RFC: auto-enabling parallel-checkout on NFS

Hi, everyone

I've been testing the parallel checkout code on some different machines, to benchmark its performance against the sequential version. As discussed in [1], the biggest speedups, on Linux, are usually seen on SSDs and NFS volumes. (I haven't got the chance to benchmark on Windows or OSX yet.)

Regarding NFS, I got some 2~3.4x speedups even when the NFS client and server were both running on single-core machines. Here are some runtimes for a linux-v5.8 clone (means of 5 cold-cache executions):

    nfs 3.0              nfs 4.0              nfs 4.1
1:  183.708 s ± 3.290 s  205.851 s ± 0.844 s  217.317 s ± 3.047 s
2:  130.510 s ± 3.917 s  139.124 s ± 0.772 s  142.963 s ± 0.765 s
4:   89.611 s ± 1.032 s  102.701 s ± 1.665 s   94.728 s ± 1.014 s
8:   68.097 s ± 0.820 s  104.914 s ± 1.239 s   69.359 s ± 0.619 s
16:  63.999 s ± 0.820 s  104.808 s ± 2.279 s   64.843 s ± 0.587 s
32:  62.316 s ± 2.095 s  102.105 s ± 1.537 s   64.122 s ± 0.374 s
64:  63.699 s ± 0.841 s  103.103 s ± 1.319 s   63.532 s ± 0.734 s

The parallel version was also faster for some smaller checkouts. For example, the following numbers come from a bat-v0.16.0 clone (251 files, ~3MB):

     nfs 3.0             nfs 4.0             nfs 4.1
1:   0.853 s ± 0.080 s   0.814 s ± 0.020 s   0.876 s ± 0.065 s
2:   0.671 s ± 0.020 s   0.702 s ± 0.030 s   0.705 s ± 0.030 s
4:   0.530 s ± 0.024 s   0.595 s ± 0.020 s   0.570 s ± 0.030 s
8:   0.470 s ± 0.033 s   0.609 s ± 0.025 s   0.510 s ± 0.031 s
16:  0.469 s ± 0.037 s   0.616 s ± 0.022 s   0.513 s ± 0.030 s
32:  0.487 s ± 0.030 s   0.639 s ± 0.018 s   0.527 s ± 0.028 s
64:  0.520 s ± 0.022 s   0.680 s ± 0.028 s   0.562 s ± 0.026 s

While looking at these numbers with Geert (in CC), he had the idea that we could try to detect when the checkout path is within an NFS mount, and auto-enable paralellism for this case. This way, users in NFS would get the best performance by default. And it seems that using ~16 workers would produce good results regardless of the NFS version that they might be running.

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

Any thoughts on this idea? Or alternative suggestions?

Thanks, Matheus

[1]: https://lore.kernel.org/git/815137685ac3e41444201316f537db9797dcacd2.1604521276.git.matheus.bernardino@usp.br/