linux-kernel - Re: [PATCH 0/8] readahead stats/tracing, backwards prefetching and more

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Message-ID: <20111121095638.GA5084@infradead.org>
Date:	Mon, 21 Nov 2011 04:56:38 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 0/8] readahead stats/tracing, backwards prefetching and
 more

On Mon, Nov 21, 2011 at 05:18:19PM +0800, Wu Fengguang wrote:
> Andrew,
> 
> I'm getting around to pick up the readahead works again :-)
> 
> This first series is mainly to add some debug facilities, to support the long
> missed backwards prefetching capability, and some old patches that somehow get
> delayed (shame me).
> 
> The next step would be to better handle the readahead thrashing situations.
> That would require rewriting part of the algorithms, this is why I'd like to
> keep the backwards prefetching simple and stupid for now.
> 
> When (almost) free of readahead thrashing, we'll be in a good position to lift
> the default readahead size. Which I suspect would be the single most efficient
> way to improve performance for the large volumes of casually maintained Linux
> file servers.

Btw, if you work actively in that area I have a todo list item I was
planning to look into sooner or later:  instead of embedding the ra
state into the struct file allocate it dynamically.  That way files that
either don't use the pagecache, or aren't read from won't need have to
pay the price for increasing struct file size, and if we have to we
could enlarge it more easily.  Besides removing f_version in the common
struct file and also allocting f_owner separately that seem to be the
easiest ways to get struct file size down.

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