mpv has a problem with 3D Render Engine in VAAPI

[Heinz Assmus]

When I check hardware acceleration in mpv for intel graphics drivers using intel-gpu-tools, it seems like mpv has a problem with the 3d render engine in the vaapi module.
I use Ubuntu MATE 20.04 with Compiz as a window manager (used plugins Scale, Expo, Desktop Wall, etc.). When I work with several virtual desktops, the load on the 3D rendering engine of the Intel driver drops when I switch to a different desktop than the one on which mpv is active.
In the Scale plug-in (overview of all windows on all workspaces) the graphics output then stutters in the reduced video image from mpv. In a normal HD video (1280x720) the load drops from approx. 32% to approx. 8%. The video engine (hardware acceleration) has no errors.
Occupancy continuously at 4% to 6%.

Mpv works perfectly under Nvidia graphics (no stuttering graphic image in the reduced mpv video window with an overview of all windows on all workspaces).

Gstreamer (Ver. 1.18) works fine with gst-play-1.0 on vaapi and nvidia-vdpau. There is no drop in the utilization of the 3D rendering engine with an overview of all windows on all workspaces in the Scale plug-in. (Checked with intel_gpu_top at Intelgraphics).
The graphics output runs without stuttering in the reduced vlc window and the Utilization in 3D Render Engine is continuously around 33% throughout.

This issue exist already a long time before up to now and in other distros and flavors with Compiz.

snapshot from intel_gpu_top with gst-play-1.0
Render/3D/0 Engine is continously used when switching to different workspaces from active vs.
Scaled video window shows normal (fluently) scaled video in overview of all windows of all workspaces.

intel-gpu-top - 208/ 208 MHz; 35% RC6; 0.86 Watts; 137 irqs/s

      IMC reads: 0 MiB/s
     IMC writes: 677 MiB/s

          ENGINE BUSY MI_SEMA MI_WAIT
     Render/3D/0 37.98% |█████████████▎ | 0% 0%
       Blitter/0 0.00% | | 0% 0%
         Video/0 13.00% |████▌ | 0% 0%
  VideoEnhance/0 0.00% | | 0% 0%

snapshot from intel_gpu_top with mpv
Render/3D/0 Engine breaks down when switched from active workspace to another workspace where mpv is not active.
scaled video is stuttering when all windows of all workspaces are shown in overview!

intel-gpu-top - 420/ 421 MHz; 48% RC6; 0.47 Watts; 50 irqs/s

      IMC reads: 0 MiB/s
     IMC writes: 201 MiB/s

          ENGINE BUSY MI_SEMA MI_WAIT
     Render/3D/0 3.00% |█ | 0% 0%
       Blitter/0 0.00% | | 0% 0%
         Video/0 6.49% |██▎ | 0% 0%
  VideoEnhance/0 0.00% | | 0% 0%

^C

snapshots not at the exactly same position in the video!

My machine:

tux@tux-Aspire-V5-573G:~$ neofetch
            .-/+oossssoo+/-. tux@tux-Aspire-V5-573G
        `:+ssssssssssssssssss+:` ----------------------
      -+ssssssssssssssssssyyssss+- OS: Ubuntu 20.04.2 LTS x86_64
    .ossssssssssssssssssdMMMNysssso. Host: Aspire V5-573G TBD by
   /ssssssssssshdmmNNmmyNMMMMhssssss/ Kernel: 5.4.0-74-generic
  +ssssssssshmydMMMMMMMNddddyssssssss+ Uptime: 2 hours, 10 mins
 /sssssssshNMMMyhhyyyyhmNMMMNhssssssss/ Packages: 2927 (dpkg), 12 (snap)
.ssssssssdMMMNhsssssssssshNMMMdssssssss. Shell: bash 5.1.8
+sssshhhyNMMNyssssssssssssyNMMMysssssss+ Resolution: 1920x1080
ossyNMMMNyMMhsssssssssssssshmmmhssssssso DE: MATE
ossyNMMMNyMMhsssssssssssssshmmmhssssssso WM: Compiz
+sssshhhyNMMNyssssssssssssyNMMMysssssss+ WM Theme: Radiant-MATE
.ssssssssdMMMNhsssssssssshNMMMdssssssss. Theme: Mojave-light [GTK2/3]
 /sssssssshNMMMyhhyyyyhdNMMMNhssssssss/ Icons: Radiant-MATE [GTK2/3]
  +sssssssssdmydMMMMMMMMddddyssssssss+ Terminal: mate-terminal
   /ssssssssssshdmNNNNmyNMMMMhssssss/ Terminal Font: Ubuntu Mono 13
    .ossssssssssssssssssdMMMNysssso. CPU: Intel i5-4200U (4) @ 2.600GHz
      -+sssssssssssssssssyyyssss+- GPU: NVIDIA GeForce GT 750M
        `:+ssssssssssssssssss+:` GPU: Intel Haswell-ULT
            .-/+oossssoo+/-. Memory: 2222MiB / 7845MiB