Messages in this thread | | | Date | Wed, 16 Jun 2010 17:03:22 +0200 (CEST) | From | Jiri Kosina <> | Subject | Re: Slow pty's (was Re: libdivecomputer interfaces?) |
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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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