From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Jared Hulbert" <jaredeh@gmail.com>
Subject: Re: [PATCH 04/10] AXFS: axfs_inode.c
Date: Thu, 21 Aug 2008 20:27:50 -0700
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References: <48AD00F0.5030403@gmail.com> <48AE0697.5040706@lougher.demon.co.uk>
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To: Phillip Lougher <phillip@lougher.demon.co.uk>
Cc: Linux-kernel@vger.kernel.org, linux-embedded@vger.kernel.org, linux-mtd <linux-mtd@lists.infradead.org>, =?UTF-8?Q?J=C3=B6rn_Engel?= <joern@logfs.org>, tim.bird@am.sony.com, cotte@de.ibm.com, nickpiggin@yahoo.com.au

> 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.