[FFmpeg-user] Multiple outputs worse performance than multiple instances

[Carl Lindqvist]

Hello

I am trying to optimize my encodings. I have a single input file and
transcode it to multiple bitrates with FFmpeg using x264 as codec. At
the moment I am starting one instance of FFmpeg for each output. This
seemed wasteful, so when I learned of the "multiple outputs"
(https://trac.ffmpeg.org/wiki/Creating%20multiple%20outputs) in FFmpeg
I wanted to try that. My results are very bad. Performance is much
lower than starting multiple instances. I have tried putting -threads
here and there in the command without getting good cpu usage.

The problem is that cpu usage is hovering at 15-20% (machine has 12
cores, 24 threads with hyperthreading). When I run multiple instances
I get 98-100% utilization and much higher encode speeds.

I would think running one input (and therefore one
read/decode/deinterlace) should be much faster. Does anyone have any
experience with this? Is it a bug or am I simply doing it wrong?

Best regards
Carl Lindqvist