Subject: Re: bug#45898: 27.1; wedged in redisplay again

From: Lars Ingebrigtsen <[email-masked]>
Date: Mon, 13 Jun 2022 14:10:19 +0200

Eli Zaretskii <[email-masked]> writes:

> Here's the first cut.  It still needs polishing and some testing, but
> let me know what you think:

[...]

> +/* Update the redisplay-tick count for a window, and signal an error
> +   if the tick count is above some threshold, indicating that
> +   redisplay of the window takes "too long".

Ah, instead of measuring the time elapsed, we use the number of iterator
"executions" as a proxy.  That sounds promising.

One problem that occurs to me is if you're, say, only displaying a shell
buffer, and it's outputting data -- then I don't think we'll be changing
windows, but just accruing ticks?  But I think that should be easy
enough to fix, since we'll be returning to command_loop and we could
just have that nixing out the tick count, too.  Probably.

A different problem is when we don't have many ticks, but each tick
takes a long time to execute.  The classic problem here is when we have
a font-locking regexp that's very complicated (with lots of
backtracking).  Then we don't update anything on the screen much -- we
spend (virtually) all the time in the regexp matcher.  I don't see an
easy fix to that using this scheme...

> For testing purposes, is it possible to have examples of files that
> could benefit from this feature, i.e. files where Emacs becomes not
> responsive enough?  I'm not sure the few examples I have cover all the
> popular reasons for the slowness, as I think there are more than one
> or two.

I don't have anything handy...  anybody else have a setup that will
freeze Emacs redisplay that we can test with?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no