Re: [patch 6/6] mm: bdi: allow setting a maximum for the bdi dirtylimit

From: Miklos Szeredi
Date: Thu Jan 31 2008 - 04:47:23 EST


> On Tue, 29 Jan 2008 16:49:06 +0100
> Miklos Szeredi <miklos@xxxxxxxxxx> 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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