FFmpeg – Default Number of Threads Used

The E5-2650 has 8 cores and can process 2 threads per code. That is 16 threads per CPU. Not 16 per core.

Two CPUs have a total of 32 threads. Assuming the rest of the computer is fast enough to keep feeding the CPU then the optimal use would be near 32 threads. In praxis this is not true, there is always some other delay, either when reading from memory, when reading or writing to disk or from somewhere else. So optimal usage is not always the same as the maximum number of usable threads.

In your case, 72 threads are probably used because some got started, then had to wait for other data. Meanwhile, other threads can continue. So there are actually more threads active that can be computed on at the same time.

The option flag you want is really just -threads and you would use it like this (for just one thread):

ffmpeg -i somefile.wmv -c:a libfdk_aac -c:v libx264  -threads 1 transcoded.mp4

However, there are quite a few subtleties that will raise your server load and ops time, such as rescaling, applying filters and final frame quality / frame rate - not to mention the fact that some VM architectures actually read and write everything twice (once natively and once virtually!!!)

Here are a few tips to get your speed up:

1. use a queue, so that only one item is ever being transcoded at a time
2. request smaller files from your users
3. use the full horsepower of your machine by:
   • reading and writing from a ramdisk
   • switching to bare metal for transcoding tasks
   • use -threads 0

Whatever you do, keep your users informed about the transcoding process, because it just takes time. (I.J.T.T.)

[edited command to reflect LordNeckbeard's comment]