monitor cpu load and launch my parallel task when it is light

Dear unix shell group members,

I want to have a script which can monitors the cpu load of a server
which has 8 processors. At the same time, I want to benchmark one
parallel program which will fork 8 processes. I don't want to do it
when the machine is busy because the bench mark will be inaccurate.
What I want is that I squeeze in my program when the 8 cpus are
relatively idle on the server.

I am pretty novice to shell scripting but any pointers is greatly

Thank you,