Subject: Re: [PATCH 0/7] pack-revindex: enable on-disk reverse indexes by default

From: Taylor Blau <[email-masked]>
Date: Tue, 11 Apr 2023 17:40:17 -0400

On Tue, Apr 11, 2023 at 09:54:08AM -0400, Derrick Stolee wrote:
> On 4/10/2023 6:53 PM, Taylor Blau wrote:
> > In the vast majority of cases, this trade-off favors the on-disk ".rev"
> > files. But in certain cases, the in-memory variant performs more
> > favorably. Since these cases are narrow, and performance is machine- and
> > repository-dependent, this series also introduces a new configuration
> > option to disable reading ".rev" files in the third commit.
>
> I agree that the performance trade-off indicates that having the .rev
> files is preferred. It makes operations that _can_ be very fast as fast
> as possible (inspecting a small number of objects is much faster because
> we don't generate the in-memory index in O(N) time) and you create a knob
> for disabling it in the case that we are already doing something that
> inspects almost-all objects.

Sweet; I'm glad that you agree.

FWIW for non-GitHub folks, observing a slow-down here has never been an
issue for us. So much so that I wrote the pack.readReverseIndex knob
yesterday for the purpose of sending this series.

That said, I think that including it here is still worthwhile, since the
cases where performance really suffers (e.g., `git cat-file
--batch-all-objects --batch-check='%(objectsize:disk)'`) isn't something
that GitHub runs regularly if at all.

To accommodate different workflows, I think having the option to opt-out
of reading the on-disk ".rev" files is worthwhile.

> This was an easy series to read. I applied the patches locally and poked
> around in the resulting code as I went along. This led to a couple places
> where I recommend a few changes, including a new patch that wires
> repository pointers through a few more method layers.

Thanks for taking a look. Based on your review, there are only a couple
of things on my mind:

  - I amended the last patch to more clearly state when we would want to
    run the suite GIT_TEST_NO_WRITE_REV_INDEXES=1 set, and kept it in
    the linux-TEST-vars configuration.

  - How do you want to handle that extra patch? As I noted in [1], I
    think squashing the two together in one way or another makes sense.
    So really we have to figure out (a) if you think that is the right
    way to go, and (b) if so, how to handle attribution / the commit
    message.

Thanks,
Taylor

[1]: https://lore.kernel.org/git/[email-masked]/