Re: Debouncing slow mode line constructs (was: Emacs design and architecture. How about copy-on-write?)

[Dmitry Gutov]

On 22/09/2023 13:23, Ihor Radchenko wrote:
> What I have in mind is the following:
>
> 1. Every time Emacs processes :eval construct it (a) measures the time
>     taken; (b) caches return value.
>     The total mode line render time is also recorded.
>
> 2. If the total render time exceeds configurable threshold, processing
>     the most time-consuming :eval constructs will be suspended until
>     "debounce" time since the last full processing.
>
>     By "suspended", I mean that cached :eval value is reused instead of
>     invoking :eval again. Possibly, not reused verbatim, but by replacing
>     the cached value with some kind of placeholder that has the same
>     total height/width as the cached value.
>
>     For example, consider :eval segment invoking external git command and
>     producing branch name: "⛬ master".
>     If that command takes too long, Emacs will display
>     "⏳       " instead of
>     "⛬ master", until the mode-line redisplay that is some fixed time
>     ahead.
>
> 3. After "debounce" time since the last "suspend", all the :eval are
>     processed in full again, generating up-to-date mode-line.

I think it can be worth a try, although this general thing sounds easier 
to do (in a different way) with parallel rendering of 
buffers/mode-lines/etc. If we ever get to that, of course.

> In terms of values, I am thinking something like 0.2 sec for threshold
> and 1.0 sec for debounce. This way, mode line will be outdated no longer
> than 1 second.

FWIW, the "slow" mode-lines I had tried in the past were still much 
faster than the above.