[FFmpeg-user] High CPU usage during scale_npp to low resolutions with multiple instances

[Valentin Schweitzer]

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Hi,

when using scale_npp to scale a test video down from 1920x1080 to
1024x576 or lower with multiple processes in parallel, CPU usage is
unusually high.For context, when scaling the same video down to
1280x720, CPU usage stays at about0.5% per FFmpeg instance. When
scaling down too 1024x576 or lower, CPU usage per FFmpeg process rises
to about 3.0%. The values listed here appear when starting 29
instances of FFmpeg in parallel. The effect is less pronounced but
still visible at 10 instances in parallel. Hardware used for this
is an AMD EPYC 7401P 24 Core + NVIDIA Quadro RTX 4000.

To generate 100s of random noise in 1080p (which will be the test video):

ffmpeg -y -hide_banner -f lavfi -i nullsrc=s=1920x1080 -filter_complex 
"geq=random(1)*255:128:128;aevalsrc=-2+random(0)" -vcodec rawvideo 
-acodec pcm_s16le -t 100 noise.mkv

Now rescale the test video to 720p:

ffmpeg -hide_banner -y -i noise.mkv -vf 
hwupload_cuda,scale_npp=w=1280:h=720:format=nv12 -vcodec h264_nvenc -an 
-f null NUL

This should not cause very high CPU usage. Now rescale the same video to 
576p:

ffmpeg -hide_banner -y -i noise.mkv -vf 
hwupload_cuda,scale_npp=w=1024:h=576:format=nv12 -vcodec h264_nvenc -an 
-f null NUL

This should cause about 5 or 6 times as much CPU usage.

This might be caused by some NVIDIA optimizations, but it does not
seem to be documented and I have yet to find a good place to ask more
in-depth questions about NVIDIA encoding hardware.
So, if anyone has encountered a similar issue or knows why this issue
might occur, I would be grateful about any advice.

Greetings,
Valentin