Performance of "git gc..." is extremely bad in some cases

[Anthony Muller <anthony@monospace.sh>]

What did you do before the bug happened? (Steps to reproduce your issue)

git clone https://github.com/notracking/hosts-blocklists
cd hosts-blocklists
git reflog expire --all --expire=now && git gc --prune=now --aggressive


What did you expect to happen? (Expected behavior)

Running gc on a ~300 MB repo should not take 1 hour 55 minutes when
running gc on a 2.6 GB repo (LLVM) only takes 24 minutes.


What happened instead? (Actual behavior)

Command took 1h 55m to complete on a ~300MB repo and used enough
resources that the machine is almost unusable.


What's different between what you expected and what actually happened?

Compression stage uses the majority of the resources and time. Compression
itself, when compared to something like zlib or lzma, should not take very long.
While more may be happening as objects are compressed, the amount of time
gc takes to compress the objects and the resources it consumed are both
unreasonable.

Memory: RSS = 3451152 KB (3.29 GB), VSZ = 29286272 KB (27.92 GB)
Time: 12902.83s user 8995.41s system 315% cpu 1:55:36.73 total

I've seen this issue with a number of repos and size of the repo does not
determine if this happens. LLVM @ 2.6 GB worked flawlessly, a 900 MB
repo never finished, this 300 MB repo takes forever, and if you test something
like chromium git will just crash.


[System Info]
hardware: 2.9Ghz Quad Core i7
git version:
git version 2.30.0
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Darwin 19.6.0 Darwin Kernel Version 19.6.0: Tue Jan 12 22:13:05 PST 2021; root:xnu-6153.141.16~1/RELEASE_X86_64 x86_64
compiler info: clang: 12.0.0 (clang-1200.0.32.28)
libc info: no libc information available
$SHELL (typically, interactive shell): /usr/local/bin/zsh