linux-kernel - Re: [PATCH] sysctl: vfs_cache

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Message-Id: <20070321161140.457a3cd7.akpm@linux-foundation.org>
Date:	Wed, 21 Mar 2007 16:11:40 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Randy Dunlap <randy.dunlap@...cle.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, "J.H." <warthog9@...nel.org>,
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sysctl: vfs_cache_divisor

On Wed, 21 Mar 2007 16:01:32 -0700
Randy Dunlap <randy.dunlap@...cle.com> wrote:

> On Mon, 19 Mar 2007 21:22:33 -0700 H. Peter Anvin wrote:
> 
> > Randy Dunlap wrote:
> > >>
> > >> The we duplicate all the relevant /proc knobs:
> > >>
> > >> cat /proc/sys/vm/dirty_ratio
> > >> 30
> > >> cat /proc/sys/vm/hires-dirty_ratio/
> > >> 300000
> > >>
> > >> Or we do something else ;)
> > > 
> > > Sounds better.  I wasn't very keen on the userspace interface that this
> > > exposed.  Will look at those.
> > > 
> > 
> > Okay... may be I could throw a spanner in the machinery, and suggest 
> > another option: perhaps we should add a way to do sysctl which can 
> > handle fractional (fixed-point) values... more coherent/detailed message 
> > tomorrow.
> 
> I prefer the fixed-point values for pressure and dirty* to having
> duplicated entries for each of them.  I'll proceed with that idea.
> 

Problem is, if a read of /proc/sys/vm/dirty_ratio is changed to return
7.457 then existing userspace might get confused.

This might be acceptable if we are careful to ensure that reads of
/proc/sys/vm/dirty_ratio will always return an integer if it was previously
initialised with an integer.
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