Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759847Ab0FJWZn (ORCPT ); Thu, 10 Jun 2010 18:25:43 -0400 Received: from sprinkles.athenacr.com ([64.95.46.210]:9439 "EHLO sprinkles.inp.in.athenacr.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1759709Ab0FJWZm (ORCPT ); Thu, 10 Jun 2010 18:25:42 -0400 Message-ID: <4C116652.8010408@athenacr.com> Date: Thu, 10 Jun 2010 18:25:22 -0400 From: Brian Bloniarz User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100423 Thunderbird/3.0.4 MIME-Version: 1.0 To: Chris Wedgwood CC: Linus Torvalds , Greg KH , Alan Cox , OGAWA Hirofumi , Jef Driesen , linux-kernel@vger.kernel.org Subject: Re: Slow pty's (was Re: libdivecomputer interfaces?) References: <20100610181042.GA19210@puku.stupidest.org> In-Reply-To: <20100610181042.GA19210@puku.stupidest.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2410 Lines: 62 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. > 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 > -- > 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/ -- 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/