https://bugs.freedesktop.org/show_bug.cgi?id=102683
Bug ID: 102683 Summary: mpv confuses the frequency scaling, leading to freqyency flapping and missed vsyncs Product: DRI Version: unspecified Hardware: x86-64 (AMD64) OS: Linux (All) Status: NEW Severity: normal Priority: medium Component: DRM/AMDgpu Assignee: dri-devel@lists.freedesktop.org Reporter: bugs.freedesktop@haasn.xyz
When rendering e.g. 24 Hz video on 60 Hz, `mpv`'s usage pattern consists of one “fresh” frame (e.g. 20ms rendering time) followed by two “redraw” 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 leads to bad performance (under-scaling), inconsistent performance (“flapping” 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 reported `GPU Load:` varies between 0% and 100% from frame to frame. I can consistently 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 (Note: 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 this user.
I expect the solution would be adding a tiny bit of (top-weighted) smoothing of this performance state / GPU load estimation across frames. The nvidia driver, for example, gets this right: If I alter between 'high' performance and 'low' performance states, it recognizes that and sticks to 'high' performance mode, instead of varying the frequency wildly like amdgpu. This results in very flat 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 560.