git-annex/doc/todo/optimise_git-annex_merge.mdwn
Joey Hess 6d74830afd note
2011-12-26 14:41:20 -04:00

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Typically `git-annex merge` is fast, but it could still be sped up.
`git-annex merge` runs `git-hash-object` once per file that needs to be
merged. Elsewhere in git-annex, `git-hash-object` is used in a faster mode,
reading files from disk via `--stdin-paths`. But here, the data is not
in raw files on disk, and I doubt writing them is the best approach.
Instead, I'd like a way to stream multiple objects into git using stdin.
Sometime, should look at either extending git-hash-object to support that,
or possibly look at using git-fast-import instead.
---
`git-annex merge` also runs `git show` once per file that needs to be
merged. This could be reduced to a single call to `git-cat-file --batch`,
There is already a Git.CatFile library that can do this easily. --[[Joey]]
> This is now done, part above remains todo. --[[Joey]]
---
Merging used to use memory proportional to the size of the diff. It now
streams data, running in constant space. This probably sped it up a lot,
as there's much less allocation and GC action. --[[Joey]]
----
Another option is to stop doing the automatic merging. Once the git
tweak-fetch hook is widely available, the automatic mergcing won't be
needed when pulling from remotes. The only remaining use would be if
a sibling git-annex branch appeared in some other way, and git annex merge
could be manually run in such an unlikely case. Making this change
would require a hard dependency on an appropriate version of git, and a
version number bump to ensure the tweak-fetch hooks are set up. --[[Joey]]