From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kara Subject: Re: [PATCH 05/18] writeback: per task dirty rate limit Date: Wed, 7 Sep 2011 01:27:38 +0200 Message-ID: <20110906232738.GC31945@quack.suse.cz> References: <20110904015305.367445271@intel.com> <20110904020915.240747479@intel.com> <1315324030.14232.14.camel@twins> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Wu Fengguang , linux-fsdevel@vger.kernel.org, Andrew Morton , Jan Kara , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Vivek Goyal , Andrea Righi , linux-mm , LKML To: Peter Zijlstra Return-path: Content-Disposition: inline In-Reply-To: <1315324030.14232.14.camel@twins> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org 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... Honza -- Jan Kara SUSE Labs, CR -- 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