On Tue, Jan 29, 2013 at 4:14 PM, Jeff King <p...@peff.net> wrote:
> The timings from this one are roughly similar to what I posted earlier.
> Unlike the earlier version, this one keeps the data for a single commit
> together for better cache locality (though I don't think it made a big
> difference in my tests, since my cold-cache timing test ends up touching
> every commit anyway).  The short of it is that for an extra 31M of disk
> space (~4%), I get a warm-cache speedup for "git rev-list --all" of
> ~4.2s to ~0.66s.

Some data point on caching 1-parent vs 2-parent commits on webkit
repo, 26k commits. With your changes (caching 2-parent commits), the
.commits file takes 2241600 bytes. "rev-list --all --quiet":

0.06user 0.00system 0:00.08elapsed 95%CPU (0avgtext+0avgdata 26288maxresident)k
0inputs+0outputs (0major+2094minor)pagefaults 0swaps

With caching 1-parent commits only, the .commits file takes 1707900
bytes (30% less), the same rev-list command:

0.07user 0.00system 0:00.07elapsed 96%CPU (0avgtext+0avgdata 24144maxresident)k
0inputs+0outputs (0major+1960minor)pagefaults 0swaps

Compared to the timing without caching at all:

0.72user 0.02system 0:00.76elapsed 98%CPU (0avgtext+0avgdata 108976maxresident)k
0inputs+0outputs (0major+7272minor)pagefaults 0swaps

The performance loss in 1-parent case is not significant while disk
saving is (although it'll be less impressive after you do Shawn's
suggestion not storing SHA-1 directly)
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