Scrolling commands and skipping redisplay, was: Re: emacs rendering comparisson between emacs23 and emacs26.3

[Dmitry Gutov]

Hi Eli,

On 07.04.2020 17:26, Eli Zaretskii wrote:

> > If so, I don't understand how we get the apparent performance improvement from 
> > using fast-but-imprecise-scrolling.
>
> To explain that, I need to step back and describe how jit-lock
> mechanism works, from the display engine POV, and then how the scroll
> commands work.  (See also a large commentary in keyboard.c around line
> 250.) <...>

Thank you, what was a lot of additional detail.

The overall approach seems well-considered, but maybe there are ways to 
make it more... predictable, so to speak. Or more helpful.

> If you lean on C-v, then the scroll command is repeatedly called to
> find the next window-start, and repeatedly fontifies the text it goes
> through.  If you scroll N screen-fulls, the fontification of the first
> N-1 screen-fulls (if they were never displayed before, which happens
> when you visit a file and immediately start scrolling) is "wasted
> effort" for the purposes of the final window redisplay, because those
> N-1 screen-fulls will not be displayed.  Only the fontification of the
> last screen-full is "useful", because it saves some, sometimes all, of
> the fontification job of the stuff to be eventually displayed when you
> stop scrolling.

As you noted, when fontification takes a considerable amount of time, 
redisplay should be "free", relatively speaking. And if we didn't skip 
it, it would provide the user with the feedback that something is indeed 
happening.

> Enter fast-but-imprecise-scrolling.  What it does is disable jit-lock
> during the part where the scroll command walked the buffer to find the
> next window-start position.  Crucially, it also disables putting the
> non-nil 'fontified' property on the text traversed by the scroll
> command (because that is done by jit-lock).  So when you stop
> scrolling, redisplay of the stuff that ends up in the window will have
> to fontify all of what is visible in the window.

The obvious downside here is that it's "imprecise". And that it doesn't 
really know whether redisplay is going to be skipped, even though it 
could take a pretty good guess (using the input_was_pending variable, 
for example). If the user has leaned on C-v, they are probably not 
looking for much precision. But they might want it if they just pressed 
C-v once.

> Of course, if fast-but-imprecise-scrolling makes the job of scroll
> commands so much easier that Emacs is capable of keeping up with the
> keyboard auto-repeat rate, then every screen-full you scroll through
> _will_ be displayed, and therefore will be fontified by the calls to
> jit-lock from redisplay itself -- and that will again make scrolling
> slower.  So fast-but-imprecise-scrolling is most efficient when it
> makes scroll commands faster, but not too fast...

In my testing with an '-Og -g3' build (from emacs-27; master is much 
faster with the recent tweaks) it allows for redisplay once every 
several screenfuls or so. So it seems like a good tradeoff, in those 
particular conditions.

I wonder if we could make it more coordinated. The two problems I see are:

1. Scrolling commands can skip jit-lock when redisplay is going to apply 
it (loss of precision for no actual perf improvement), or they don't 
skip it (when fast-but-... is nil) even though redisplay will (precision 
seems unimportant in most pending-input scenarios).
2. If we accumulate a large enough input queue of C-v keypresses, Emacs 
won't react to anything (even to C-g, even hit three times) until it 
processes these events. Even though the user most probably wanted it to 
stop scrolling just as soon as they stopped leaning on C-v.

I think #2 can have good solutions, but I only have rough suggestions 
about that.

But let's look at #1. We could fix the discrepancy in two ways: either 
skip jit-lock in window_scroll if and only if redisplay will likely be 
skipped (as shown in the attached patch) (a). Or never skip it there, as 
well as indicate to redisplay that it shouldn't be skipped either (b).

(a) will increase reactivity, as well as make it harder to overwhelm the 
input queue, which will be perceived by a number of users (who manage to 
hit these conditions and have slow enough machines) as better 
responsiveness.

(b) will let the user see what's happening instead, even though the 
scrolling will become slower. I think this is a plus in its own right, 
because scrolling is a visual activity; there's little point in doing it 
if you can't see where you scrolled to. We can probably implement this 
by setting input_was_pending unconditionally to false inside window_scroll.

Logically, I think I'd prefer (b), especially if a solution for #2 was 
added as well. Because as soon as #2 is solved, (a)'s benefit isn't so 
clear anymore.

But either (a) or (b), both look more responsive to me than the current 
state of affairs.

fast_scroll_on_pending_input.diff
Description: Text Data