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

[Jeff Hostetler <git@jeffhostetler.com>]

On 11/19/20 4:01 AM, Ævar Arnfjörð Bjarmason wrote:
> 
>> 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.
> 

I kinda like this idea.  It would give us a chance to let maintenance
periodically probe the repo/system and improve some of these tuning
knobs.

Jeff