git gc taking forever to reflog expire --all

On my recent MacBook Pro, I gave up after reflog expire --all had chewed up 30 minutes of perfectly fine CPU time.

Basically, I have a SVN import of a single branch of a repository with just a single directory with 3K files or so (this SVN import is part of a bigger repository). There are about 117K commits in the GIT history. I have a few local branches of just a couple commits each. The total .git size is 450MB, with about 3K loose objects.

This seems something that should be "git gc"-ed in about a minute or less, so I'm puzzled what is going on in this git reflog expire. From a cursory look it seems that we're in a loop finding a merge base for each commit, or similar. I'm including a log of a random interruption (using Ubuntu here instead of MacOS for better debugging).

  -Geert

#0  0x080b1c8c in clear_commit_marks (commit=0x87b2d60, mark=983040)
    at commit.c:403
#1  0x080b2d44 in get_merge_bases_many (one=0x90a9120, n=1, twos=0xbffff344, 
    cleanup=1) at commit.c:655
#2  0x080b2e67 in get_merge_bases (one=0x90a9120, two=0x819f720, cleanup=1)
    at commit.c:704
#3  0x080b2e9b in in_merge_bases (commit=0x90a9120, reference=0xbffff51c, 
    num=1) at commit.c:728
#4  0x08092ffa in unreachable (cb=0xbffff514, commit=0x5, 
    sha1=0xbffff484 "\264\252\244\232\320=Q\213\243Ҋ\323\070%[T\232\360,\032\214=\226\340\001\253\335tZ ", <incomplete sequence \303>) at builtin/reflog.c:233
#5  0x080930f5 in expire_reflog_ent (
    osha1=0xbffff498 "\232\360,\032\214=\226\340\001\253\335tZ ", <incomplete sequence \303>, 
    nsha1=0xbffff484 "\264\252\244\232\320=Q\213\243Ҋ\323\070%[T\232\360,\032\214=\226\340\001\253\335tZ ", <incomplete sequence \303>, 
    email=0x81c0ada "Geert Bosch <bosch@gnat.com>", timestamp=1265908541, 
    tz=0, message=0x81c0b08 "r4741\n", cb_data=0xbffff514)
    at builtin/reflog.c:298
#6  0x080e2c1f in for_each_recent_reflog_ent (
    ref=0x818066c "refs/remotes/git-svn@130115", 
    fn=0x8093030 <expire_reflog_ent>, ofs=0, cb_data=0xbffff514) at refs.c:1623
#7  0x080e2d27 in for_each_reflog_ent (
    ref=0x818066c "refs/remotes/git-svn@130115", 
    fn=0x8093030 <expire_reflog_ent>, cb_data=0xbffff514) at refs.c:1635
#8  0x080924d7 in expire_reflog (ref=0x818066c "refs/remotes/git-svn@130115", 
    sha1=0x819f720 "\005", unused=<value optimized out>, cb_data=0xbffff594)
    at builtin/reflog.c:353
#9  0x08092ab6 in cmd_reflog_expire (argc=2, argv=0xbffffb2c, prefix=0x0)
    at builtin/reflog.c:599
#10 0x0804b374 in run_builtin (argc=3, argv=0xbffffb28) at git.c:260
#11 handle_internal_command (argc=3, argv=0xbffffb28) at git.c:416
#12 0x0804b572 in run_argv (argc=3, argv=0xbffffb28) at git.c:458
#13 main (argc=3, argv=0xbffffb28) at git.c:529