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List:       linux-kernel
Subject:    Re: Slow pty's (was Re: libdivecomputer interfaces?)
From:       Mike Galbraith <efault () gmx ! de>
Date:       2010-06-16 15:16:51
Message-ID: 1276701411.30669.6.camel () marge ! simson ! net
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On Wed, 2010-06-16 at 17:03 +0200, Jiri Kosina wrote:
> 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.

Chris already gave me a heads up, it's on my todo.  The old P4 box I use
for a serial console box is exploding on boot, or I would have already
had a look.
  
> > > and looking at the only two interesting hunks it's not clear why:

Complete mystery to me.

> > > +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

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