Re: [RFH] git cherry-pick takes forever

[Junio C Hamano <gitster@pobox.com>]

Michal Vitecek <fuf@mageo.cz> writes:

>  Hello everyone,
>
>  I have two git repositories: one is the origin of the other. However no
>  merging is being done as the projects in the repositories quite differ
>  but still use the same core. So to propagate changes I cherry-pick
>  those which are useful from one repository to another.
>
>  however 'git cherry-pick' has lately started to last almost forever:

Can you define "lately"?  Is it a function of your git version, or is it a
function of the age of your repositories?

>  $ time git cherry-pick b42b77e66a83f1298d9900a9bb1078b9b42e8618
>  Finished one cherry-pick.
>  Created commit 7caef83: - removed some superfluous newlines
>  2 files changed, 0 insertions(+), 2 deletions(-)
>  git cherry-pick b42b77e66a83f1298d9900a9bb1078b9b42e8618  282.97s user 34.69s system 100% cpu 5:17.63 total
>
>  Both repositories have approximately 16k commits and their forking
>  point (merge base) is 250 to 490 commits far away.

When talking about cherry-pick, the size of the history (unless the
repository has too many objects and badly packed) does not matter; the
operation is purely about your current state, the cherry-picked commit
itself, and the parent commit of the cherry-picked one.

Taking 5 minutes to cherry-pick a change to only two paths, one line
deletion each, is plain ridiculous, but if the tree state of cherry-picked
commit and the tree state of the target is vastly different (e.g. almost
no common pathnames), the behaviour is certainly understandable.  Ancient
git used straight three-way merge for cherry-pick, but recent ones use
more expensive "recursive-merge", which tries to detect renames.  If the
states of trees are very dissimilar, you can end up wasting a lot of time.

    $ H=$(git rev-parse 7caef83^) ;# the commit before cherry-pick
    $ C=b42b77e6 ;# the cherry-picked one

cherry-pick operation roughly runs these two diffs:

    $ time git diff --shortstat -M $H $C
    $ time git diff --shortstat -M $H $C^1

and uses the result to perform its work.  Can you clock these?

If you rarely have renames, it may be much more efficient to run "git
format-patch -1 --stdout $C | git am -3" instead of cherry-pick.