[Libav-user] Timing problems while decoding

[stefan berke]

Hello there,

i'm writing a high-quality video player using ffmpeg. Basic functioning 
is: a worker thread pulls video frames from libavcodec/format and 
buffers them. A window thread uploads the frames (YUV420) to the GPU and 
displays them. This works at least up to 4k*4k at 60hz on good hardware.
It's most important to maintain a constant refresh rate of the window to 
avoid tearing and flickering. Normally as long as the decoder is fast 
enough there should not be any problem. However, the timing of the 
window refresh is badly affected when the decoder (h264/h265) is 
running. Even if it's run as a separate application. I just can't have 
constant (eg 60hz) update (regardless of the graphics content) when 
ffmpeg is working on the same machine.
The problem appears on Windows and (not as much but still) on Linux.
It happens in forced vsync mode (through Nvidia-Settings) or with 
handmade timing using a high-precision spin-wait (using 
QueryPerformanceCounter [win] or clock_gettime(CLOCK_MONOTONIC..) [linux]).
Systems tried: Ubuntu 14.04, Windows 7 & 10, I7 and Xeon CPUs and 
various Nvidia Quadro cards.
The decoder does not use the GPU.
I also reduced the AVCodecContext::thread_count which does not help. 
Reducing to 1 makes the fluctuation in timing less pronounced but still 
visible. So both GPU and CPU are not fully loaded but the timing is 
affected.
Running dummy tasks (like calculating std::sin()) on all available 
processors does not influence the timing at all.
Has anyone an idea what ffmpeg is doing internally that affects the 
screen refresh?

Thanks in advance,
Stefan