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bug#21028: Performance regression in revision af1a69f4d17a482c359d98c00e

From: Eli Zaretskii
Subject: bug#21028: Performance regression in revision af1a69f4d17a482c359d98c00ef86fac835b5fac (Apr 2014).
Date: Thu, 22 Dec 2016 20:04:35 +0200

> Cc: address@hidden
> From: Clément Pit--Claudel <address@hidden>
> Date: Thu, 22 Dec 2016 11:49:09 -0500
> I did; append works fine here. Everything is displayed with XITS Math, as 
> expected.  prepend uses Segoe or XITS for all text, which I don't want.  It's 
> good for testing, though, since I guess that otherwise your Emacs picks other 
> fonts that are available on your machine.

Yes, that's why I used that.  I have Symbola installed, and it is used
for all those characters unless I use prepend, and the problem doesn't

> If you see the slowdown, then it seems that managed to reproduce the problem, 
> right?

No, not really.  It doesn't seem useful to define a font that is only
used for blanks and can support no other characters in the buffer,
because that probably means Emacs is desperately trying to use it and
fails, which can explain the slowdown.

> Is there anything preventing us from applying the fix that you described last 
> month?

You have to convince me it's needed ;-)

Is the other font a necessary part of this problem?  IOW, if you only
setup the fontset with the Segoe UI Emoji, does the problem still
happen?  I didn't try to install the other font, but if doing so is
necessary, I will try that.

> The example only shows arrows because I tried to make it minimal; my bug 
> report is about the fact that, when I set up Emacs to display characters the 
> way I want them, it gets slow.  I readily admit that changing the font set-up 
> may make the problem go away, but it's not a satisfactory solution (I still 
> get support requests from users of my packages about degraded performance due 
> to prettify-symbols-mode, and it's hard to tell them to use different fonts).

I agree that changing the font is not a satisfactory solution (unless
it turns out the font you've been using is simply broken in some
way).  But neither does it make sense to make changes in the code we
don't understand well enough when it turns out the offending font is
not used for any of the characters in the buffer.

> The fixes that you suggested to my minimal example do work around the issue 
> in that case (another work around for me is to just remove the Segoe UI 
> line), but these fixes are not applicable to my full font configuration.  I 
> can post my full configuration if you want to suggest improvements that would 
> work around the issue in my specific case (alternatively, it's available 
> here: https://github.com/cpitclaudel/.emacs.d/blob/master/init/fonts.el), but 
> this seems like a waste of your time: there's no doubt that others will run 
> into the same issues (indeed, they already do), and fixing their 
> configurations one by one isn't practical (assuming we even hear about them).

I would like to try and understand the problem.  For that I need a
minimal setup that will show the problem while still making sense,
i.e. when the characters in the buffer are displayed by the offending
font.  A setup with a font that is not actually used for display
doesn't seem to be sensible; if showing the problem with real
characters requires a somewhat more elaborate setup, please show it.

One aspect that might make this issue easier to analyze is not to use
'unicode as the target of the fontset, but instead specify range(s) of
characters where you want it used.  Then the prepend/append issue will
go away, and the differences between our systems wrt other fonts will
not get in the way of reproducing the problem.


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