linux-kernel - Re: [PATCH 04/10] AXFS: axfs

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Message-ID: <6934efce0808212027q412c4cbbp6ea8673a7d3bc1b9@mail.gmail.com>
Date:	Thu, 21 Aug 2008 20:27:50 -0700
From:	"Jared Hulbert" <jaredeh@...il.com>
To:	"Phillip Lougher" <phillip@...gher.demon.co.uk>
Cc:	Linux-kernel@...r.kernel.org, linux-embedded@...r.kernel.org,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	"Jörn Engel" <joern@...fs.org>,
	tim.bird@...sony.com, cotte@...ibm.com, nickpiggin@...oo.com.au
Subject: Re: [PATCH 04/10] AXFS: axfs_inode.c

> I assume compressed blocks can be larger than PAGE_CACHE_SIZE?  This suffers
> from the rather obvious inefficiency that you decompress a big block >
> PAGE_CACHE_SIZE, but only copy one PAGE_CACHE_SIZE page out of it.  If
> multiple files are being read simultaneously (a common occurrence), then
> each is going to replace your one cached uncompressed block
> (sbi->current_cnode_index), leading to decompressing the same blocks over
> and over again on sequential file access.
>
> readpage file A, index 1 -> decompress block X
> readpage file B, index 1 -> decompress block Y (replaces X)
> readpage file A, index 2 -> repeated decompress of block X (replaces Y)
> readpage file B, index 2 -> repeated decompress of block Y (replaces X)
>
> and so on.

Yep.  Been thinking about optimizing it.  So far it hasn't been an
issue for my customers.  Most fs traffic being on the XIP pages.  Once
I get a good automated performance test up we'll probably look into
something to improve this.
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