Recommend Git's manyFiles feature

[michaelkuhn]

Git 2.24 introduced a feature flag for repositories with many files, see:
https://github.blog/2019-11-03-highlights-from-git-2-24/#feature-macros

Since Spack's Git repository contains roughly 8,500 files, it can be
worthwhile to enable this, especially on slow file systems such as NFS:

$ hyperfine --warmup 3 'cd spack-default; git status' 'cd spack-manyfiles; git status'
Benchmark #1: cd spack-default; git status
  Time (mean ± σ):      3.388 s ±  0.095 s    [User: 256.2 ms, System: 625.8 ms]
  Range (min … max):    3.168 s …  3.535 s    10 runs

Benchmark #2: cd spack-manyfiles; git status
  Time (mean ± σ):     168.7 ms ±  10.9 ms    [User: 98.6 ms, System: 126.1 ms]
  Range (min … max):   144.8 ms … 188.0 ms    19 runs

Summary
  'cd spack-manyfiles; git status' ran
   20.09 ± 1.42 times faster than 'cd spack-default; git status'

[sethrj]

Will the config flag be ignored on older git versions that don't support the feature?

[michaelkuhn]

Will the config flag be ignored on older git versions that don't support the feature?

I tested it with Git 2.17 and it just doesn't do anything then. git clone will still add it to the config file, so it will start working once Git is updated.

[adamjstewart]

It's not possible to ship a git config file within Spack where this is preset, is it?

[michaelkuhn]

It's not possible to ship a git config file within Spack where this is preset, is it?

I don't think so. It needs to be set in .git/config as far as I know.

Edit: To be precise, we could also put it into a separate config file (see https://stackoverflow.com/questions/18329621/how-to-store-a-git-config-as-part-of-the-repository) but then this file needs to be referenced in .git/config. Moreover, if I read the man page correctly, at least some of the implied options should be set before initializing the repository:

index.version
Specify the version with which new index files should be initialized. This does not affect existing repositories. If feature.manyFiles is enabled, then the default is 4.

[adamjstewart]

As long as older versions of git don't crash on this unsupported config setting, I agree with this feature!

[haampie]

@spackbot run pipeline

[spackbot-app]

I've started that pipeline for you!