Speed up `git log --graph` 18x times

Published 2020-04-07

For large repositories 'git log --graph' can be slow, but for git v2.20+ it can be sped up.

$ time git lg
git lg  13.34s user 0.87s system 84% cpu 16.845 total

# True by default as of git v2.24
git config --global core.commitGraph true
git config --global gc.writeCommitGraph true

# Added in git v2.29, pre-compute file paths, so that git log commands
# that are scoped to files also benefit from this cache.
git commit-graph write --reachable --changed-paths

$ time git lg
git lg  0.72s user 0.14s system 74% cpu 1.154 total

This is a speed up of ~18x, compared to the older versions.

The way this works is that the commit-graph file stores the commit graph structure along with some extra metadata to speed up graph in the .git/objects/info directory.