On Fri, Nov 05, 2021 at 03:07:18AM -0400, Jeff King wrote:
> 2. It's not clear what the performance implications will be,
> especially on a busy server doing a lot of ref updates, or on a
> filesystem where fsync() ends up syncing everything, not just the
> one file (my impression is ext3 is such a system, but not ext4).
> Whereas another solution may be journaling data and metadata writes
> in order without worrying about the durability of writing them to
> disk.
>
> I suspect for small updates (say, a push of one or two refs), this
> will have little impact. We'd generally fsync the incoming packfile
> and its idx anyway, so we're adding may one or two fsyncs on top of
> that. But if you're pushing 100 refs, that will be 100 sequential
> fsyncs, which may add up to quite a bit of latency. It would be
> nice if we could batch these by somehow (e.g., by opening up all of
> the lockfiles, writing and fsyncing them, and then renaming one by
> one).
So here's a quick experiment that shows a worst case: a small push that
updates a bunch of refs. After building Git with and without your patch,
I set up a small repo like:
git init
git commit --allow-empty -m foo
for i in $(seq 100); do
git update-ref refs/heads/$i HEAD
done
To give a clean slate between runs, I stuck this in a script called
"setup":
#!/bin/sh
rm -rf dst.git
git init --bare dst.git
sync
And then ran:
$ hyperfine -L v orig,fsync -p ./setup '/tmp/{v}/bin/git push dst.git refs/heads/*'
Benchmark 1: /tmp/orig/bin/git push dst.git refs/heads/*
Time (mean ± σ): 9.9 ms ± 0.2 ms [User: 6.3 ms, System: 4.7 ms]
Range (min … max): 9.5 ms … 10.5 ms 111 runs
Benchmark 2: /tmp/fsync/bin/git push dst.git refs/heads/*
Time (mean ± σ): 401.0 ms ± 7.7 ms [User: 9.4 ms, System: 15.2 ms]
Range (min … max): 389.4 ms … 412.4 ms 10 runs
Summary
'/tmp/orig/bin/git push dst.git refs/heads/*' ran
40.68 ± 1.16 times faster than '/tmp/fsync/bin/git push dst.git refs/heads/*'
So it really does produce a noticeable impact (this is on a system with
a decent SSD and no other disk load, so I'd expect it to be about
average for modern hardware).
Now this test isn't entirely fair. 100 refs is a larger than average
number to be pushing, and the effect is out-sized because there's
virtually no time spent dealing with the objects themselves, nor is
there any network latency. But 400ms feels like a non-trivial amount of
time just in absolute numbers.
The numbers scale pretty linearly, as you'd expect. Pushing 10 refs
takes ~40ms, 100 takes ~400ms, and 1000 takes ~4s. The non-fsyncing
version gets slower, too (there's more work to do), but much more slowly
(6ms, 10ms, and 50ms respectively).
So this will definitely hurt at edge / pathological cases.
-Peff
