From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755150Ab1HJVmG (ORCPT ); Wed, 10 Aug 2011 17:42:06 -0400 Received: from mx1.redhat.com ([209.132.183.28]:12895 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755117Ab1HJVmE (ORCPT ); Wed, 10 Aug 2011 17:42:04 -0400 Date: Wed, 10 Aug 2011 17:40:57 -0400 From: Vivek Goyal To: Wu Fengguang Cc: Peter Zijlstra , "linux-fsdevel@vger.kernel.org" , Andrew Morton , Jan Kara , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Andrea Righi , linux-mm , LKML Subject: Re: [PATCH 2/5] writeback: dirty position control Message-ID: <20110810214057.GA6576@redhat.com> References: <20110806084447.388624428@intel.com> <20110806094526.733282037@intel.com> <1312811193.10488.33.camel@twins> <20110808141128.GA22080@localhost> <1312814501.10488.41.camel@twins> <20110808230535.GC7176@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110808230535.GC7176@localhost> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 09, 2011 at 07:05:35AM +0800, Wu Fengguang wrote: > On Mon, Aug 08, 2011 at 10:41:41PM +0800, Peter Zijlstra wrote: > > On Mon, 2011-08-08 at 22:11 +0800, Wu Fengguang wrote: > > > @@ -538,11 +538,6 @@ static unsigned long bdi_position_ratio( > > > goal = thresh - thresh / DIRTY_SCOPE; > > > origin = 4 * thresh; > > > > > > - if (unlikely(origin < limit && dirty > (goal + origin) / 2)) { > > > - origin = limit; /* auxiliary control line */ > > > - goal = (goal + origin) / 2; > > > - pos_ratio >>= 1; > > > - } > > > pos_ratio = origin - dirty; > > > pos_ratio <<= BANDWIDTH_CALC_SHIFT; > > > do_div(pos_ratio, origin - goal + 1); > > FYI I've updated the fix to the below one, so that @limit will be used > as the origin in the rare case of (4*thresh < dirty). > > --- linux-next.orig/mm/page-writeback.c 2011-08-08 21:56:11.000000000 +0800 > +++ linux-next/mm/page-writeback.c 2011-08-09 06:34:25.000000000 +0800 > @@ -536,13 +536,8 @@ static unsigned long bdi_position_ratio( > * global setpoint > */ > goal = thresh - thresh / DIRTY_SCOPE; > - origin = 4 * thresh; > + origin = max(4 * thresh, limit); Hi Fengguang, Ok, so just trying to understand this pos_ratio little better. You have following basic formula. origin - dirty pos_ratio = -------------- origin - goal Terminology is very confusing and following is my understanding. - setpoint == goal setpoint is the point where we would like our number of dirty pages to be and at this point pos_ratio = 1. For global dirty this number seems to be (thresh - thresh / DIRTY_SCOPE) - thresh dirty page threshold calculated from dirty_ratio (Certain percentage of total memory). - Origin (seems to be equivalent of limit) This seems to be the reference point/limit we don't want to cross and distance from this limit basically decides the pos_ratio. Closer we are to limit, lower the pos_ratio and further we are higher the pos_ratio. So threshold is just a number which helps us determine goal and limit. goal = thresh - thresh / DIRTY_SCOPE limit = 4*thresh So goal is where we want to be and we start throttling the task more as we move away goal and approach limit. We keep the limit high enough so that (origin-dirty) does not become negative entity. So we do expect to cross "thresh" otherwise thresh itself could have served as limit? If my understanding is right, then can we get rid of terms "setpoint" and "origin". Would it be easier to understand the things if we just talk in terms of "goal" and "limit" and how these are derived from "thresh". thresh == soft limit limit == 4*thresh (hard limit) goal = thresh - thresh / DIRTY_SCOPE (where we want system to be in steady state). limit - dirty pos_ratio = -------------- limit - goal Thanks Vivek From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vivek Goyal Subject: Re: [PATCH 2/5] writeback: dirty position control Date: Wed, 10 Aug 2011 17:40:57 -0400 Message-ID: <20110810214057.GA6576@redhat.com> References: <20110806084447.388624428@intel.com> <20110806094526.733282037@intel.com> <1312811193.10488.33.camel@twins> <20110808141128.GA22080@localhost> <1312814501.10488.41.camel@twins> <20110808230535.GC7176@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Peter Zijlstra , "linux-fsdevel@vger.kernel.org" , Andrew Morton , Jan Kara , Christoph Hellwig , Dave Chinner , Greg Thelen , Minchan Kim , Andrea Righi , linux-mm , LKML To: Wu Fengguang Return-path: Content-Disposition: inline In-Reply-To: <20110808230535.GC7176@localhost> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Tue, Aug 09, 2011 at 07:05:35AM +0800, Wu Fengguang wrote: > On Mon, Aug 08, 2011 at 10:41:41PM +0800, Peter Zijlstra wrote: > > On Mon, 2011-08-08 at 22:11 +0800, Wu Fengguang wrote: > > > @@ -538,11 +538,6 @@ static unsigned long bdi_position_ratio( > > > goal = thresh - thresh / DIRTY_SCOPE; > > > origin = 4 * thresh; > > > > > > - if (unlikely(origin < limit && dirty > (goal + origin) / 2)) { > > > - origin = limit; /* auxiliary control line */ > > > - goal = (goal + origin) / 2; > > > - pos_ratio >>= 1; > > > - } > > > pos_ratio = origin - dirty; > > > pos_ratio <<= BANDWIDTH_CALC_SHIFT; > > > do_div(pos_ratio, origin - goal + 1); > > FYI I've updated the fix to the below one, so that @limit will be used > as the origin in the rare case of (4*thresh < dirty). > > --- linux-next.orig/mm/page-writeback.c 2011-08-08 21:56:11.000000000 +0800 > +++ linux-next/mm/page-writeback.c 2011-08-09 06:34:25.000000000 +0800 > @@ -536,13 +536,8 @@ static unsigned long bdi_position_ratio( > * global setpoint > */ > goal = thresh - thresh / DIRTY_SCOPE; > - origin = 4 * thresh; > + origin = max(4 * thresh, limit); Hi Fengguang, Ok, so just trying to understand this pos_ratio little better. You have following basic formula. origin - dirty pos_ratio = -------------- origin - goal Terminology is very confusing and following is my understanding. - setpoint == goal setpoint is the point where we would like our number of dirty pages to be and at this point pos_ratio = 1. For global dirty this number seems to be (thresh - thresh / DIRTY_SCOPE) - thresh dirty page threshold calculated from dirty_ratio (Certain percentage of total memory). - Origin (seems to be equivalent of limit) This seems to be the reference point/limit we don't want to cross and distance from this limit basically decides the pos_ratio. Closer we are to limit, lower the pos_ratio and further we are higher the pos_ratio. So threshold is just a number which helps us determine goal and limit. goal = thresh - thresh / DIRTY_SCOPE limit = 4*thresh So goal is where we want to be and we start throttling the task more as we move away goal and approach limit. We keep the limit high enough so that (origin-dirty) does not become negative entity. So we do expect to cross "thresh" otherwise thresh itself could have served as limit? If my understanding is right, then can we get rid of terms "setpoint" and "origin". Would it be easier to understand the things if we just talk in terms of "goal" and "limit" and how these are derived from "thresh". thresh == soft limit limit == 4*thresh (hard limit) goal = thresh - thresh / DIRTY_SCOPE (where we want system to be in steady state). limit - dirty pos_ratio = -------------- limit - goal Thanks Vivek -- 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