How to still kill git fetch with too many refs

I have often reported problems with git fetch when there are many refs in a repo, and I have been pleasantly surprised how many problems I reported were so quickly fixed. :) With time, others have created various synthetic test cases to ensure that git can handle many many refs. A simple synthetic test case with 1M refs all pointing to the same sha1 seems to be easily handled by git these days. However, in our experience with our internal git repo, we still have performance issues related to having too many refs, in our kernel/msm instance we have around 400K.

When I tried the simple synthetic test case and could not reproduce bad results, so I tried something just a little more complex and was able to get atrocious results!!! Basically, I generate a packed-refs files with many refs which each point to a different sha1. To get a list of valid but unique sha1s for the repo, I simply used rev-list. The result, a copy of linus' repo with a million unique valid refs and a git fetch of a single updated ref taking a very long time (55mins and it did not complete yet). Note, with 100K refs it completes in about 2m40s. It is likely not linear since 2m40s * 10 would be ~26m (but the difference could also just be how the data in the sha1s are ordered).

Here is my small reproducible test case for this issue:

git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git cp -rp linux linux.1Mrefs-revlist

cd linux echo "Hello" > hello ; git add hello ; git ci -a -m 'hello' cd ..

cd linux.1Mrefs-revlist git rev-list HEAD | for nn in $(seq 0 100) ; do for c in $(seq 0 10000) ; do read sha ; echo $sha refs/c/$nn/$c$nn ; done ; done > .git/packed-refs

time git fetch file:///$(dirname $PWD)/linux refs/heads/master

Any insights as to why it is so slow, and how we could possibly speed it up?

Thanks,

-Martin

PS: My tests were performed with git version 1.8.2.1 on 
linux 2.6.32-37-generic #81-Ubuntu SMP