When rendering e.g. 24 Hz video on 60 Hz, `mpv`'s usage patter=
n consists of one
=E2=80=9Cfresh=E2=80=9D frame (e.g. 20ms rendering time) followed by two =
=E2=80=9Credraw=E2=80=9D frames, each
of which are essentially just blits/mixes of already-rendered frames. This
results in a high-low-low-high-low GPU activity pattern.
Apparently this confuses the GPU frequency scaling quite heavily, which lea=
ds
to bad performance (under-scaling), inconsistent performance (=E2=80=9Cflap=
ping=E2=80=9D SCLK)
and missed vblanks (delayed/dropped frames and vsync jitter as measured by
mpv). If I `watch -n0.1 cat /sys/kernel/debug/dri/*/amdgpu_pm_info`, I can =
see
it varying wildly between `SCLK: 500 MHz` and `SCLK: 1000 MHz`, as the repo=
rted
`GPU Load:` varies between 0% and 100% from frame to frame. I can consisten=
tly
solve the issue by setting `power_dpm_force_performance_level` to `high`.
A graphical explanation of the issue, with mpv performance graphs:
`auto`: https://0x0.st/7qz.jpg
`high`: https://0x0.st/7qi.jpg
This is not just cosmetic, since it results in an increase in the number of
missed vsyncs, due to the occasional spikes. I've also had a different user
report significantly worse performance with 'auto', even more extreme than =
my
example: https://0x0.st/7Aw.jpg (Not=
e: the third step in this image, going from
10k ms to 5k ms is due to switching from mesa 17.2 to 17.1; but that's an
unrelated, cosmetic bug). Using 'auto' makes mpv completely unusable for th=
is
user.
I expect the solution would be adding a tiny bit of (top-weighted) smoothin=
g of
this performance state / GPU load estimation across frames. The nvidia driv=
er,
for example, gets this right: If I alter between 'high' performance and 'lo=
w'
performance states, it recognizes that and sticks to 'high' performance mod=
e,
instead of varying the frequency wildly like amdgpu. This results in very f=
lat
graphs, much like the 'high' screenshot I uploaded.
Kernel version is 4.12.4, mesa version is 17.2.0, device is a Sapphire RX 5=
60.
![https://0x0.st/7qz.jpg](3D"https://0x0.st/7qz.jpg"){kind=link}
![https://0x0.st/7qi.jpg](3D"https://0x0.st/7qi.jpg"){kind=link}
![https://0x0.st/7Aw.jpg](3D"https://0x0.st/7Aw.jpg"){kind=link}