq->nohz_stamp;
> > +
> > + rq->nohz_stamp = rq->clock;
> > +
> > + return diff < (NSEC_PER_SEC / HZ) >> 1;
> > +}
> >
> > + if (nohz_ratelimit(cpu))
> > + goto end;
> > +
> >
> > network latnecy is fine, and if i create lots of wakeups (network IO
> > is fine) then the serial port latency is noticable

--
Jiri Kosina
SUSE Labs, Novell Inc.



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