Re: pack operation is thrashing my server

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sat, 6 Sep 2008, Jon Smirl wrote:
> 
> Some alternative algorithms are here...
> http://cs.fit.edu/~mmahoney/compression
> It is possible to beat zlib by 2x at the cost of CPU time and memory.

Jon, you're missing the point.

The problem with zlib isn't that it doesn't compress well. It's that it's 
too _SLOW_.

> Turning a 500MB packfile into a 250MB has lots of advantages in IO
> reduction so it is worth some CPU/memory to create it.

..and secondly, there's no way you'll find a compressor that comes even 
close to being twice as good. 10% better yes - but then generally much 
MUCH slower.

Take a look at that web page you quote, and then sort things by 
decompression speed. THAT is the issue. 

And no, LZO isn't even on that list. I haven't tested it, but looking at 
the code, I do think LZO can be fast exactly because it seems to be 
byte-based rather than bit-based, so I'd not be surprised if the claims 
for its uncompression speed are true.

The constant bit-shifting/masking/extraction kills zlib performance (and 
please realize that zlib is at the TOP of the list when looking at the 
thing you pointed to - that silly site seems to not care about compressor 
speed at all, _only_ about size). So "kills" is a relative measure, but 
really - we're looking for _faster_ algorithms, not slower ones!

			Linus