Font-related slowness after update to Qt6

[Aster89]

Since I updated qutebrowser last time (to Qt6) I find that when I type keys after :open [-t] , the reaction of the filtering is extremely slow, eventaully the browser visually freezes for several seconds, and it must eat so much RAM, because I hear the vent spinning more loudly.

With the help of #7850, I manged to attribute this performance regression to some of my settings. Specifically, these (which I honestly don't even remember why I put there; proabaly I wanted to show emojis somwhere, who knows 😄)

# in $HOME/.config/qutebrowser/config.py
# ...
c.fonts.default_family = ['DejaVuSansMono Nerd Font Mono', 'xos4 Terminus', 'Terminus']
# ...

# in $HOME/.config/qutebrowser/autoconfig.yml
  # ...
  fonts.default_family:
    global:
    - DejaVuSansMono Nerd Font Mono
    - xos4 Terminus
    - Terminus
  # ...

If I comment out those lines, everything seems to be better (probably as far as before, even if I don't really feel like that, probably because my perception has been altered by the frustrating slowness I've observed in the last few days).

Is this behavior normal? If not, why am I observing it?

[toofar]

Damn you lead me on a diversion looking into (#2812) (my conclusion on that so far is that drawing cell-by-cell, and having to make styling/highlighting decisions within the cells, with all the PyQt indirection, just doesn't scale - although if we could make it async somehow that would get rid of a lot of the pain).

No, in my opinion this isn't normal. I couldn't reproduce it myself, but it's probably setup dependant. I installed "DejaVuSansM Nerd Font Mono" and switched that that and didn't notice any difference in speed. (And I was profiling stuff!)

Some debugging questions, if you are willing to indulge me:

1. can you tell which of those fonts it is causing the issue?
2. can you do fc-match 'the font name' to see what it actually resolves too?
3. do you feel up to doing a py-spy record while you are banging away at the completion and seeing if that shows anything?
4. you can see what the default font is by running :debug-console and putting from qutebrowser.config import configutils; configutils.FontFamilies.from_system_default() in there (for me it just says 'monospace', which fc-match can resolved to a real font) - there might be a better way, idk

Other tips for just plain slow completion:

• bump completion.height down a bit so there are less rows for the poor thing to draw
• set completion.delay so it gets to take a rest between keypresses

[Aster89]

Currently I'm on 6.5.2:

qutebrowser v3.0.0
Git commit: 
Backend: QtWebEngine 6.5.2, based on Chromium 108.0.5359.220 (from api)
Qt: 6.5.2

CPython: 3.11.3
PyQt: 6.5.2

Qt wrapper info:
  PyQt6: success
  PyQt5: not imported
  -> selected: PyQt6 (via autoselect)

colorama: no
jinja2: 3.1.2
pygments: 2.16.1
yaml: 6.0.1
adblock: no
objc: no
PyQt6.QtWebEngineCore: 6.5.0
PyQt6.sip: 6.7.10
pdf.js: 3.7.107 (/usr/share/pdf.js/build/pdf.js)
sqlite: 3.42.0
QtNetwork SSL: OpenSSL 3.1.2 1 Aug 2023

Style: QFusionStyle
Platform plugin: xcb
OpenGL: Intel, 4.6 (Compatibility Profile) Mesa 23.1.6
Platform: Linux-6.4.11-arch2-1-x86_64-with-glibc2.38, 64bit
Linux distribution: Arch Linux (arch)
Frozen: False
Imported from /usr/lib/python3.11/site-packages/qutebrowser
Using Python from /usr/bin/python3
Qt library executable path: /usr/lib/qt6, data path: /usr/share/qt6

Paths:
cache: /home/enrico/.cache/qutebrowser
config: /home/enrico/.config/qutebrowser
data: /home/enrico/.local/share/qutebrowser
runtime: /run/user/1000/qutebrowser
system data: /usr/share/qutebrowser

Autoconfig loaded: yes
Config.py: /home/enrico/.config/qutebrowser/config.py has been loaded
Uptime: 1 day, 16:58:51

[toofar]

Do you mind doing another experiment for me? To see if long strings of emojis are a contributing factor. From #7872 (comment)

1. set fonts.default_family to "xos4 Terminus" again
2. put the completion to :open dock and see if it reproduces the issue - hopefully it doesn't, maybe try in a temp basedir if it does
3. load https://github.com/cirosantilli/china-dictatorship
4. put the completion to :open dock and see if it reproduces the issue now

[Aster89]

What!? :open dock? For me that searches the word dock on DuckDuckGo.

Also what do you mean at all by "put the completion to :open dock"? Do you mean I should type the 10 keys :open dock and see if the slowdown is observable while I type? Or after that?

Can you clarify?

[toofar]

type the 10 keys :open dock and see if the slowdown is observable while I type? Or after that?

Yes, that. Just type it and let the completion do it's thing, maybe type some more stuff after it so you can see if it's frozen. No need to press Enter.

[Aster89]

Sorry, but I really don't understand what your trying to make me do, for these reasons:

• As regards your step 2,
  • yes I notice the sluggishness when I write :open dock (for some reason I don't know, it doesn't seem as slow as I described in point ii in my first comment to your first comment, but it's kind of how I described the slowness of Terminus at point iii;
  • if I try qutebrowser --temp-basedir, I don't have history nor quick/bookmarks, so how could anything be slow?
• As regards point 3, why? Are you trying to make al those kanjis show in my history? If that's the case, then I can't observe any difference in my normal qutebrowser session, because I have many other entries in the history that have emojis and/or kanjis; and the basedir session, see point above.