linux-kernel - Re: [patch 6/6] mm: bdi: allow setting a maximum for the bdi dirty limit

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Date:	Thu, 31 Jan 2008 10:46:50 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	akpm@...ux-foundation.org
CC:	miklos@...redi.hu, a.p.zijlstra@...llo.nl,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch 6/6] mm: bdi: allow setting a maximum for the bdi dirty
 limit

> On Tue, 29 Jan 2008 16:49:06 +0100
> Miklos Szeredi <miklos@...redi.hu> wrote:
> 
> > Add "max_ratio" to /sys/class/bdi.  This indicates the maximum
> > percentage of the global dirty threshold allocated to this bdi.
> 
> Maybe I'm having a stupid day, but I don't understand the semantics of this
> min and max at all.  I've read the code, and I've read the comments (well,
> I've hunted for some) and I've read the docs.
> 
> I really don't know how anyone could use this in its current state without
> doing a lot of code-reading and complex experimentation.  All of which
> would be unneeded if this tunable was properly documented.
> 
> So.  Please provide adequate documentation for this tunable.  I'd suggest
> that it be pitched at the level of a reasonably competent system operator. 
> It should help them understand why the tunable exists, why they might
> choose to alter it, and what effects they can expect to see.  Hopefully a
> reaonably competent kernel developer can then understand it too.

OK.  I think what's missing from some docs, is a high level
description of the per-bdi throttling algorithm, and how it affects
writeback.  Because with info, I think the min and max ratios are
trivially understandable: they just override the result of the
algorithm, in case it would mean too high or too low threshold.

Peter, could you write something about that?

Thanks,
Miklos
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