Re: [RFC] Breaking data compatibility with userspace bzlib

[Jeff Garzik]

On Fri, Jun 20, 2003 at 08:59:15PM +0200, J?rn Engel wrote:
> Now, the cost of the underlying BWT is O(n) in memory and O(n*ln(n))
> in time.  That given, I consider it odd to use a linear semantic of
> blockSizeXXXX and would prefer an exponential one, as the zlib uses
> here and there.  Thus blockSizeBits would now give the blockSize as
> 1 << blockSizeBits, allowing to go well below 100k, resulting in lower
> memory consumption for some and well above 900k, giving better
> compression ratios.
> 
> 
> Long intro, short question: Jay O Nay?

The big question is whether the bzip2 better compression is actually
useful in a kernel context?  Patches to do bzip2 for initrd, for
example, have been around for ages:

	http://gtf.org/garzik/kernel/files/initrd-bzip2-2.2.13-2.patch.gz

But the compression and decompression overhead is _much_ larger
than gzip.  It was so huge for maximal compression that dialing back
compression reaching a point of diminishing returns rather quickly,
when compared to gzip memory usage and compression.

I talked a bit with the bzip2 author a while ago about memory usage.
He eventually added the capability to only require small blocks
for decompression (64K IIRC?), but there was a significant loss in
compression factor.

So... even in 2003, I really don't know of many (any?) tasks which
would benefit from bzip2, considering the additional memory and
cpu overhead.

	Jeff