# High CPU when idle with no diagram loaded on Linux/Wayland **URL:** \ **Category:** QCAD Troubleshooting and Problems\ **Created:** [July 28, 2026, 10:01am UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895 "2026-07-28T10:01:19Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![toddklaus](http://forum.qcad.org/letter_avatar_proxy/v4/letter/t/919ad9/32.png) [@toddklaus](http://forum.qcad.org/u/toddklaus)\ **Post date:** [July 28, 2026, 10:01am UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/1 "2026-07-28T10:01:20Z")
**Environment** - QCAD Professional 3.32.9 / Qt 6.11.0 / Linux x86\_64 / xcb (Flatpak, ‘‘org.qcad.QCAD’’) - Ubuntu, GNOME on Wayland, single 6144x3456 display, devicePixelRatio 1 - Reproduces on a completely stock configuration **Problem** With QCAD open, no drawing loaded and no user interaction, ‘‘qcad-bin’’ sits at 50-70% CPU indefinitely. Xwayland simultaneously consumes a further ~60%. The cause is a self-perpetuating repaint loop on the ruler viewport margins. QCAD reserves a 53px margin above and to the left of the graphics view for ‘‘RRulerQt’’ (‘‘HorizontalRuler’’ / ‘‘VerticalRuler’’). Those two strips are repainted approximately **2175 times per second**. **The repaints produce no visible change.** I captured both damage regions directly from the X server with GetImage: each is a single uniform colour, RGB(239,239,239), 100% of pixels, and byte-identical between captures taken one second apart. The rulers render no ticks, no labels, nothing. **Evidence** Per-iteration X11 request sequence, repeated ~2175x/sec on the main window: ` opcode 59 SetClipRectangles (2 rects) opcode 130 MIT-SHM PutImage x2 (the two ruler strips) opcode 56 ChangeGC (clip-mask -> None) opcode 43 GetInputFocus (blocking XSync round-trip) ` Damage rectangles, in main-window coordinates (window 3864x2545): - horizontal: x=266 y=302 w=3475 h=53 (above the canvas) - vertical: x=213 y=355 w=53 h=1876 (left of the canvas) ‘‘strace -c’’ over 5s: 57163 write, 45730 poll, 22866 writev, 22864 read. 2.2M voluntary context switches. Qt’s event dispatcher polls with a **0 ms timeout** continuously, i.e. the event queue is never empty and the application never goes idle. **Ruled out** (each tested independently) - Platform plugin: ~62% under both ‘’-platform xcb’’ and ‘’-platform wayland’’ - Widget style: default / ‘’-style fusion’’ / ‘’-style windows’’ → 50 / 50 / 60% - Document state: loading a drawing makes it worse (flange.dxf 60%, example01.dxf 70%) - Configuration: pristine ‘‘QCAD3.conf’’ still reproduces at 60% **Additional note on the Wayland plugin** Under ‘’-platform wayland’’ the cost shifts to the compositor and gets worse: QCAD calls ‘‘memfd\_create’’ + ‘‘ftruncate(11272664)’’ to allocate a **new 11 MB wl\_shm buffer 36 times per second** rather than reusing a pool, alongside ~1835 surface commits/sec. This pins gnome-shell at 100% CPU and makes the entire desktop unresponsive. Under xcb the damage is absorbed by Xwayland instead. **Not attached:** no DXF/DWG is needed - this reproduces on the empty “Untitled 1” document at startup. ---
**Author:** ![andrew](http://forum.qcad.org/user_avatar/forum.qcad.org/andrew/32/14926_2.png) [@andrew](http://forum.qcad.org/u/andrew)\ **Post date:** [July 28, 2026, 1:09pm UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/2 "2026-07-28T13:09:12Z")
Thanks for your detailed report. Unfortunately, I cannot reproduce any of this here (Ubuntu, 64bit Linux, Gnome Desktop, Wayland, QCAD 3.32.9). The rulers only paint when the mouse moves and, as expected, there’s virtually no CPU usage if there is no user interaction. Please indicate what exact Linux distribution and version thereof you are using. Also, do you have any “exotic” pointer devices such as a tablet, 3D mouse, touchpad, etc? One thing you could try it to set Gnome GUI scaling to 100% and disable high DPI scaling in QCAD: ```auto QT_ENABLE_HIGHDPI_SCALING=0 flatpak run org.qcad.QCAD ``` It’s a long shot, but given your high res specs, if this fixes the CPU problem, this might indicate that there’s some devicePixelRatio rounding mismatch. ---
