bug#16129: 24.3.50; Emacs slow with follow-mode when buffer ends before last window

[Anders Lindgren]

Hi!

I agree that we would need to find out why the patch makes Emacs slow. (In fact, I only supplied the information about the internals of follow-mode to help you track down the problems with the slowdown.)

However, I don't agree with Eli -- it is possible to place window-start at point-max! However, there is code in the display engine that explicitly recenters such windows, after a while, or when something happens. For example:

     emacs -Q

     C-x 3

     C-x o

     M-: (set-window-start (selected-window) (point-max)) RET

     C-x o

     M-<

     blablabla     (type some text)

As you type text in the left window at the beginning of the scratch buffer, the right window is recentered. Follow-mode needs its windows to stay put (even the empty ones), as this is essential in creating the illusion that a number of windows make up a very tall virtual window.

When I originally wrote follow-mode (some 18 years ago), I suggested to the Emacs maintainers to add a feature to make the recentering of empty windows conditional, so that follow-mode could control this. However, at the time they were not interested so I continued with the current system, which has worked flawlessly since then.

If you are interested in making the change in the display engine, follow-mode should of course be rewritten to use it. Otherwise, I suggest that we keep it as it is today -- solutions using overlays etc. don't appeal to me at all.

    -- Anders

On Fri, Dec 13, 2013 at 5:38 PM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:

> I am the original author of follow-mode, so I can share one interesting
> implementation detail. When the viewed buffer ends before the last window,
> follow-mode tries to display this window without any content (by setting
> the window start to point-max). Unfortunately, the Emacs display engine

> always tries ensure that windows are not empty so it repositions it... So,
> follow-mode hammers in its view of the world every chance it gets,
> currrently in post-command hook and window-scroll-functions.

Hmm.. so we have 2 things to do:
1- figure out why my patch slowed things down so much.
2- change follow-mode to use a different approach.  Maybe a good way is
   to do the following: put window-point at point-max, and add an overlay
   on window-start...point-max that makes the text invisible (with
   a `window' property, so it's only invisible in that window).
   Of course, maybe that won't work either.  But hooking everywhere
   doesn't sound like a good idea.


-- Stefab