Re: Removing redisplay-dont-pause

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> Assume the incoming events come every T seconds and take T1 seconds to
>> run, and the subsequent redisplay takes time T2+T3 (where T2 is the time
>> in the middle where `redisplay-dont-pause` is consulted).
>
> I think you meant to say that T3 is the phase at the end where
> redisplay-dont-pause is checked, the update phase that writes to the
> screen. T2 would then be the phase where the glyphs are produced.

My "analysis" doesn't care what's done when betwen T2 and T3, but yes,
T2 is the part that's affected by long lines, jit-lock, invisible text,
whereas T3 is about actual "drawing", so it can be affected by your 1200
bauds serial line (if you're sufficiently unlucky to be stuck behind
such a thing).

My impression is that, historically, T3 has been on a mostly decreasing
slope over time, whereas T2 has been on a rather increasing slope.
So back in Emacs-18, I suspect that T2 was usually dwarfed by T3,
whereas nowadays it's rather the reverse.

>> - If T < T1, then we plain and simply can't keep up at all, the
>>   redisplay is just not run at all and incoming events keep accumulating
>>   as we fall ever further behind the incoming events until the incoming
>>   events stop (at which point Emacs will only refresh its redisplay
>>   after it has finally caught up with the buffered input).
> The direct_output_for_insert optimization was though for that, ensuring
> that at least the line one types in is made visible on the screen.

That's my understanding as well.
But the change in hardware performance has made it so that in the
`direct_output_for_insert` case nowadays T is almost never > T1+T2+T3,
which is why we could remove that optimization.

Nowadays to get T<T1 you need a high repeat rate together with
a "costly" operation, such as scrolling through a buffer.
[ Tho, admittedly, with enough bells&whistles attached to things like
  `post-command-hook` and `mode-line-format`, even a plain
  `self-insert-command` can be made costly again, admittedly.  ]


        Stefan