bug#17124: 24.3.50; Occasional Extremely Slow Redraws in OSX Emacs

[Eli Zaretskii <eliz@gnu.org>]

> From: Jan Djärv <jan.h.d@swipnet.se>
> Date: Fri, 27 Jun 2014 19:13:00 +0200
> Cc: ericfroemling@gmail.com,
>  17124@debbugs.gnu.org
> 
> >> With the "shake the divider" recepie (see below), redisplay_internal is called more than 30 times per second.  On an old computer (end of 2008) I get about 37 times per second.
> >> But each redisplay results in multiple draw_begin/end, so for drawing, it is more than 37 times per second.
> > 
> > Does it help to set redisplay-dont-pause to nil?
> 
> The "shake the divider" case becomes much worse, lots of flickering and incomplete text.

But do you see less drawing requests sent to the backend?

> > Incidentally, I don't think your suggestion will help in the "shake
> > the divider" scenario: when window dimensions are changed, we toss the
> > glyph matrices of the affected windows, and then allocate them anew
> > (and perform a thorough redisplay of those windows).  So this will
> > anyhow require to redisplay the entire window completely, and the
> > backend will not be able to save us any redraws.
> 
> Not by itself, but if the backend is responsible for when actual drawing happens we can make sure we don't draw faster than the screen can update.

I actually find it hard to believe that we overwhelm the backend,
except, maybe, when the X client is on a remote machine.  E.g., on
Windows the "shake the divider" recipe doesn't show any signs of a
problem, and the CPU load is never more than a single execution unit
being busy, which means not much is at work except Emacs itself.  With
today's multi-core fast machines, how come it's impossible to redraw a
region 37 times a second?

> >> I think there is room for optimizations in the generic display also, for example moving the mouse redraws the entire mode line, even if the mouse pointer is outside the frame.
> > 
> > Please show the recipe to reproduce this.  I just tried a naive way of
> > doing that, and didn't see any mode-line updates even when the mouse
> > is inside the frame.  So I must be missing something.
> > 
> 
> I had problems seeing what was drawn and when so I added debug code to see what the font driver actually draws.  But I see now that it is different for X11.  So it might be NS specific.  What can make the modeline redraw in one version of Emacs and not in another?  I thought all that was generic code.

It _is_ generic code.  Perhaps we are not talking about the same
things: when you say that the mode line is redrawn, what exactly do
you see?

> Well, shake the divider is not really something normal a user does.  It is just a way to force the issue.  But slow redraws happens in normal usage also, i.e. switching buffers and editing.  It solves the second case, but makes shake the divider worse in terms of smooth redraws.

We need to compare the performance with this proposed feature with the
current implementation.  I think it's hard to talk about this without
some measurements, and probably also some reasonably important use
cases (which "shake the divider" isn't, IMO).