**Author:** ![toddklaus](http://forum.qcad.org/letter_avatar_proxy/v4/letter/t/919ad9/32.png) [@toddklaus](http://forum.qcad.org/u/toddklaus)\ **Post date:** [July 28, 2026, 8:00pm UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/3 "2026-07-28T20:00:53Z")
Thanks for the response. Resetting the Gnome UI scaling (Settings → Accessibility → Text Size) to 100% (was 125%) fixes the problem; the CPU usage goes away and the rulers are drawn correctly. Is this fixable or do I need to stick with no scaling? At my resolution, UI elements are small and hard to read at 100% scaling. Thanks ===== Packaging ===== - Flatpak, ‘‘org.qcad.QCAD’’, ref ‘‘app/org.qcad.QCAD/x86\_64/master’’ - Branch ‘‘master’’, origin ‘‘qcad-origin’’, **user** installation - Commit ‘‘e05eec7f2f20948ef4a97f24519343a8463142333c388a3a775beb3088477104’’ - Build/export date 2026-05-06 - Runtime ‘‘org.freedesktop.Platform/x86\_64/25.08’’ (‘‘freedesktop-sdk-25.08.14’’) - Flatpak 1.16.6 - Sandbox permissions (default, **no user overrides applied** ): ‘‘shared=ipc; sockets=x11;wayland; devices=dri; filesystems=home;’’ ===== Operating System ===== - Ubuntu **26.04 LTS** (Resolute Raccoon), x86\_64 - Kernel ‘‘7.0.0-27-generic’’ (‘’#27-Ubuntu SMP PREEMPT\_DYNAMIC’') - Locale ‘‘en\_US.UTF-8’’ ===== Desktop Session ===== - GNOME Shell **50.1** (‘‘gnome-shell 50.1-0ubuntu1.1’’) - Session type: **Wayland** (‘‘XDG\_SESSION\_TYPE=wayland’’, ‘‘XDG\_CURRENT\_DESKTOP=ubuntu:GNOME’’) - Xwayland **24.1.10** — QCAD runs under ‘’-platform xcb’', therefore through Xwayland - ‘‘QT\_QPA\_PLATFORM’’ unset, ‘‘QT\_SCALE\_FACTOR’’ unset - GTK/GNOME ‘‘text-scaling-factor’’ = 1.0, ‘‘scaling-factor’’ = 0 ===== Display ===== - Single monitor: **ASUS PG32UCDP** on ‘‘DP-3’’ - Native mode **3840x2160 @ 59.997 Hz** - Mutter logical scale: **1.5** (fractional) → logical desktop 2560x1440 - Xwayland presents this monitor to X11 clients as **5120x2880** - Layout mode: ‘‘logical’’ **Mutter experimental features enabled:** ` ['scale-monitor-framebuffer', 'xwayland-native-scaling'] ` Both of these affect how an Xwayland client’s surface is sized, scaled and damaged, so they are likely relevant to a repaint/damage issue. ===== Hardware ===== - CPU: **AMD Ryzen 9 3900X** 12-Core / 24 threads, x86\_64 - RAM: 30 GiB total - GPU: **NVIDIA GeForce RTX 3080 Ti** , 12288 MiB - NVIDIA driver **595.71.05** - Flatpak GL extensions in use: ‘‘org.freedesktop.Platform.GL.nvidia-595-71-05’’, Mesa 26.1.4 (‘‘org.freedesktop.Platform.GL.default’’ 25.08 and 25.08-extra) ---
**Author:** ![toddklaus](http://forum.qcad.org/letter_avatar_proxy/v4/letter/t/919ad9/32.png) [@toddklaus](http://forum.qcad.org/u/toddklaus)\ **Post date:** [July 28, 2026, 8:12pm UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/4 "2026-07-28T20:12:28Z")
To be clear, increasing Settings → Accessibility \> Text Size beyond 100% causes the problem, but increasing Settings → Display → Scale does not. Scaling the UI vs just the fonts looks better anyway. ---
**Author:** ![EdbO](http://forum.qcad.org/letter_avatar_proxy/v4/letter/e/f14d63/32.png) [@EdbO](http://forum.qcad.org/u/EdbO)\ **Post date:** [August 7, 2026, 6:20pm UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/5 "2026-08-07T18:20:53Z")
OK, I noticed that when I close the open document “Untitled 1”`./qcad-bin` doesn’t show up in the list of processes anymore. When starting a new document it’s on top again. Because the rulers are at suspicion, I disabled them in “Application Preferences” → “Graphics View” → “Appearance” → “Show rulers”. After a restart `./qcad-bin` is out of the list of processes which basically means it is using 0% CPU. So there is really something going on with the rulers. For me not a big deal because I don’t use them. ---
