Aaron Lewis wrote:
quoted text
> 	In chapter 5.3 , it talks about a schedule algorithm: SJF
> 	SJF means shortest jobs schedules firstly.
> 	I think it's impossible to let OS know exactly how long a process will
> take to run.

http://en.wikipedia.org/wiki/Shortest_job_next
"Shortest job next scheduling is rarely used outside of specialized 
environments because it requires accurate estimations of the runtime of 
all processes that are waiting to execute."

 From what I've read the scheduler tries to keep a statistic about how 
much CPU time a job used during its slice and then predicts the future 
behaviour.

This also has some ideas about scheduling:
http://www.capricorn.org/~akira/cgi-bin/scheduler/explain/scheduling.html

kind regards,
Robert