linux-kernel - Re: O_DIRECT question

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Message-ID: <20070116034113.GB27177@lazybastard.org>
Date:	Tue, 16 Jan 2007 03:41:14 +0000
From:	Jörn Engel <joern@...ybastard.org>
To:	Aubrey <aubreylee@...il.com>
Cc:	Linus Torvalds <torvalds@...l.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Hua Zhong <hzhong@...il.com>, Hugh Dickins <hugh@...itas.com>,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	kenneth.w.chen@...el.com, akpm@...l.org, mjt@....msk.ru,
	Carsten Otte <cotte@...enet.de>
Subject: Re: O_DIRECT question

On Fri, 12 January 2007 00:19:45 +0800, Aubrey wrote:
>
> Yes for desktop, server, but maybe not for embedded system, specially
> for no-mmu linux. In many embedded system cases, the whole system is
> running in the ram, including file system. So it's not necessary using
> page cache anymore. Page cache can't improve performance on these
> cases, but only fragment memory.

You were not very specific, so I have to guess that you're referring to
the problem of having two copies of the same file in RAM - one in the
page cache and one in the "backing store", which is just RAM.

There are two solutions to this problem.  One is tmpfs, which doesn't
use a backing store and keeps all data in the page cache.  The other is
xip, which doesn't use the page cache and goes directly to backing
store.  Unlike O_DIRECT, xip only works with a RAM or de-facto RAM
backing store (NOR flash works read-only).

So if you really care about memory waste in embedded systems, you should
have a look at mm/filemap_xip.c and continue Carsten Otte's work.

Jörn

-- 
Fantasy is more important than knowledge. Knowledge is limited,
while fantasy embraces the whole world.
-- Albert Einstein
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