**Author:** ![EdbO](http://forum.qcad.org/letter_avatar_proxy/v4/letter/e/f14d63/32.png) [@EdbO](http://forum.qcad.org/u/EdbO)\ **Post date:** [August 7, 2026, 6:21pm UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/6 "2026-08-07T18:21:09Z")
I just want to chime in here because I just purchased QCad Professional for GNU/Linux and noticed the same. I’m not running the Flatpak but the `qcad-3.32.9-pro-linux-qt6-x86_64.tar.gz` version. When just running `./qcad` the cpu usage of `qcad-bin` goes to 80% and `Xwayland` to 50%. The messages on the console if it is of any use: ```bash QCAD version 3.32.9 22:17:28: Debug: loading plugins... 22:17:28: Debug: RCavalierPlugin::init 22:17:28: Debug: RHelpPlugin::init 22:17:28: Debug: RJSApiPlugin::init 22:17:28: Debug: RPolygonPlugin::init 22:17:28: Debug: RProScriptsPlugin::init 22:17:28: Debug: RProxiesPlugin::init 22:17:28: Debug: plugin not initialized 22:17:28: Debug: RTracePlugin::init 22:17:28: Debug: RDwgPlugin::init 22:17:28: Debug: loading static plugins... Warning: Unimplemented code. Warning: Unimplemented code. Warning: Unimplemented code. Warning: Unimplemented code. 22:17:29: Debug: trying to load theme stylesheet: themes/Dark Grey/stylesheet.css 22:17:29: Debug: trying to load theme stylesheet: themes/Dark Grey/stylesheet_linux.css Warning: Unimplemented code. Warning: Unimplemented code. Warning: Unimplemented code. Warning: Unimplemented code. Warning: Unimplemented code. Warning: Unimplemented code. Warning: Unimplemented code. Warning: Unimplemented code. 22:17:31: Debug: loading plugins... 22:17:31: Debug: loading static plugins... 22:17:32: Debug: loading plugins... 22:17:32: Debug: loading static plugins... 22:17:32: Debug: openFiles: Warning: Unimplemented code. Warning: Unimplemented code. Warning: Unimplemented code. ``` The system I’m running is Fedora 44 with KDE Plasma 6.7.3 on Wayland. ---
**Author:** ![andrew](http://forum.qcad.org/user_avatar/forum.qcad.org/andrew/32/14926_2.png) [@andrew](http://forum.qcad.org/u/andrew)\ **Post date:** [August 9, 2026, 1:14pm UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/7 "2026-08-09T13:14:28Z")
@EdbO It’s not clear to me if this is related to this topic. Are you also using a text scale factor that is not 100%? ---
**Author:** ![andrew](http://forum.qcad.org/user_avatar/forum.qcad.org/andrew/32/14926_2.png) [@andrew](http://forum.qcad.org/u/andrew)\ **Post date:** [August 9, 2026, 1:15pm UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/8 "2026-08-09T13:15:15Z")
Bug report at: [FS#2739 : High CPU usage under Gnome with Text Size other than 100%](https://qcad.org/bugtracker/index.php?do=details&task_id=2739) ---
**Author:** ![EdbO](http://forum.qcad.org/letter_avatar_proxy/v4/letter/e/f14d63/32.png) [@EdbO](http://forum.qcad.org/u/EdbO)\ **Post date:** [August 10, 2026, 8:18am UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/9 "2026-08-10T08:18:43Z")
@andrew I haven’t made any changes to QCad. I just downloaded the `.tar.gz`, unpacked it and started `./qcad`. IMO no text scaling done. I did look into "Application Preferences” → “General” → “User Interface” and saw “Auto screen scale factor” enabled and the “Scale factor:” 1. I disabled the “Auto screen scale factor”, restarted QCad but nothing changed. Enabling / disabling the “Show rulers” shows that there is a problem there. I also want to mention that I’m using KDE and not Gnome. ---
**Author:** ![andrew](http://forum.qcad.org/user_avatar/forum.qcad.org/andrew/32/14926_2.png) [@andrew](http://forum.qcad.org/u/andrew)\ **Post date:** [August 10, 2026, 8:41am UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/10 "2026-08-10T08:41:05Z")
Okay, hard to tell if this is related to the original report or not. The original issue was fixed for the upcoming 3.33.0 release. Please let us know if the problem persists on your setup. ---
**Author:** ![EdbO](http://forum.qcad.org/letter_avatar_proxy/v4/letter/e/f14d63/32.png) [@EdbO](http://forum.qcad.org/u/EdbO)\ **Post date:** [August 10, 2026, 10:47am UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/11 "2026-08-10T10:47:43Z")
Thanks, is it already possible to download the new release or any estimate of release? ---
**Author:** ![andrew](http://forum.qcad.org/user_avatar/forum.qcad.org/andrew/32/14926_2.png) [@andrew](http://forum.qcad.org/u/andrew)\ **Post date:** [August 19, 2026, 9:42am UTC](http://forum.qcad.org/t/high-cpu-when-idle-with-no-diagram-loaded-on-linux-wayland/11895/12 "2026-08-19T09:42:11Z")
An update (QCAD 3.33) is scheduled before end of August.