Actual Refresh Rate is 40Hz when Refresh Rate set to 60Hz in System Settings and xrandr

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: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.

1 Answer

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