Subject: Re: RFC: auto-enabling parallel-checkout on NFS

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Thu, 19 Nov 2020 10:01:07 +0100

> The major downside is that detecting the file system type is quite
> platform-dependent, so there is no simple and portable solution. (Also,
> I'm not sure if the optimal number of workers would be the same on
> different OSes). But we decided to give it a try, so this is a
> rough prototype that would work for Linux:
> https://github.com/matheustavares/git/commit/2e2c787e2a1742fed8c35dba185b7cd208603de9

I'm not intrinsically opposed to hardcoding some "nr_threads = is_nfs()
? x : y" as a stopgap.

I do think we should be thinking about a sustainable way of doing this
sort of thing, this method of testing once and hardcoding something
isn't a good approach.

It doesn't anticipate all sorts of different setups, e.g. in this case
NFS is not a FS, but a protocol, there's probably going to be some
implementations where parallel is much worse due to a quirk of the
implementation.

I think integrating an optimization run with the relatively new
git-maintenance is a better way forward.

You'd configure e.g.:

    maintenance.performanceTests.enabled=true
    maintenance.performanceTests.writeConfig=true

Which would run e.g.:

    git config --type bool core.untrackedCache $(git update-index --test-untracked-cache && echo true || echo false)
    git config checkout.workers $(git maintenance--helper auto-discover-config checkout.workers)

Such an implementation can be really basic at first, or even just punt
on the test and use your current "is it NFS?" check.

But I think we should be moving to some helper that does the actual test
locally when asked/configured by the user, so we're not making a bunch
of guesses in advance about the size/shape of the repository, OS/nfs/fs
etc.