From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933544AbXKPDxi (ORCPT ); Thu, 15 Nov 2007 22:53:38 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758773AbXKPDx3 (ORCPT ); Thu, 15 Nov 2007 22:53:29 -0500 Received: from gw1.cosmosbay.com ([86.65.150.130]:49926 "EHLO gw1.cosmosbay.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758072AbXKPDx2 (ORCPT ); Thu, 15 Nov 2007 22:53:28 -0500 Message-ID: <473D131A.5000200@cosmosbay.com> Date: Fri, 16 Nov 2007 04:48:42 +0100 From: Eric Dumazet User-Agent: Thunderbird 2.0.0.9 (Windows/20071031) MIME-Version: 1.0 To: Arjan van de Ven CC: Linux Kernel Mailing List , davem@davemloft.net Subject: Re: [NET]: rt_check_expire() can take a long time, add a cond_resched() References: <200711150401.lAF41mSs021898@hera.kernel.org> <20071115193802.4ec64eef@laptopd505.fenrus.org> In-Reply-To: <20071115193802.4ec64eef@laptopd505.fenrus.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-1.6 (gw1.cosmosbay.com [86.65.150.130]); Fri, 16 Nov 2007 04:48:48 +0100 (CET) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Arjan van de Ven a écrit : > On Thu, 15 Nov 2007 04:01:48 GMT > Linux Kernel Mailing List wrote: > >> Gitweb: >> http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=d90bf5a976793edfa88d3bb2393f0231eb8ce1e5 >> Commit: d90bf5a976793edfa88d3bb2393f0231eb8ce1e5 Parent: >> 66ba886254edbbd9442d30f1eef6f6fb0145027d Author: Eric Dumazet >> AuthorDate: Wed Nov 14 16:14:05 2007 -0800 >> Committer: David S. Miller >> CommitDate: Wed Nov 14 16:14:05 2007 -0800 >> >> [NET]: rt_check_expire() can take a long time, add a >> cond_resched() >> On commit 39c90ece7565f5c47110c2fa77409d7a9478bd5b: > >> When the IP route cache is big, rt_check_expire() can take a long >> time to run. (default settings : 20% of the hash table is scanned at >> each invocation) >> >> Adding cond_resched() helps giving cpu to higher priority tasks if >> necessary. >> >> Using a "if (need_resched())" test before calling >> "cond_resched();" is necessary to avoid spending too much time doing >> the resched check. > > int __sched cond_resched(void) > { > if (need_resched() && ..... > > somehow I wonder why the second if() is useful at all; it's another > spot for a branch predictor to miss... and a void function call is > really really cheap... Its not that cheap. The ChangeLog included my own numbers, on a Pentium M machine. (i686, 1.6 GHz, 1.5 GB ram) Without "if (need_resched())" (so calling need_resched() X.XXX.XXX times), each run takes 88ms With the extra check (and *much* less function calls), each run takes 25ms