linux-kernel - Re: O_DIRECT question

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Message-Id: <20070110225720.7a46e702.akpm@osdl.org>
Date:	Wed, 10 Jan 2007 22:57:20 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Aubrey <aubreylee@...il.com>
Cc:	"Linus Torvalds" <torvalds@...l.org>,
	"Hua Zhong" <hzhong@...il.com>, "Hugh Dickins" <hugh@...itas.com>,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	kenneth.w.chen@...el.com, mjt@....msk.ru
Subject: Re: O_DIRECT question

On Thu, 11 Jan 2007 14:45:12 +0800
Aubrey <aubreylee@...il.com> wrote:

> >
> > In the interim you could do the old "echo 3 > /proc/sys/vm/drop_caches"
> > thing, but that's terribly crude - drop_caches is really only for debugging
> > and benchmarking.
> >
> Yes. This method can drop caches, but will fragment memory.

That's what page reclaim will do as well.

What you want is Mel's antifragmentation work, or lumpy reclaim.

> This is
> not what I want. I want cache is limited to a tunable value of the
> whole memory. For example, if total memory is 128M, is there a way to
> trigger reclaim when cache size > 16M?

If there was, it'd "fragment memory" as well.

You might get a little benefit from increasing /proc/sys/vm/min_free_kbytes,
but not much.  Some page allocation tweaks would aid that.

But basically, to do this well, serious work is needed.

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