[FFmpeg-user] very low cpu usage when ffmpeg include "-re" option

[Huang Soho]

Hi All,

the command I use to stream output:
ffmpeg -sn -re -f video4linux2 -r 30 -s 1280x720 -input_format mjpeg -i
/dev/video0 -vcodec copy http://localhost:8090/feed1.ffm > /var/log/ffmpeg1
2>&1 &

I see the frame rate about 5fps only, cpu usage is about 10%,
but I set 30fps for target frame rate.
if I remove "-re" option, then the output is about 30fps. but it will
occupy a lot of cpu time (about 95%) when ffserver output the video stream.

Why "-re" option cause ffmpeg use very low cpu time?
it is very strange!
if I remove "-re" option, then ffmpeg+ffserver will occupy most of cpu time,
If I add "-re" option, then ffmpeg+ffserver will NOT use most of cput time,
Does any one have idea?
how to get the properly frame rate output and cpu usage with
ffserver+ffmpeg?