From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752760Ab1IGQdI (ORCPT ); Wed, 7 Sep 2011 12:33:08 -0400 Received: from casper.infradead.org ([85.118.1.10]:38261 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752337Ab1IGQdE convert rfc822-to-8bit (ORCPT ); Wed, 7 Sep 2011 12:33:04 -0400 Subject: Re: [PATCH 05/18] writeback: per task dirty rate limit From: Peter Zijlstra To: Jan Kara Cc: Wu Fengguang , linux-fsdevel@vger.kernel.org, Andrew Morton , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Vivek Goyal , Andrea Righi , linux-mm , LKML Date: Wed, 07 Sep 2011 09:27:50 +0200 In-Reply-To: <20110906232738.GC31945@quack.suse.cz> References: <20110904015305.367445271@intel.com> <20110904020915.240747479@intel.com> <1315324030.14232.14.camel@twins> <20110906232738.GC31945@quack.suse.cz> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT X-Mailer: Evolution 3.0.2- Message-ID: <1315380470.11101.1.camel@twins> Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2011-09-07 at 01:27 +0200, Jan Kara wrote: > On Tue 06-09-11 17:47:10, Peter Zijlstra wrote: > > On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote: > > > /* > > > + * After a task dirtied this many pages, balance_dirty_pages_ratelimited_nr() > > > + * will look to see if it needs to start dirty throttling. > > > + * > > > + * If dirty_poll_interval is too low, big NUMA machines will call the expensive > > > + * global_page_state() too often. So scale it near-sqrt to the safety margin > > > + * (the number of pages we may dirty without exceeding the dirty limits). > > > + */ > > > +static unsigned long dirty_poll_interval(unsigned long dirty, > > > + unsigned long thresh) > > > +{ > > > + if (thresh > dirty) > > > + return 1UL << (ilog2(thresh - dirty) >> 1); > > > + > > > + return 1; > > > +} > > > > Where does that sqrt come from? > He does 2^{log_2(x)/2} which, if done in real numbers arithmetics, would > result in x^{1/2}. Given the integer arithmetics, it might be twice as > small but still it's some approximation... Right, and I guess with a cpu that can do the fls its slightly faster than our int_sqrt(). From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 05/18] writeback: per task dirty rate limit Date: Wed, 07 Sep 2011 09:27:50 +0200 Message-ID: <1315380470.11101.1.camel@twins> References: <20110904015305.367445271@intel.com> <20110904020915.240747479@intel.com> <1315324030.14232.14.camel@twins> <20110906232738.GC31945@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Cc: Wu Fengguang , linux-fsdevel@vger.kernel.org, Andrew Morton , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Vivek Goyal , Andrea Righi , linux-mm , LKML To: Jan Kara Return-path: In-Reply-To: <20110906232738.GC31945@quack.suse.cz> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Wed, 2011-09-07 at 01:27 +0200, Jan Kara wrote: > On Tue 06-09-11 17:47:10, Peter Zijlstra wrote: > > On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote: > > > /* > > > + * After a task dirtied this many pages, balance_dirty_pages_ratelim= ited_nr() > > > + * will look to see if it needs to start dirty throttling. > > > + * > > > + * If dirty_poll_interval is too low, big NUMA machines will call th= e expensive > > > + * global_page_state() too often. So scale it near-sqrt to the safet= y margin > > > + * (the number of pages we may dirty without exceeding the dirty lim= its). > > > + */ > > > +static unsigned long dirty_poll_interval(unsigned long dirty, > > > + unsigned long thresh) > > > +{ > > > + if (thresh > dirty) > > > + return 1UL << (ilog2(thresh - dirty) >> 1); > > > + > > > + return 1; > > > +} > >=20 > > Where does that sqrt come from?=20 > He does 2^{log_2(x)/2} which, if done in real numbers arithmetics, woul= d > result in x^{1/2}. Given the integer arithmetics, it might be twice as > small but still it's some approximation... Right, and I guess with a cpu that can do the fls its slightly faster than our int_sqrt(). -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org