Re: Commit cache to speed up rev-list and merge

On Thu, Sep 27, 2012 at 10:51 PM, Shawn Pearce [off-list ref] wrote:

In Linus' Linux kernel tree there are currently about 323,178 commits.
If we store just the pre-parsed commit time as an int32 field this is
an additional 1.2 MiB of data in the pack-*.idx file, assuming we can
use additional data like pack offset position to correlate commit to
the parsed int. If we stored parent pointers in a similar way you
probably need at least 3.6 MiB of additional disk space on the index.
For example, use 12 bytes for each commit to store enough of the
parsed commit time to sort commits, and up to 2 parent pointers per
commit.... with a reserved magic value for octopus merges to mean the
commit itself has to be parsed to get the graph structure correct.

This is much better than my naive approach (storing sha-1 and
timestamps). We could use less space by storing parent pointer of
non-merge commits only. Merge commits linux-2.6 is 6% the number of
commits. git.git has higher percentage, 21%. I bet many projects do
not merge as much and the number of merge commits is less than 5%.
-- 
Duy