Re: Significant performance waste in git-svn and friends

Mike Hommey [off-list ref] wrote:

Hi,

Hi Mike,

Being a pervert abusing the way subversion doesn't deal with branches
and tags, I'm actually not a user of git-svn or git-svnimport, because
they just can't deal easily with my perversion. So I'm writing a script
to do the conversion for me, and since I also like to learn new things
when I'm coding, I'm writing it in ruby.

Anyways, one of the things I'm trying to convert is my svk repository
for debian packaging of xulrunner (so, a significant subset of the
mozilla tree), which doesn't involve a lot of revisions (around 280,
because I only imported releases or CVS snapshots), but involves a lot
of files (roughly 20k).

The first thing I noticed when twisting around the svk repo so that
git-svn could somehow import it a while ago, is that running git-svn
was in my case significantly slower than svnadmin dump | svnadmin load
(more than 2 times slower).

And now, with my own script, I got the same kind of "slowdown". So I
investigated it, and it didn't take long to realize that replacing
git-hash-object by a simple reimplementation in ruby was *way* faster.
git-hash-object being more than probably what you do the most when you
import a remote repository, it is not much of a surprise that forking
thousands of times is a huge performance waste.

I haven't looked at the times in a while, but I suspect that exec()
is the (much bigger) culprit.

Since I usually import off remote repositories, so I notice network
latency way before I notice local performance problems with git-svn.

So, just for the record, I did a lame hack of git-svn to see what kind
of speedup could happen in git-svn. You can find this lame hack as a
patch below. I did some tests (with a 1.5.2.1 release) and here are the
results, importing only the trunk (192 revisions), with no checkout, and
redirecting stdout to /dev/null:

original git-svn:
real    25m1.871s
user    8m51.593s
sys     12m31.659s

patched git-svn:
real    14m45.870s
user    7m31.928s
sys     4m1.047s

That's awesome.

- It might be worth testing if git-cat-file is called a lot. If so,
  implementing a simple git-cat-file equivalent that would work for
  unpacked objects could improve speed.

IIRC git-cat-file is called a lot.  Every modified file needs the
original cat-ed to make use of the delta.

The same things obviously apply to git-cvsimport and other scripts
calling git-hash-object a lot.

Making git-svn use fast-import would be very nice.  I've got a bunch
of other git-svn things that I need to work on, but having git-svn
converted to use fast-import would be nice.  Or allowing Git.pm
to access more of the git internals...

However, how well/poorly would fast-import work for incremental
fetches throughout the day?

-- 
Eric Wong