Significant performance waste in git-svn and friends
- From
Mike Hommey <mh@glandium.org>
- Date
- Sep 5, 2007, 18:47 UTC
- Message-ID
- <20070905184710.GA3632@glandium.org>
Hi,
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.
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
Some notes about the patch: - I've not looked at the rest of the code to see if there was a way to get the size of the file so that SHA-1 sum and compression could be done in one pass and without copying the whole file in memory. - The object creation in the .git/objects directory is not as safe as what git-hash-object does.
Some notes about git-svn: - A few lines above the patched zone, the file is already read once to do the MD5 sum. It should be possible to do SHA-1, MD5 sums and deflate in just one pass. - 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.
The same things obviously apply to git-cvsimport and other scripts calling git-hash-object a lot.
Mike
diff --git a/git-svn.perl b/git-svn.perl index d3c8cd0..202c228 100755 --- a/git-svn.perl +++ b/git-svn.perl @@ -2417,6 +2417,8 @@ use warnings; use Carp qw/croak/; use IO::File qw//; use Digest::MD5; +use Digest::SHA1; +use Compress::Zlib; # file baton members: path, mode_a, mode_b, pool, fh, blob, base sub new { @@ -2603,15 +2605,26 @@ sub close_file { $buf eq 'link ' or die "$path has mode 120000", "but is not a link\n"; } - defined(my $pid = open my $out,'-|') or die "Can't fork: $!\n"; - if (!$pid) { - open STDIN, '<&', $fh or croak $!; - exec qw/git-hash-object -w --stdin/ or croak $!; + my $size = 0; + my $buf = ""; + while (my $read = sysread $fh, my $tmp, 4096) { + $size += $read; + $buf .= $tmp; } - chomp($hash = do { local $/; <$out> }); - close $out or croak $!; + my $sha1 = Digest::SHA1->new; + $sha1->add("blob $size\0"); + $sha1->add($buf); + $hash = $sha1->hexdigest; close $fh or croak $!; $hash =~ /^[a-f\d]{40}$/ or die "not a sha1: $hash\n"; + my $blob_dir = "$ENV{GIT_DIR}/objects/" . substr($hash, 0, 2); + my $blob_file = $blob_dir . "/" . substr($hash, 2); + if (! -f $blob_file) { + mkdir $blob_dir unless -d $blob_dir; + open BLOB, ">$blob_file"; + print BLOB compress("blob $size\0" . $buf); + close BLOB; + } close $fb->{base} or croak $!; } else { $hash = $fb->{blob} or die "no blob information\n";