On 9/6/08, Linus Torvalds  wrote:

quoted text
>

>

>  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_.

When I was playing with those giant Mozilla packs speed of zlib wasn't

a big problem. Number one problem was the repack process exceeding 3GB

which forced me to get 64b hardware and 8GB of memory. If you start

swapping in a repack, kill it, it will probably take a month to

finish.
I'm forgetting the numbers now but on a quad core machine (with git

changes to use all cores) and 8GB I believe I was able to repack the

Mozilla repo in under an hour. At that point I believe I was being

limited by disk IO.
Size and speed are not unrelated. Buy reducing the pack size in half

you reduce the IO and memory demands (cache misses) a lot. For example

if we went to no compression we'd be killed by memory and IO

consumption. It's not obvious to me what's the best trade off for git

without trying several compression algorithms and comparing. They were

feeding 100MB into PAQ on that site, I don't know what PAQ would do

with a bunch of 2K objects.
Most delta chains in the Mozilla data were easy to process. There was

a single 2000 delta chain that consumed 15% of the total CPU time to

process. Something causes performance to fall apart on really long

chains.
>  > Turning a 500MB packfile into a 250MB has lots of advantages in IO

quoted text
>  > 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

>

--

Jon Smirl

jonsmirl@gmail.com

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