[FFmpeg-user] ffmpeg bottleneck, thread-safe ?

Thu Mar 25 18:04:16 EET 2021

Hello,

I share with you a performance trouble I get while transcoding some high resolution inputs.
I'm currently using a "huge" VM to transcode an hevc 8K movie to a lower bitrate to perform some tests.

The VM :
76 vCPUs (Skylake Gold 6161 v5 bi-pro 2.2Ghz) | 304 GB | cc3.19xlarge.4 | OBS CentOS 7.7

Transcoding :

*        Command :

o   /home/local/ffmpeg_build/bin/ffmpeg -i /data/testjulien/iss_ep01telco_DT_video360_h265_7680x7680_10b_250Mbps_stereo_v001.mp4 -maxrate 25M -bufsize 200M  -b:v 25000k -c:v libx265 -pix_fmt yuvj420p -c:a copy  -f hls -hls_time 4 -hls_flags independent_segments -hls_list_size 5 -strftime 1 -hls_segment_filename /data/www/httpsvmmalo/test_capa/file_%m-%d_%H-%M-%S.m4s /data/www/httpsvmmalo/test_capa/stream.m3u8

*        Version :

o   ffmpeg version 4.3.2

*        input file :

o   Stream #0:0(und): Video: hevc (Main 10) (hev1 / 0x31766568), yuv420p10le(tv, bt709/bt709/unknown), 7680x7680 [SAR 1:1 DAR 1:1],                 202195 kb/s, 30 fps, 30 tbr, 30k tbn, 30 tbc (default)

*        Output file :

o   Stream #0:0(und): Video: hevc (libx265),                                          yuvj420p(pc),                                                  7680x7680 [SAR 1:1 DAR 1:1], q=-1--1, 25000 kb/s,   30 fps, 90k tbn,              30 tbc (default)

During the transcoding job, I see a very low speed, probably processing the huge 8K source file.

frame=   32 fps=3.1 q=0.0  size=N/A time=00:00:01.53 bitrate=N/A speed=0.148x
frame=  333 fps=2.6 q=1.9  size=N/A time=00:00:11.58 bitrate=N/A speed=0.0911x
frame= 1133 fps=2.6 q=17.9 size=N/A time=00:00:38.25 bitrate=N/A speed=0.0869x
frame= 2023 fps=2.7 q=26.0 size=N/A time=00:01:07.90 bitrate=N/A speed=0.0909x
frame= 2807 fps=2.8 q=13.2 size=N/A time=00:01:34.03 bitrate=N/A speed=0.0923x

I see my machine is not totally used at a system performances point of view (see htop below) !

Questions :

*        Do you know what is limiting the transcoding speed of ffmpeg ?

o   Are some tasks monothreaded in the ffmpeg code ?

o   Are there some missing improvements in my ffmpeg command ?

o   Are FUTEX communications (shown by strace) limiting the transcoding speed?

[cid:image003.jpg at 01D72198.E24865A0]

Best regards,

[Logo Orange]<http://www.orange.com/>

Julien Gibory
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