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

From: Jeff Hostetler <git@jeffhostetler.com>
Date: Nov 16, 2020, 15:19 UTC
Message-ID: <9c999e38-34db-84bb-3a91-ae2a62b964b5@jeffhostetler.com>
In-Reply-To: <20201115194359.67901-1-matheus.bernardino@usp.br>

On 11/15/20 2:43 PM, Matheus Tavares wrote:

Show 55 quoted lines

> 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/
>

I can't really speak to NFS performance, but I have to wonder if there's not something else affecting the results -- 4 and/or 8 core results are better than 16+ results in some columns. And we get diminishing returns after ~16.

I'm wondering if during these test runs, you were IO vs CPU bound and if VM was a problem. I'm wondering if setting thread affinity would help here.

Jeff