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bug#21028: Slow font rendering in emacs

From: Ralf Jung
Subject: bug#21028: Slow font rendering in emacs
Date: Tue, 14 Mar 2017 16:57:33 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0


>> I know very little about emacs, so I wouldn't know how to make any
>> measurements.
> If some reaction time is slow, just estimating how many seconds it
> takes might be good enough, if you describe the slow command and what
> is on the screen when it is slow.

Okay, so what I do as actually quite simple: I open the aforementioned
file in emacs, and then I use the scroll wheel on my mouse to scroll
around quickly (move wheel up fast - move wheel down fast - repeat).
When I do this with the "good" setup, text keeps flying over my screen
and everything reacts instantaneously.  When I do this with the "bad"
setup, nothing moves until I stop turning the scroll wheel and wait a
little more, where the waiting time is roughly half the time I spent
scrolling quickly.

>>> Also, what is your value of system-configuration-options and
>>> system-configuration-features?  And which version of Emacs is that?
>> Sorry, I have no idea how to read those variables
> Type
>   M-: system-configuration-options RET
> and tell what Emacs displays.  the same with the other one.

The -options variable says

"--build x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
--libexecdir=/usr/lib --localstatedir=/var/lib --infodir=/usr/share/info
--mandir=/usr/share/man --with-pop=yes
--build x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
--libexecdir=/usr/lib --localstatedir=/var/lib --infodir=/usr/share/info
--mandir=/usr/share/man --with-pop=yes
--with-x=yes --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g
-O2 -fdebug-prefix-map=/build/emacs24-OFSh0x/emacs24-24.5+1=.
-fstack-protector-strong -Wformat -Werror=format-security -Wall
-fno-PIE' 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2'
'LDFLAGS=-Wl,-z,relro -no-pie'"

And the -features one says

Symbol's value as variable is void: system-configuration-features

>> Emacs is fine in terms of speed, but it uses the wrong font for
>> characters not supported by Fira Sans Mono.
>> For example, for ∃ it picks "STIX"
> The default fontset was improved in Emacs 25.1, so there you should
> have Symbola for this character automatically.

But I don't want Symbola for this character.  Symbola is not monospace,
so it's not going to look right in a document that otherwise uses
monospace characters.

> I'm hesitant because no one understands why that patch should have any
> profound effect on performance.  We arrived at the patch by
> selectively reverting a particular commit that was fond by bisection.
> But that commit fixed an unrelated bug, so it wasn't done by mistake.

I see; thanks.

Kind regards,

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