On 2008.08.14 17:06:13 -0700, Linus Torvalds wrote:
quoted text
> The "hash" we use for looking things up is also pretty much a joke, and it 
> has no overflow capability, it just replaces the old entry with a new one.

So I added some stupid tracing to cache_or_unpack entry to see how often
we reread the same stuff. The whole thing just logs the base_offset in
case of a cache miss. I've gc'ed my linux-2.6.git before the run, so
that there's only a single packed_git around (at least I hope so), and I
can ignore that for the tracing.

The whole log for a "git rev-list --objects HEAD" has about 1.2M
entries, while the output of the rev-list command has about 870k lines.
Some postprocessing of the trace shows that the majority of objects is
read only once or twice. A few percent are read three to ten times, and
some are read more than two hundred times.

I'll attach the post-processed thing. The format is:
 x y

Meaning that there were x base_offset values for which we had y cache
misses.

Bj