linux-kernel - Re: O_DIRECT question

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Date:	Fri, 12 Jan 2007 10:12:41 +0800
From:	Aubrey <aubreylee@...il.com>
To:	"Roy Huang" <royhuang9@...il.com>
Cc:	"Nick Piggin" <nickpiggin@...oo.com.au>,
	"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,
	"Robin Getz" <rgetz@...ckfin.uclinux.org>
Subject: Re: O_DIRECT question

On 1/11/07, Roy Huang <royhuang9@...il.com> wrote:
> On a embedded systerm, limiting page cache can relieve memory
> fragmentation. There is a patch against 2.6.19, which limit every
> opened file page cache and total pagecache. When the limit reach, it
> will release the page cache overrun the limit.

The patch seems to work for me. But some suggestions in my mind:

1) Can we limit the total page cache, not the page cache per each file?
    think about if total memory is 128M, 10% of it is 12.8M, here if
one application is running, it can use 12.8M vfs cache, then the
performance will probably not be impacted. However, the current patch
limit the page cache per each file, which means if only one
application runs it can only use CONFIG_PAGE_LIMIT pages cache. It may
be small to the application.
------------------snip---------------
if (mapping->nrpages >= mapping->pages_limit)
               balance_cache(mapping);
------------------snip---------------

2) A percent number should be better to control the value. Can we add
a proc interface to make the value tunable?

Thanks,
-Aubrey
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