Subject: cat-file --batch-command info performance problem

From: Rob Browning <[email-masked]>
Date: Tue, 05 Aug 2025 16:21:36 -0500

While doing some testing before a bup release, I ran in to a performance
problem that I've narrowed down to git (2.47.2) cat-file --batch-command
(currently, bup often relies on git cat-file).

I can reproduce the problem with a ~125GB (~3M object) repository on
an external SSD and a system with 16GB RAM via "git --batch-command <
fetch-oids" where fetch-oids contains 8k "info HASH" commands.

That process runs at 32 hashes/sec (overall average), with a cold
cache (echo 3 > /proc/sys/vm/drop_caches), and does not improve over
repeated runs.  While that's running, every time I check, git's
reading about 300-400+ MB/s.

That didn't seem right, so I wrote a test command that produces the same
information via direct index access and packfile reads/seeks. With a
cold cache, that runs at 1.5k hashes/sec (even from python), and on the
second or third run, 9 hashes/seck.  Interestingly, if I run cat-file
after the test command has warmed up the cache, cat-file then reaches
53k hashes/sec.

My current guess is that cat-file's approach (all mmap?) is causing
some kind of kernel derived read amplification that's vastly
increasing the working set.

This is easily repeatable here, so I'd be happy to help test if that's
desirable, and perhaps even to pursue it if I can, and if it seems like
something that could/should be addressed in git.

If not, that's also helpful to know, and then we'll just handle the
lookups ourselves.

Thanks
-- 
Rob Browning
rlb @defaultvalue.org and @debian.org
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