linux-kernel - Re: [PATCH 07/13] writeback: explicit low bound for vm.dirty

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Message-ID: <20100810135712.0eb34759@notabene>
Date:	Tue, 10 Aug 2010 13:57:12 +1000
From:	Neil Brown <neilb@...e.de>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Mel Gorman <mel@....ul.ie>,
	Chris Mason <chris.mason@...cle.com>,
	Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 07/13] writeback: explicit low bound for vm.dirty_ratio

On Tue, 10 Aug 2010 12:12:06 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> > Subject: writeback: explicit low bound for vm.dirty_ratio
> > From: Wu Fengguang <fengguang.wu@...el.com>
> > 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.

NeilBrown

> personally I prefer to
>  - accept all value, or
>  - clipping value in dirty_ratio_handler 
> 
> Both don't have explicit ABI change.
> 
> Thanks.
> 
> 
> 
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