linux-kernel - Re: [PATCH 1/2] Squashfs: add XZ compression support

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Date:	Sat, 11 Dec 2010 00:49:18 +0200
From:	Lasse Collin <lasse.collin@...aani.org>
To:	Phillip Lougher <phillip@...gher.demon.co.uk>
Cc:	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	Linux Embedded Maillist <linux-embedded@...r.kernel.org>
Subject: Re: [PATCH 1/2] Squashfs: add XZ compression support

On 2010-12-09 Phillip Lougher wrote:
> +	stream->state = xz_dec_init(XZ_PREALLOC, block_size);

This sets the preallocated LZMA2 dictionary size to Squashfs block size, 
which allows the best compression ratio for the given block size (bigger 
values would be waste of memory). XZ decompressor needs about 30 KiB of 
memory plus the dictionary. So with 1 MiB Squashfs block size a little 
over 1 MiB of memory is needed by the XZ decompressor.

Would it be useful to support smaller dictionaries without reducing the 
Squashfs block size? It would save RAM while increasing the Squashfs 
image size less than if also the block size was reduced. Reasonable 
dictionary sizes could be e.g. 75 %, 50 %, or 25 % of the Squashfs block 
size. The size increase of the Squashfs image will vary a lot depending 
on the files in it, but e.g. dict_size = block_size / 2 won't 
necessarily increase the image size more than 1 %.

I don't know if this kind of RAM savings matter on embedded systems. I 
hope someone else can comment. My point in this email is only to let 
others know that reducing the RAM usage is possible without reducing the 
Squashfs block size.

-- 
Lasse Collin  |  IRC: Larhzu @ IRCnet & Freenode
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