On 24-01-2022 22:41, Eric Gross wrote:
Hi all,
I'd love to understand better some of the performance differences I'm seeing in
various situations (and hopefully use that contribute some patches upstream!).
My use case I'm exploring is using ffmpeg to decode video to pass to another
process. I see dramatically different performance in a few situations that I'd
like to better understand.
My test setup is on a 5950x CPU (Windows), with hardware offload off just to
keep things simple. I'm decoding a 4k video stream (h264 (Main),
yuv420p(progressive), 3848x2168, 30 fps, 30 tbr, 1200k tbn),
Here are some quick benchmarks:
Null renderer ("-f null -") - 643 fps
Yuv420p rawvideo ("-pix_fmt yuv420p -f rawvideo - > nul") - 290 fps
Rgb8 rawvideo ("-pix_fmt rgb8 -f rawvideo - > nul") - 276 fps
Yuyv422 rawvideo ("-pix_fmt yuyv422 -f rawvideo - > nul) - 173 fps
Rgb24 rawvideo ("-pix_fmt rgb24 -f rawvideo - > nul") - 128 fps
Rgba rawvideo ("-pix_fmt rgba -f rawvideo - > nul") - 102 fps
This brought up some a few questions:
- What does the null renderer skip that costs so much once we want to get video
out? What is the native format out of x264?
- For the RGB formats, it almost seems proportional to the output data size.
I'd imagine the main calculation to generate RGB from YUV is similar, and just
the layout/bits are different. But RGB8 is 2.5x faster than RGBA, even though
one would think the extra bit shifting might add some overhead.
- RGB formats seem more expensive than YUV formats, but more than I'd expect.
Does ffmpeg use any vectorized implementations to convert between the two?
I also noticed some other things like ffmpeg being very sensitive to the
latency of the thing on the other end of the pipe. Is it possible to increase
the buffering that ffmpeg does on the output side?
Let me know if I can provide any more information. At some point I'll get out
the profiler myself, but hoped to see if I could get some insight into anything
obvious first.
Thanks!
Eric Gross
INTERNAL - NI CONFIDENTIAL
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As far as the documentation goes, the NULL renderer does not generate
output. So it only has to read (and probably decode) the input.
For all the other formats, some amount of recoding has to be done after
decoding the input stream depending on the format of the h264 stream.
Read more about that here:
https://en.wikipedia.org/wiki/YUV#Y.E2.80.B2UV420p_.28and_Y.E2.80.B2V12_or_YV12.29_to_RGB888_conversion
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