Re: [PATCH WIP] sha1-lookup: make selection of 'middle' less aggressive

[Jeff King <peff@peff.net>]

On Mon, Dec 31, 2007 at 12:37:36PM -0800, Linus Torvalds wrote:

> And quite honestly I'm not really even sure that the performance downside 
> is entirely about zlib itself: I suspect a lot of the reason zlib shows up 
> in the profiles is that the source data is usually cold in the cache, so 
> it probably takes a lot of cache misses (it also will take all the page 
> faults!).

zlib makes a noticeable impact in real world cases. On a git.git repo,
fully packed with stock config, warm cache:

  $ /usr/bin/time git whatchanged >/dev/null
  4.12user 0.37system 0:04.50elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
  0inputs+0outputs (0major+6452minor)pagefaults 0swaps

  $ git config pack.compression 0
  $ git repack -a -d -f
  $ /usr/bin/time git whatchanged >/dev/null
  2.93user 0.43system 0:03.36elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
  0inputs+0outputs (0major+8501minor)pagefaults 0swaps

More pagefaults, but a 25% improvement in wall clock time.  The packfile
is noticeably larger (55M versus 40M), so I'm sure the cold cache case
sucks. It may also change with larger repos, where the packfile size
difference kills your cache.

-Peff