How to benchmark the decoding of the frame by ffmpeg?

Question

[ What I have done ]

I am trying to measure a performance of different ffmpeg decoders by

• timing how long the call to function avcodec_decode_video2(..) takes in ffmpeg.c and running ffmpeg binary the following way

~/bin/ffmpeg -benchmark_all -loglevel debug -threads 0 -i ~/Documents/video-input.h264 -c:v libx265 -x265-params crf=25 video-output.hevc

• and by timing how long the same function takes in ffplay.c and running the ffplay binary the following way

~/bin/ffplay ~/Documents/video-input.h264

In my understanding the average time for the call to that function should be the same whether I am converting the video or playing it, because I am only measuring how long it takes to decode the frame for that video. Is this a wrong way of doing it? Please let me know if I am incorrect. The results I am getting are strange to me - the call to the aforementioned function takes twice as much in ffmpeg binary compared to ffplay binary. I have tried to run ffmpeg binary with -threads 0 and without it, but the results are still the same(twice as long as ffplay). Could it be because ffplay binary simply utilizes more threads? When I try it with -threads 1 , ffmpeg takes about 10 times as long as ffplay (which makes sense to me since before it was using several threads and now it is only using 1)

Before I ask my question, I want you to know that I am a beginner in video processing and video encoding/decoding processes.

[My question]

I am wondering what would be an accurate way to measure how long it takes to decode a frame (using 1 thread)? Should I simply measure only how long it takes to call avcodec_decode_video2(..) function using the ffmpeg binary, and not the ffplay binary? Would the results be more accurate that way? I also tried to enable -benchmark_all -loglevel debug options, but it seems like the following message bench: 64537 decode_video 0.0 is not very helpful if 0.0 is supposed to mean time. (Not sure what the other number means).

Answer 1

Null muxer

If you want a simple way to benchmark decoding use the null muxer :

ffmpeg -i input -f null -

Timing decoding

Linux and macOS users can add the time command:

$ time ffmpeg -i input -f null -
[...]
real    0m5.343s
user    0m20.290s
sys     0m0.230s

See man time for more info.

-benchmark option

The -benchmark option can be added to output CPU time and maximum memory consumption:

$ time ffmpeg -i input -benchmark -f null -
[...]
bench: utime=7.314s
bench: maxrss=72280kB

Choosing streams

If you want to just decode a particular stream then use the map option :

ffmpeg -i input -map 0:a:0 -f null -

Threads

You can decode with one thread if you want to:

ffmpeg -threads 1 -i input -f null -

Not all decoders have threading capabilities and some have several. You can check decoder details such as ffmpeg -h decoder=h264 .

Choosing a decoder

There can be several decoders available for a format. You can name the decoder if you don't want to rely on the default:

ffmpeg -c:v vp8 -i input -f null -
ffmpeg -c:v libvpx -i input -f null -