Bug#909818:

[Kochin Chang]

I believe this issue is caused by a new behaviour in fontconfig 2.13.1, which removes .uuid file from directories containing no font files after scanned them. On my computer, because of that behaviour and interactions between xfsettingsd and fontconfig, system performance takes a big hit. Running Firefox is almost impossible.

What happens seems to start with xfsettingsd using g_file_monitor to watch font directories. When any of the font folders are changed, it gets notified and in turn tells fontconfig to rescan those directories. Now, due to the new behaviour in fontconfig 2.13.1, the .uuid file is deleted and the modification time is changed for those font directories containing no fonts or only symbolic links after scanned by fontconfig. Since the modification time changed, xfsettingsd gets notified and the cycle begins.

It's possible Firefox has similar interactions with fontconfig and suffers the same performance impact.

A patch has been committed to the master branch of fontconfig so that it doesn't remove .uuid from directories without any fonts. But, there is no binary release yet. See https://gitlab.freedesktop.org/fontconfig/fontconfig/commit/5f12f564f8748deaa603adb7a4b8f616b6390ad4
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