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Here are 5 output lines from glxgears:

219 frames in 5.0 seconds = 43.674 FPS
201 frames in 5.0 seconds = 40.007 FPS
201 frames in 5.0 seconds = 40.006 FPS
201 frames in 5.0 seconds = 40.005 FPS
201 frames in 5.0 seconds = 40.008 FPS

Clearly it stabilized at 40 FPS (vsync is on by default, using vblank_mode=0 results in 7500+ FPS).

It, and everything else including animations and the cursor, also definitely seem like they are refreshing at less than 60Hz.

Output of xrandr:

Screen 0: minimum 320 x 200, current 1366 x 768, maximum 8192 x 8192
eDP-1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 344mm x 193mm
   1366x768      60.00*+  40.00  
   1360x768      59.80    59.96  
   1280x720      60.00    59.99    59.86    59.74  
   1024x768      60.04    60.00  
   960x720       60.00  
   928x696       60.05  
   896x672       60.01  
   1024x576      59.95    59.96    59.90    59.82  
   960x600       59.93    60.00  
   960x540       59.96    59.99    59.63    59.82  
   800x600       60.00    60.32    56.25  
   840x525       60.01    59.88  
   864x486       59.92    59.57  
   800x512       60.17  
   700x525       59.98  
   800x450       59.95    59.82  
   640x512       60.02  
   720x450       59.89  
   700x450       59.96    59.88  
   640x480       60.00    59.94  
   720x405       59.51    58.99  
   684x384       59.88    59.85  
   680x384       59.80    59.96  
   640x400       59.88    59.98  
   576x432       60.06  
   640x360       59.86    59.83    59.84    59.32  
   512x384       60.00  
   512x288       60.00    59.92  
   480x270       59.63    59.82  
   400x300       60.32    56.34  
   432x243       59.92    59.57  
   320x240       60.05  
   360x202       59.51    59.13  
   320x180       59.84    59.32  
HDMI-1 disconnected (normal left inverted right x axis y axis)

You can see that the chosen refresh rate is 60.00Hz, and 40.00Hz is an available option, but not selected.

Also in System Settings: System Settings Again, 60.00Hz selected, 40.00Hz is an available option but not selected.

When I change the resolution to 1280*720, the refresh rate changes to 120Hz and everything becomes smoother, but I know that my laptop's display is capable of 60Hz maximum, so it just gets there and doesn't refresh any faster.

Does anyone have an idea about what is happening here, and what I can do about it?

System specs:

  • Ubuntu 18.04, Linux Kernel 4.15.0-22-generic (uname -r)
  • Intel HD Graphics 520 / AMD Radeon R5 M330 (HAINAN)

Model Name: HP Notebook 15-ac141ne

EDIT:

I changed the resolution to 1360*768, which gives a refresh rate of 59.96Hz according to the xrandr output above and System Settings, then I ran glxgears, and the output was stable at 60 FPS. Also the mouse cursor movement and everything overall is definitely much smoother, which again confirms the problem. I think I will keep it at this resolution for now as losing just 6 bars of 768 pixels is much better than losing 20Hz from the refresh rate.

Another EDIT: This has been fixed in the mainstream kernel (4.19), which confirms that it is a bug in 4.15. I am not sure when exactly it was fixed between 4.15 - 4.19.

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Refresh Rate and Frames Per Second are two different things.

Refresh Rate refers to the frequency at which the pixels that make up the view on the screen are updated (i.e. Refreshed). On a static picture, the refresh is not noticeable because the newly displayed pixels are the same as previous pixels.

Frames Per Second is a measure of how often the screen picture is changed to create motion. You can think of it as the pages of cartoon drawing on paper in which each piece of paper has the drawing change slightly, so that as you flip through the pages (frames on the computer screen) you perceive motion. The faster the Frames Per Second the smoother the perceived motion.

There is another explanation here. FPS vs Hz

  • They might be different but they are related. A 60Hz refresh rate means a monitor can display a maximum of 60 frames per second, as 60Hz means refreshed 60 times a second. When vsync caps framerates at 40, then it is like the refresh rate is 40Hz. It should cap framerates at 60 for 60Hz. – Tooniis Sep 22 '18 at 17:41
  • The explanation you linked says just what I said about vsync. Vsync limits the GPU output so it outputs no more FPS than the refresh rate. In my case, glxgears with vsync on outputs no more than 40 FPS when stabilised, which is a clear sign that the actual refresh rate is 40Hz. – Tooniis Sep 23 '18 at 9:59
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    It could also be the sign of a bug rather than the refresh rate. – Chai T. Rex Sep 24 '18 at 13:09
  • A bug that affects the refresh rate possibly. Everything is refreshing at 40Hz not just glxgears, and it is obvious for me. I know that I can differentiate only using my eyes between 40Hz and 60Hz. – Tooniis Sep 24 '18 at 13:33

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