Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758966Ab0FPPD2 (ORCPT ); Wed, 16 Jun 2010 11:03:28 -0400 Received: from cantor2.suse.de ([195.135.220.15]:53117 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755114Ab0FPPD0 (ORCPT ); Wed, 16 Jun 2010 11:03:26 -0400 Date: Wed, 16 Jun 2010 17:03:22 +0200 (CEST) From: Jiri Kosina X-X-Sender: jikos@twin.jikos.cz To: Brian Bloniarz Cc: Chris Wedgwood , Linus Torvalds , Greg KH , Alan Cox , OGAWA Hirofumi , Jef Driesen , linux-kernel@vger.kernel.org, Mike Galbraith Subject: Re: Slow pty's (was Re: libdivecomputer interfaces?) In-Reply-To: <4C116652.8010408@athenacr.com> Message-ID: References: <20100610181042.GA19210@puku.stupidest.org> <4C116652.8010408@athenacr.com> User-Agent: Alpine 2.00 (LRH 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2366 Lines: 66 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 > > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/