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

[Ihor Radchenko]

Eli Zaretskii <eliz@gnu.org> writes:

>> What about automatically debouncing slow :eval constructs in the mode
>> line? If we make sure that :eval constructs running longer than a
>> threshold do not run too frequently, it can certainly improve user
>> experience.
>
> How do you propose to evaluate just portions of the mode line?  What
> will the :eval whose processing is bypassed yield, and how will that
> affect processing of the rest of mode-line-format, given its
> highly-recursive processing structure and the fact that the text
> produced by this processing is laid out as it is produced?

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.

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 This way, mode line will be outdated no longer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.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
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