Fwd: w32-pipe-read-delay (was: vc-dir operation is very slow on large git repositories in Emacs 26.1)

[Alex Harsanyi <alexharsanyi@gmail.com>]

On Sat, Jun 23, 2018 at 3:21 PM, Eli Zaretskii <eliz@gnu.org> wrote:

> (I'm surprised that this variable had such a profound effect with
> Git's ls-files, but maybe that has something to do with the MSYS
> components in Git on Windows.)
>

I don't think this has anything to do with Git or MSYS: I wrote a C++ program
to output 1 Mb of data (similar to the perl program I wrote earlier) and I
compiled this using Visual Studio, to get a native windows console
application.  I than set w32-pipe-buffer-size to 0 and experimented with
various settings for the w32-pipe-read-delay, using the `time-process`
function (see my previous email).  Here are the results:

* when w32-pipe-read-delay is 50 milliseconds, it takes 13.2 seconds to read 1
  Mb of piped data from the C++ executable.

* when it is 25 milliseconds, it takes 6.8 secons to read the same data

* when it is 12 milliseconds, it takes 3.47 seconds to read the same data

* when it is 0, it takes 0.13 seconds

Increasing w32-pipe-buffer-size had no effect on the speed -- I know I said in
a previous email that it had an effect, but I only tried to increase it after
dropping w32-pipe-read-delay to 0 and tried it once -- sometimes the time
drops further from 0.13 to 0.8 seconds, but this seems random -- at this
speed, probably other factors, such as process startup, have an effect on the
read speed.

On Monday, I will test it against my work repository with 50000 files, and
will report back on that.

Alex.