bug#19266: 24.4; Font-related window redrawing delays on OS X

[Kirill Ignatiev]

On 4 December 2014 at 02:30, Eli Zaretskii <eliz@gnu.org> wrote:
>> Date: Thu, 4 Dec 2014 02:13:30 -0500
>> From: Kirill Ignatiev <kirill.ignatiev@gmail.com>
>>
>> [This is a copy of this question on Emacs.SE:
>> http://emacs.stackexchange.com/questions/4061]
>> At least one other person has the same problem.
>
> Please ask those persons to report here directly, and to read the
> responses and discussions.
>
>> Often when I'm editing a file with many uncommon unicode symbols (e.g.,
>> in languages like haskell, julia or c++), and (I think) especially with
>> an ornate color theme, I experience delays of maybe about five seconds
>> when switching between buffers. Running emacs under lldb show a stack
>> trace below; I get a similar stack trace from running `emacs -Q` and
>> typing `C-h h` (`view-hello-file`), which takes quite a while to display
>> the hello file.
>
> AFAICT, these delays are due to Emacs searching the system for
> appropriate fonts needed to display those unusual symbols.

I understand that this needs to happen once. Is it reasonable that the
delay happens many times during normal editing?

The description of what triggers the delays is unclear because for me
it's only reproducible incidentally, during normal work. I have
fold-mode that hides large portions of text, and a number of frames.
If I hide some previously-displayed text, go work in another frame,
then come back and unhide the text, then there is the font-searching
delay. Something similar happens if I switch to a different buffer,
then some time later switch back.

>> What can I do about these window redrawing delays? I am not sure what I
>> have misconfigured.
>
> Optimize your font configuration, so that the font search becomes
> faster.
>
>> Does emacs reload all fonts every time I switch to a different frame or
>> buffer? It also seems to have a delay sometimes when a previously
>> invisible overlay is shown.
>
> Emacs only looks for a font when it needs to display something.  when
> previously invisible portion is about to be displayed, Emacs needs the
> fonts to display it.

The reason I suspect this may be an actual bug is not that the delay
happens at all, but rather that it happens quite often, and the text
that causes the search for fonts used to be displayed only a short
time ago.

That said, I'm not familiar with how emacs manages fonts. Is it
reasonable to expect to keep all recently used fonts in memory and not
search for them again?

Thanks for the very quick reply.