Re: [PATCH v2 0/3] refs: sync loose refs to disk before committing them

[Jeff King <peff@peff.net>]

On Wed, Nov 10, 2021 at 12:40:50PM +0100, Patrick Steinhardt wrote:

> Please note that I didn't yet add any performance numbers or tests.
> Performance tests didn't show any conclusive results on my machine given
> that I couldn't observe any noticeable impact at all, and I didn't write
> tests yet given that I first wanted to get some feedback on this series.
> If we agree that this is the correct way to go forward I'll of course
> put in some more time to add them.

Here's a mild adjustment of the quick test I showed earlier in [1]. It
uses your version for all cases, but swaps between the three config
options, and it uses --atomic to put everything into a single
transaction:

  $ hyperfine -L v false,true,batch -p ./setup 'git -C dst.git config core.fsyncreffiles {v}; git.compile push --atomic dst.git refs/heads/*'
  Benchmark 1: git -C dst.git config core.fsyncreffiles false; git.compile push --atomic dst.git refs/heads/*
    Time (mean ± σ):      10.5 ms ±   0.2 ms    [User: 7.8 ms, System: 3.9 ms]
    Range (min … max):    10.1 ms …  11.0 ms    108 runs
   
  Benchmark 2: git -C dst.git config core.fsyncreffiles true; git.compile push --atomic dst.git refs/heads/*
    Time (mean ± σ):     402.5 ms ±   6.2 ms    [User: 13.9 ms, System: 10.7 ms]
    Range (min … max):   387.3 ms … 408.9 ms    10 runs
   
  Benchmark 3: git -C dst.git config core.fsyncreffiles batch; git.compile push --atomic dst.git refs/heads/*
    Time (mean ± σ):      21.4 ms ±   0.6 ms    [User: 8.0 ms, System: 5.2 ms]
    Range (min … max):    20.7 ms …  24.9 ms    99 runs
   
  Summary
    'git -C dst.git config core.fsyncreffiles false; git.compile push --atomic dst.git refs/heads/*' ran
      2.05 ± 0.07 times faster than 'git -C dst.git config core.fsyncreffiles batch; git.compile push --atomic dst.git refs/heads/*'
     38.51 ± 1.06 times faster than 'git -C dst.git config core.fsyncreffiles true; git.compile push --atomic dst.git refs/heads/*'

So yes, the "batch" case is much improved. It's still more expensive
than skipping the fsync entirely, but it's within reason. The trick,
though is the --atomic. If I drop that flag, then "batch" takes as long
as "true". And of course that's the default.

I wonder if that means we'd want an extra mode: do the WRITEOUT_ONLY
flush, but _don't_ do a HARDWARE_FLUSH for each transaction. That
doesn't give you durability, but it (in theory) solves the out-of-order
journal problem without paying any performance cost.

I say "in theory" because I'm just assuming that the WRITEOUT thing
works as advertised. I don't have an easy way to test the outcome on a
power failure at the right moment here.

-Peff

[1] https://lore.kernel.org/git/YYTaiIlEKxHRVdCy@coredump.intra.peff.net/