[RFC] commit-reach: skip STALE drain when only one merge-base needed

[spkrka]

Context for what this is all about.

I am working with a very large git monorepo and have been investigating performance issue. After some digging I ended up looking more deeply into git merge-base. I saw it had an --all parameter but the default is to only return a single merge-base. Looking through the code and adding debug timing, I realized that although the total time to compute the merge-base was high, a very small amount of time was spent finding the initial merge-base value that was later returned.

The optimization is actually quite dramatic in a large repo - runtime went down from 5000ms to 50ms, so it's roughly a 100x optimization. This comes from an exploding frontier of STALE commits to drain.

Thus, my idea is simply to return early from the function once we know what will be returned. This only works if we find a candidate that we know will not be pruned later - but fortunately if we have a commit graph with generations we will visit commits in order such that it will actually not be pruned.

CC: Derrick Stolee stolee@gmail.com

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[gitgitgadget]

There is an issue in commit a43f270:
commit-reach: skip STALE drain in paint_down_to_common when only one merge-base needed

• First line of commit message is too long (> 76 columns)

[gitgitgadget]

There is an issue in commit 09c61b0:
commit-reach: skip STALE drain in paint_down_to_common when only one merge-base needed

• First line of commit message is too long (> 76 columns)

[derrickstolee]

Meta comments:

• You can use your PR description to introduce yourself as a new contributor. It will not impact the commit message that is stored in the final repository, but helps folks get to know you.
• I like your custom tests. I'd be interested to know if any performance tests in t/perf/ would help. You can cd t/perf/ && ./run HEAD~1 HEAD -- p<num>-<name>.sh. However: I checked the existing tests and I don't think that any will actually show meaningful results in the way your manual test does.

[derrickstolee]

/allow

[gitgitgadget]

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[spkrka]

/preview

[gitgitgadget]

Preview email sent as pull.2109.git.1778252509243.gitgitgadget@gmail.com

[spkrka]

/submit

[gitgitgadget]

Submitted as pull.2109.git.1778252837132.gitgitgadget@gmail.com

To fetch this version into FETCH_HEAD:

git fetch https://github.com/gitgitgadget/git/ pr-2109/spkrka/merge-base-early-exit-v1

To fetch this version to local tag pr-2109/spkrka/merge-base-early-exit-v1:

git fetch --no-tags https://github.com/gitgitgadget/git/ tag pr-2109/spkrka/merge-base-early-exit-v1

[gitgitgadget]

This patch series was integrated into seen via git@0b5178e.

[gitgitgadget]

This patch series was integrated into seen via git@b7cd8e6.