From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with ESMTP id A709E6B0071 for ; Wed, 24 Nov 2010 06:18:38 -0500 (EST) Subject: Re: [PATCH 10/13] writeback: make reasonable gap between the dirty/background thresholds From: Peter Zijlstra In-Reply-To: <20101117042850.482907860@intel.com> References: <20101117042720.033773013@intel.com> <20101117042850.482907860@intel.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Date: Wed, 24 Nov 2010 12:18:18 +0100 Message-ID: <1290597498.2072.458.camel@laptop> Mime-Version: 1.0 Sender: owner-linux-mm@kvack.org To: Wu Fengguang Cc: Andrew Morton , Jan Kara , Christoph Hellwig , Dave Chinner , Theodore Ts'o , Chris Mason , Mel Gorman , Rik van Riel , KOSAKI Motohiro , linux-mm , linux-fsdevel@vger.kernel.org, LKML List-ID: On Wed, 2010-11-17 at 12:27 +0800, Wu Fengguang wrote: > plain text document attachment > (writeback-fix-oversize-background-thresh.patch) > The change is virtually a no-op for the majority users that use the > default 10/20 background/dirty ratios. For others don't know why they > are setting background ratio close enough to dirty ratio. Someone must > set background ratio equal to dirty ratio, but no one seems to notice or > complain that it's then silently halved under the hood.. >=20 > Signed-off-by: Wu Fengguang > --- > mm/page-writeback.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) >=20 > --- linux-next.orig/mm/page-writeback.c 2010-11-15 13:12:50.000000000 +08= 00 > +++ linux-next/mm/page-writeback.c 2010-11-15 13:13:42.000000000 +0800 > @@ -403,8 +403,15 @@ void global_dirty_limits(unsigned long * > else > background =3D (dirty_background_ratio * available_memory) / 100; > =20 > - if (background >=3D dirty) > - background =3D dirty / 2; > + /* > + * Ensure at least 1/4 gap between background and dirty thresholds, so > + * that when dirty throttling starts at (background + dirty)/2, it's at > + * the entrance of bdi soft throttle threshold, so as to avoid being > + * hard throttled. > + */ > + if (background > dirty - dirty * 2 / BDI_SOFT_DIRTY_LIMIT) > + background =3D dirty - dirty * 2 / BDI_SOFT_DIRTY_LIMIT; > + > tsk =3D current; > if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) { > background +=3D background / 4; Hrm,.. the alternative is to return -ERANGE or somesuch when people try to write nonsensical values. I'm not sure what's best, guessing at what the user did mean to do or forcing him to actually think. -- 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 policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: email@kvack.org