ffmpeg not using full CPU power

Question

I have a Dell Precision 490 workstation, running Ubuntu 11.10, with two dual core Xeon 5160 CPUs (so 2 physical CPUs, with 2 cores each), which I am using to encode TV recordings (MPEG2) to H264 using ffmpeg & x264. ffmpeg is encoding at approx 180fps and everything works well, but CPU usage is little low, hovering around 30% per core, when I would expect close to 100%. Anyone know why this is?

These are the inputs I'm giving ffmpeg

time ffmpeg -i ch31.m2t -s 640x360 -acodec libfaac -ac 1 -ar 44100 -b:a 56k -vcodec libx264 -preset superfast -b:v 744k ch31_superfast_800.mp4

Output of uname -a

Linux dell 3.0.0-14-generic #23-Ubuntu SMP Mon Nov 21 20:28:43 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux

Output of grep -i core /proc/cpuinfo

core id     : 0
cpu cores   : 2
core id     : 0
cpu cores   : 2
core id     : 1
cpu cores   : 2
core id     : 1
cpu cores   : 2

Output of mpstat -P ALL

Linux 3.0.0-14-generic (dell)   18/12/11    _x86_64_    (4 CPU)

15:46:52     CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest   %idle
15:46:52     all   16.37   10.29    1.20    3.46    0.00    0.13    0.00    0.00   68.54
15:46:52       0   21.43   10.91    1.74    1.06    0.00    0.39    0.00    0.00   64.47
15:46:52       1   15.64    9.95    1.08    5.29    0.00    0.07    0.00    0.00   67.97
15:46:52       2   14.65   10.29    1.03    5.88    0.00    0.03    0.00    0.00   68.11
15:46:52       3   13.79   10.03    0.97    1.59    0.00    0.02    0.00    0.00   73.60

Output of top whilst ffmpeg is running

top - 15:35:59 up  1:11,  3 users,  load average: 0.00, 0.15, 0.29
Tasks: 162 total,   2 running, 159 sleeping,   0 stopped,   1 zombie
Cpu0  : 29.9%us,  1.7%sy, 31.6%ni, 36.9%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Cpu1  : 30.4%us,  1.3%sy, 25.8%ni, 42.5%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Cpu2  : 38.7%us,  2.0%sy, 22.7%ni, 36.0%id,  0.7%wa,  0.0%hi,  0.0%si,  0.0%st
Cpu3  : 27.2%us,  1.0%sy, 27.9%ni, 43.5%id,  0.3%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   4056484k total,  3939144k used,   117340k free,    16532k buffers
Swap:  4190204k total,     9348k used,  4180856k free,  3024616k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                                                              
 3559 decrypti  20   0  321m  91m 4708 R  240  2.3  45:52.11 ffmpeg

Answer 1

Like most code, this code is not doing a good job of taking advantage of multiple cores. It may switch which core it's placing load on very rapidly (around 100 times per second), but it is only able to take full advantage of one core at a time.