bug#75342: [PATCH] Speed up asynchronous man page fontifying

[Eli Zaretskii <eliz@gnu.org>]

> Cc: Juri Linkov <juri@linkov.net>
> From: Stefan Kangas <stefankangas@gmail.com>
> Date: Sat, 4 Jan 2025 01:22:03 -0600
> 
> We seem to call `man--maybe-fontify-manpage` many times on very small
> chunks: on my machine, it processes ~30 characters at a time.  Things
> get substantially faster if we make `Man-bgproc-filter` fontify the
> buffer in much larger chunks.  The main drawback of doing this is that
> we risk very briefly seeing an incorrect display flash by in the man
> buffer (e.g., with `end-of-buffer`).

Man-bgproc-filter also affects how we process sections of the man
page, see bug#36927.  Wouldn't larger chunks increase the probability
of making an error there?

> My measurements show that 32 KiB might be a good choice, and gives a
> ~95% speedup:
> 
> | Chunk size (KiB) | Completion time (s) |
> |------------------+---------------------|
> |                4 |                29.3 |
> |                8 |                18.2 |
> |               16 |                12.7 |
> |               32 |                 7.4 |
> |               64 |                 6.5 |
> |              128 |                 5.4 |
> 
> Is this the best approach to optimize something like this, or am I
> overlooking something obvious?  Note that I didn't add a variable for
> the chunk size, but we could easily add one, if that'd be useful.
> 
> When I set `Man-prefer-synchronous-call` to t, it takes ~1.8 seconds to
> process the same page.  I guess that this is the lower bound for how
> fast we could make the asynchronous call.

Why not make Man-prefer-synchronous-call t by default, then?