From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 039356B0302 for ; Tue, 10 Aug 2010 09:30:12 -0400 (EDT) Date: Tue, 10 Aug 2010 15:29:18 +0200 From: Jan Kara Subject: Re: [PATCH 07/13] writeback: explicit low bound for vm.dirty_ratio Message-ID: <20100810132918.GA3351@quack.suse.cz> References: <20100805163401.e9754032.akpm@linux-foundation.org> <20100806124452.GC4717@localhost> <20100809235652.7113.A69D9226@jp.fujitsu.com> <20100810135712.0eb34759@notabene> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100810135712.0eb34759@notabene> Sender: owner-linux-mm@kvack.org To: Neil Brown Cc: KOSAKI Motohiro , Wu Fengguang , Andrew Morton , LKML , Peter Zijlstra , Dave Chinner , Christoph Hellwig , Mel Gorman , Chris Mason , Jens Axboe , Jan Kara , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" List-ID: On Tue 10-08-10 13:57:12, Neil Brown wrote: > On Tue, 10 Aug 2010 12:12:06 +0900 (JST) > KOSAKI Motohiro wrote: > > > > Subject: writeback: explicit low bound for vm.dirty_ratio > > > From: Wu Fengguang > > > Date: Thu Jul 15 10:28:57 CST 2010 > > > > > > Force a user visible low bound of 5% for the vm.dirty_ratio interface. > > > > > > This is an interface change. When doing > > > > > > echo N > /proc/sys/vm/dirty_ratio > > > > > > where N < 5, the old behavior is pretend to accept the value, while > > > the new behavior is to reject it explicitly with -EINVAL. This will > > > possibly break user space if they checks the return value. > > > > Umm.. I dislike this change. Is there any good reason to refuse explicit > > admin's will? Why 1-4% is so bad? Internal clipping can be changed later > > but explicit error behavior is hard to change later. > > As a data-point, I had a situation a while back where I needed a value below > 1 to get desired behaviour. The system had lots of RAM and fairly slow > write-back (over NFS) so a 'sync' could take minutes. > > So I would much prefer allowing not only 1-4, but also fraction values!!! > > I can see no justification at all for setting a lower bound of 5. Even zero > can be useful - for testing purposes mostly. If you run on a recent kernel, /proc/sys/vm/dirty_background_bytes and dirty_bytes is what was introduced exactly for these purposes. Not that I would think that magic clipping at 5% is a good thing... 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/ . Don't email: email@kvack.org