bug#21028: Performance regression in revision af1a69f4d17a482c359d98c00ef86fac835b5fac (Apr 2014).

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 21028@debbugs.gnu.org
> From: Clément Pit--Claudel <clement.pitclaudel@live.com>
> Date: Wed, 15 Mar 2017 16:46:33 -0400
> 
> > The problem here is that opening a font and looking up a character is
> > very expensive, certainly when there are hundreds of fonts installed.
> > So Emacs filters the fonts according to the scripts they claim to
> > support, and only opens those which appear to be valid candidates for
> > the script of the character it needs to display.  By using 'unicode as
> > the script when you set up your fontset, you actually trip Emacs by
> > telling it to try this font for every character it needs to display.
> > You should instead specify the scripts which the fonts supports well.
> 
> Ok — but why wasn't it slow in 24.3?

That's the question about the significance of the zero_vector in the
font-cache, the patch you want to apply.  I'm still struggling with
understanding why it speeds up the font search.

> And why does the number of fonts matter, 
> if I'm only adding one (Symbola) with 'unicode?

Any font has several variants, and they are all checked.  Moreover,
checking too many fonts conses a lot of Lisp data, which is likely to
trigger GC, which is likely to throw away font caches, which will then
require us to check those fonts again.

> > That shouldn't be a problem in Emacs 25: it by default uses the
> > default face's font for any punctuation and symbol characters for
> > which the font has glyphs, even if the fontset specifies a different
> > font for punctuation and symbol blocks.
> 
> Neat.  Though of course that doesn't help for Emacs < 24.

Users who care about fonts and exotic characters shouldn't stay with
Emacs 24.

> > So using more accurate scripts gives a major improvement.  Good.
> 
> It also seems to negate the previous conclusion, right?

Not really.

> In these fast example I'm still adding Symbola with 'unicode — but
> adding Ubuntu Mono with 'latin.

Ubuntu Mono _is_ the problem.  Symbola has very good coverage of
almost all Unicode blocks, so it is not the problem.

> > That could sometimes be the case, but I have found that in some cases
> > omitting the registry for characters beyond Latin-1 can be a major
> > setback.  So I recommend to always use it.
> 
> Ok.  Could Emacs not infer it?

I think Emacs still goes by the XLFD rules, which require iso8859-1 be
the default.  I don't know how important it is to keep that
compatibility.

> > You can use a font utility, such as Fontforge, to see which blocks the
> > font supports, and how many characters from each block it can display.
> > My conclusion from looking at Ubuntu Mono is that the above 3 scripts
> > are the only ones it supports well; the rest are not covered well at
> > all.  You can, of course, add more scripts if you need them, but the
> > downside will be that some of those scripts will be displayed by a mix
> > of more than one font, which I think will make the display ugly.
> 
> I don't care about the display being pretty as much as it being properly align 
> (vertically), which means that using Ubuntu Mono helps a lot.

That's fine, just don't tell Emacs Ubuntu Mono supports the entire
Unicode.

> > Moreover, Emacs cannot compose glyphs that come from different fonts,
> > so you will sometimes see decomposed display if you request a font for
> > scripts where its support is incomplete.  I think this is an important
> > factor for users of prettify-symbols-mode in particular.
> 
> Why? prettify-symbols-mode composes strings (typically) into single characters, 
> so why would this matter?

I imagine that prettify-symbols-mode is not limited to single existing
characters, but if it is, this issue is indeed not relevant to that
mode.

> >> (dolist (ft (fontset-list))
> >>   (set-fontset-font ft 'unicode (font-spec :name "YOUR-USUAL-FONT"))
> >>   (set-fontset-font ft 'unicode (font-spec :name "Symbola") nil 'append))
> > 
> > Well, maybe with the new insights you have now, you can recommend them
> > better setups which don't use 'unicode'.

> I think I still have a blurry picture of the whole process, or at least it 
> sounds very complicated.  Is it really the following?
> 
> * Pick a good monospace font.  Set that as the default Emacs font (this is easy)

Yes.

> * Pick a good math font.  Symbola is easier than others, because Emacs knows 
> about it.

Yes.

> * Download and install fontforge.  Figure out which ranges the math font 
> support, and all the characters that it supports that are not punctuation or 
> symbols, too

Not needed, as long as the default font is set up via
default-frame-alist.

> * Create a set-fontset-font rule for each range and character.  Figure out the 
> font's registry too.  Add all these rules with 'append, though for better 
> performance you can use 'prepend, but for that you'll need to know which ranges 
> the first font supports, too.

Should not be needed at all, as math symbols are already set up in the
default fontset (if I understand correctly what you need the math font
for, see below).  Definitely not needed if Symbola is to be used.  For
other Math fonts, I expect the default to just work; if it doesn't,
I'd like to see bug reports.

> In Emacs 25, given that symbols use the default font,  it looks like the 
> following should work, at least with your patch:
> 
>   25.1/src/emacs -Q --eval "(set-fontset-font \"fontset-default\" 'unicode 
> (font-spec :name \"XITS Math\") nil 'prepend)"

What happens if you don't do this?  Which font is used for math
symbols?  Or are the problems with symbols other than math symbols?
IOW, I don't understand fully why you need to set up XITS Math in the
fontset, what is missing/incorrect/ugly if you don't?