[Bug 1784650] New: Fontconfig is slow, causing stuttering and freezing

https://bugzilla.redhat.com/show_bug.cgi?id=1784650

Bug ID: 1784650
           Summary: Fontconfig is slow, causing stuttering and freezing
           Product: Fedora
           Version: 31
            Status: NEW
         Component: fontconfig
          Severity: high
          Assignee: [e-mail]
          Reporter: [e-mail]
        QA Contact: [e-mail]
                CC: [e-mail], [e-mail],
                    [e-mail],
                    [e-mail],
                    [e-mail],
                    [e-mail], [e-mail],
                    [e-mail], [e-mail],
                    [e-mail], [e-mail],
                    [e-mail]
  Target Milestone: ---
    Classification: Fedora

Description of problem:
Fontconfig is much much slower than on other distros, and it stutters or
freezes applications that use it.

Version-Release number of selected component (if applicable):

Name        : fontconfig
Version     : 2.13.92
Release     : 3.fc31
Architecture: x86_64

How reproducible:
I can reproduce this bug on a fresh Fedora 31 vm with the Xfce desktop and
google-noto-sans-* fonts installed.

Steps to Reproduce:
1. dnf install google-noto-sans-*
2. run gedit on the attached example file

alternatively
1. dnf install google-noto-sans-*
2. open firefox and browse to https://www.wikidata.org/wiki/Q52 (page with lots
of languages)

Actual results:
It takes around 60 seconds for gedit to become responsive to scrolling and
input. Mousepad is faster but still slow.
It takes firefox upwards of 5 minutes to get to first paint on a page with many
fonts or languages, compared to a simpler page.

Expected results:
Gedit should load files with many fonts at a similar speed as other distros.
The page should load quickly, like on Debian and others.

Additional info:
I have tried to diagnose the source of this issue in many ways.

Running `perf trace` on what sysprof indicated was the most busy function
(FcStrCmpIgnoreCaseAndDelims), shows that every name of every font family is
being compared to every other name of every other font family. I do not know if
this is a normal behaviour of fontconfig.

I have noticed the amount of calls to "FcStrCmpIgnoreCaseAndDelims" and program
startup time both drop to a similar amount as Debian's when all of the
"google-noto" configuration files in /etc/fonts/conf.d/ are deleted (These
files are not present in Debian). However, this might not be the source of the
problem:

In the Debian vm, with a copy of my computer's /etc/fonts/, including the
google-noto files, (I took care to ensure that there would be no broken
symlinks) and /usr/share/fonts, fontconfig does not stall any programs. The
amount of calls to FcStrCmpIgnoreCaseAndDelims is also much lower as well.

This led me to believe that it was a difference caused by compiler flags but
this does not seem to be the case. I tried to replace the optflags in the
package, except for the rpmbuild required debug ones, and found no difference.
I also checked to ensure that it was not caused by GCC version differences.

Debian results for mousepad:

1,845,449 calls to FcStrCmpIgnoreCaseAndDelims
Time: 5 seconds

Fedora results for mousepad:

11,658,380 calls to FcStrCmpIgnoreCaseAndDelims
Time: 23 seconds

https://perfht.ml/2tleJxN
Here is a link to a Firefox profiler result of the wikidata page, where in the
flame graph you can see that Firefox is spending most of its time in
fontconfig. You can also see "FirstNonBlankPaint" is at 50 seconds in the
marker table.

TLDR: Fontconfig matching is slow with all google-noto fonts installed, unless
you remove the noto config files. Using the same exact font directory and
config directory (including the noto config files) on Debian does not cause the
same problem.

[Bug 1784650] Fontconfig is slow, causing stuttering and freezing

https://bugzilla.redhat.com/show_bug.cgi?id=1784650

--- Comment #1 from [e-mail] ---
https://bugs.chromium.org/p/chromium/issues/detail?id=1025356 Related chromium
bug

[Bug 1784650] Fontconfig is slow, causing stuttering and freezing

https://bugzilla.redhat.com/show_bug.cgi?id=1784650

Akira TAGOH  changed:

What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|31                          |rawhide
           Doc Type|---                         |If docs needed, set a value

--- Comment #2 from Akira TAGOH  ---
Somewhat improved in upstream. but not yet making a release. hopefully we will
see some improvements on this in f34.

[Bug 1784650] Fontconfig is slow, causing stuttering and freezing

https://bugzilla.redhat.com/show_bug.cgi?id=1784650

Akira TAGOH  changed:

What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|34                          |35

--- Comment #4 from Akira TAGOH  ---
This should be improved in the latest release in f36.

[Bug 1784650] Fontconfig is slow, causing stuttering and freezing

https://bugzilla.redhat.com/show_bug.cgi?id=1784650

[e-mail] changed:

What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |CLOSED
         Resolution|---                         |NEXTRELEASE
        Last Closed|                            |2022-03-03 20:53:15

--- Comment #5 from [e-mail] ---
Trying to reproduce this bug in F36 fails, and when I perf trace it I get only
1,478,448 calls to the function that used to be called 10 times as much.