Re: q: faster way to integrate/merge lots of topic branches?

From: Ingo Molnar <mingo@elte.hu>
Date: Jul 23, 2008, 13:49 UTC
Message-ID: <20080723134926.GA12888@elte.hu>
In-Reply-To: <20080723131736.GA9100@elte.hu>

* Ingo Molnar <mingo@elte.hu> wrote:

> So i guess it's better to just create a separate 
> .git/refs/merge-cache/ hierarchy with timestamps of last merged 
> branches and their head sha1 ... but maybe i'm banging on open doors?

here's the git-fastmerge script i've whipped up in 10 minutes. It does the trick nicely for me:

first run:

  real    0m53.228s
  user    0m41.134s
  sys     0m11.405s

second run:

  real    0m2.751s
  user    0m1.280s
  sys     0m1.491s

or a 20x speedup. Yummie! :-)

It properly notices when i commit to a topic branch, and it maintains a proper matrix of <A> <- <B> merge timestamps. It even embedds the sha1's in the timestamp path so it should be quite complete. It should work fine across resets, re-merges, etc. too i think. It should work well with renamed branches as well i think. (although i dont do that all that often)

In fact even if i delete the whole .git/mergecache/ hierarchy and run a 'cold' merge, it's much faster:

  real    0m32.129s
  user    0m24.456s
  sys     0m7.603s

Because many of the branches have the same sha1 so it's already half-optimized even on the first run.

Much of the remaining 2.7 seconds overhead comes from the git-log runs to retrieve the sha1s, so i guess it could all be made even faster.

Now this scheme assumes that there's a sane underlying filesystem that can take these long pathnames and which has good timestamps (which i have, so it's not a worry for me).

Hm?

	Ingo

-----------------{ git-fastmerge }---------------------> #!/bin/bash

usage () {
  echo 'usage: git-fastmerge <refspec>..'
  exit -1
}

[ $# = 0 ] && usage

BRANCH=$1

MERGECACHE=.git/mergecache

[ ! -d $MERGECACHE ] && { mkdir $MERGECACHE || usage; }

HEAD_SHA1=$(git-log -1 --pretty=format:"%H") BRANCH_SHA1=$(git-log -1 --pretty=format:"%H" $BRANCH)

CACHE=$MERGECACHE/$HEAD_SHA1/$BRANCH_SHA1

[ -f "$CACHE" -a "$CACHE" -nt .git/refs/heads/$BRANCH_SHA1 ] && {
  echo "merge-cache hit on HEAD <= $1"
  exit 0
}

git-merge $1 && {
  mkdir -p $(dirname $CACHE)
  touch $CACHE
}