bug#71355: 30.0.50; [PATCH] Improve performance of buffered output in Eshell

[Stefan Kangas <stefankangas@gmail.com>]

Jim Porter <jporterbugs@gmail.com> writes:

> In Eshell, if I run "time cat config.log" from my Emacs build directory,
> it reports that it takes about 7.5s. It also doesn't show *any* output
> until it's completely finished. With my attached patches, it now takes
> about 0.6s and also shows the output iteratively, redisplaying
> periodically so users can see that something is happening.
>
> The other command most likely to be impacted by this is the built-in
> version of "ls". When I run "ls -Al /usr/bin" on my system, I go from
> 2.1s before my patch to 1.2s after. Not as big an improvement, but still
> noticeable, and it *feels* a lot faster too with the iterative redisplay.
>
> I don't usually add a NEWS entry for perf improvements, but this one
> seemed notable enough that I figured it was worth tooting my own horn. :)

Nice, thanks for working on this.  Your patch makes sense to me at first
glance, but I didn't test it.  The performance improvement definitely
seems highly worthwhile based on your measurements.

Bonus points for adding tests, as always.

> +(defcustom eshell-buffered-print-size 2048
> +  "The size of the print queue in characters, for doing buffered printing.
>  This is basically a speed enhancement, to avoid blocking the Lisp code
>  from executing while Emacs is redisplaying."

How did you decide on this value?

Could the docstring be expanded to explain what a user can expect to
happen if they increase or decrease this value?