[FFmpeg-user] Trying to understand ffmpeg cpu usage in this case of capturing rtsp streams

[Jim Ruxton]

I'm using Ubuntu 22.04 and ffmpeg version 4.4.2-0

I'm capturing 3 rtsp streams as shown at the bottom .
Stream A resolution is 1600x1200
Stream B and Stream C resolution is 384x192 .
Stream A and B are coming from one device (ip address) and Stream C is
coming from a second device (ip address)

When I start Stream A and capture it with the command below I get a cpu
usage of around 90% using the top command. If I then start stream B
simultaneously it shows about 25% cpu usage in another ffmpeg instance.

Then with the other 2 streams running I start Stream C and it shows about
90% cpu usage.

If I start Stream B on it's own it shows 90% cpu usage.

In other words it doesn't seem to matter what the resolution of the video
coming in is but rather which stream starts first that determines the cpu
usage. Does it matter that Stream A an B are coming from the same ip
address and that explains why the second stream uses less cpu? My
expectation is that Stream B and C would have the same cpu usage since they
are the same resolution just coming from different devices.  Can anyone
explain what I see as strange behaviour. Stream commands follow.


Stream A: ffmpeg -f rtsp -i rtsp://192.168.35.243:8086/?camera=world -f
v4l2 /dev/video2

Stream B: ffmpeg -f rtsp -i rtsp://192.168.35.243:8086/?camera=eyes -vf
format=yuv420p  -f v4l2 /dev/video3

Stream C: ffmpeg -f rtsp -i rtsp://192.168.35.247:8086/?camera=eyes -vf
format=yuv420p  -f v4l2 /dev/video5

Thanks,
Jim