fontconfig initialization spends most of its time parsing XML configuration files in /etc/fonts/conf.d/

For information, output of strace related to that:

open("/etc/fonts/conf.d/10-antialias.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/10-hinting-slight.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/10-hinting.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/11-lcdfilter-default.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/20-unhint-small-dejavu-lgc-sans-mono.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/20-unhint-small-dejavu-lgc-sans.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/20-unhint-small-dejavu-lgc-serif.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/20-unhint-small-dejavu-sans-mono.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/20-unhint-small-dejavu-sans.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/20-unhint-small-dejavu-serif.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/20-unhint-small-vera.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/30-metric-aliases.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/30-urw-aliases.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/40-nonlatin.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/45-latin.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/49-sansserif.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/50-user.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/51-local.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/local.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 26
open("/etc/fonts/conf.d/57-dejavu-sans-mono.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/57-dejavu-sans.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/57-dejavu-serif.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/58-dejavu-lgc-sans-mono.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/58-dejavu-lgc-sans.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/58-dejavu-lgc-serif.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/60-latin.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/64-01-tlwg-kinnari.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/65-droid-sans-fallback.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/65-fonts-persian.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/65-fonts-takao-pgothic.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/65-nonlatin.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/65-wqy-microhei.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/66-lohit-devanagari.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/66-lohit-punjabi.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/69-unifont.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 25
open("/etc/fonts/conf.d/70-no-bitmaps....

Hmm.. fontconfig must parse those XML files at startup to do its job. While you indicate that it happens at startup of QML apps, my belief was that it happens already at login.

It may well be possible to reduce the number of fontconfig config files, but that would be a large project, where you'd need to make sure fore every single language that the rendering isn't changed adversely. AFAIK, the fontconfig and fonts-* packages currently used in Ubuntu touch are the same as on the desktop.

The output of strace shows that fontconfig opens those file during every QML application startup.

A solution to this would be to introduce a caching system: after fontconfig opens and parses those files the first time it could serialize the result to disk for subsequent starts.

For a given user only a subset of these fonts are used, so this could probably be mitigated by installing fonts on demand with the language (reported in bug 1577369)

On 2016-06-14 20:41, Florian Boucault wrote:
> The output of strace shows that fontconfig opens those file during
> every QML application startup.
>
> A solution to this would be to introduce a caching system: after
> fontconfig opens and parses those files the first time it could
> serialize the result to disk for subsequent starts.

From "man fc-cache":

<quote>
fc-cache scans the font directories on the system and builds font information cache files for applications using fontconfig for their font handling.

If directory arguments are not given, fc-cache uses each directory in the current font configuration. Each directory is scanned for font files readable by FreeType. A cache is created which contains properties of each font and the associated filename. This cache is used to speed up application startup when using the fontconfig library.
</quote>

Maybe there is a simple solution available.

I tried fc-cache, but starting a qml app still continues to open up the conf files.

Digging through Qt source to understand where these calls are coming from.

From the attached backtrace, It seems that Qt calls FcInitReinitialize when populating its font database (a QGenericUnixFontDatabase instance provided by Qtubuntu) - that explains why re-parses the conf files everytime.

See http://code.qt.io/cgit/qt/qtbase.git/tree/src/platformsupport/fontdatabases/fontconfig/qfontconfigdatabase.cpp?h=v5.4.1#n439

So actually, fontconfig does not cache conf files at all... it caches fonts but not conf files...

fontconfg needs support to also cache the result of the parsing of the xml config file parsing. It seems a low priority effort for less than a 35ms improvement in startup.

Also FcInitReinitialize and FcInitBringUptoDate both will do the same thing the first time is called in the process, so wouldn't help for startup either.

Status changed to 'Confirmed' because the bug affects multiple users.