bug#64735: 29.0.92; find invocations are ~15x slower because of ignores

[Eli Zaretskii]

> Date: Tue, 25 Jul 2023 05:41:13 +0300
> Cc: luangruo@yahoo.com, sbaugh@janestreet.com, yantar92@posteo.net,
>  64735@debbugs.gnu.org
> From: Dmitry Gutov <dmitry@gutov.dev>
> 
> >> One thing to try it changing the -with-find implementation to use a
> >> synchronous call, to compare (e.g. using 'process-file'). And repeat
> >> these tests on GNU/Linux too.
> > 
> > This still uses pipes, albeit without the pselect stuff.
> 
> I'm attaching an extended benchmark, one that includes a "synchronous" 
> implementation as well. Please give it a spin as well.
> 
> Here (GNU/Linux) the reported numbers look like this:
> 
>  > (my-bench 1 default-directory "")
> 
> (("built-in" . "Elapsed time: 1.601649s (0.709108s in 22 GCs)")
>   ("with-find" . "Elapsed time: 1.792383s (1.135869s in 38 GCs)")
>   ("with-find-p" . "Elapsed time: 1.248543s (0.682827s in 20 GCs)")
>   ("with-find-sync" . "Elapsed time: 0.922291s (0.343497s in 10 GCs)"))

Almost no change on Windows:

  (("built-in" . "Elapsed time: 1.218750s (0.078125s in 5 GCs)")
   ("with-find" . "Elapsed time: 8.984375s (0.109375s in 7 GCs)")
   ("with-find-p" . "Elapsed time: 8.718750s (0.046875s in 3 GCs)")
   ("with-find-sync" . "Elapsed time: 8.921875s (0.046875s in 3 GCs)"))

I'm beginning to suspect the implementation of pipes (and IPC in
general).  How else can such slowdown be explained?