linux-kernel - Re: O_DIRECT question

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Message-ID: <6d6a94c50701102354l7ab41a3bp4761566204f1d992@mail.gmail.com>
Date:	Thu, 11 Jan 2007 15:54:05 +0800
From:	Aubrey <aubreylee@...il.com>
To:	"Nick Piggin" <nickpiggin@...oo.com.au>
Cc:	"Andrew Morton" <akpm@...l.org>,
	"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 1/11/07, Nick Piggin <nickpiggin@...oo.com.au> wrote:
> Andrew Morton wrote:
> > 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.
>
> OTOH, the antifragmentation stuff can also break down eventually,
> especially if higher order allocations are actually in common use.

That's right. When VFS cache eat up almost all of the memory, I think
no memory algorithm can help the case, including Mei's anti-fragment
patch.

>
> What you _really_ want to do is avoid large mallocs after boot, or use
> a CPU with an mmu. I don't think nommu linux was ever intended to be a
> simple drop in replacement for a normal unix kernel.

Is there a position available working on mmu CPU? Joking, :)
Yes, some problems are serious on nommu linux. But I think we should
try to fix them not avoid them.

-Aubrey
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