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

On Thu, Sep 27, 2012 at 5:17 AM, Nguyen Thai Ngoc Duy [off-list ref] wrote:

I'd like to see some sort of extension mechanism like in
$GIT_DIR/index, so that we don't have to increase pack index version
often. What I have in mind is optional commit cache to speed up
rev-list and merge, which could be stored in pack index too.

Can you share some of your ideas?

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.