Performance issues when merging branches (1000+ files being merged takes 75~ seconds)

[Matt Mackall]

On Thu, 2010-06-17 at 00:46 +0300, Idan K wrote:
> Hi,
> 
> As part of choosing a SCM, the usual debate between Mercurial and Git
> has begun in our work place, we decided to do some benchmarks. One of
> the tests was this:
> 
> 1. Clone Mercurial's repository (http://hg.intevation.org/mercurial/),
> updated to default.
> 2. Create a named branch "branch-a".
> 3. For every file in the repository, insert a new line after line #3
> (if the file contains 3 lines), containing "### ADDED TEXT ###".
> 4. Commit.
> 5. Update back to default.
> 6. Create a named branch "branch-b".
> 7. Repeat step 3, but for line #9.
> 8. Commit.
> 9. Merge "branch-a" to "branch-b".
> 
> I ran this test on a Ubuntu 10.04 with Mercurial 1.5.4 and the result
> was quite surprising to say the least: Step #9 took about 75 seconds
> (average of 5 runs) on a Q6600 with 4GB RAM, ext4 drive.
> I also ran the same test with Git (using fast-export to covert
> Mercurial's repository to Git) to get an idea of what I should be
> getting, and this time step #9 took an average of 1 second. That's
> quite a difference. I know Mercurial is written in Python (with some C
> modules) and Git in C, but can the gap be that big?

Oh, probably. This is the first time the performance of file-level
merging has been raised as a performance issue in our history, so it's
not exactly been something we've put time into optimizing.

Your profile didn't show anything interesting, I'm afraid - all the
numbers are way too small to account for the slowness. Can you send the
result of using time(1) too? Perhaps there's a lot of extra I/O
happening.

-- 
Mathematics is the supreme nostalgia of our time.