CLOCK-DRIVEN APPROACH
As the name implies, when scheduling is clock-driven (also called time-driven), decisions on what jobs execute at what times are made at specific time instants. These instants are chosen a priori before the system begins execution. Typically, in a system that uses clock-driven scheduling, all the parameters of hard real-time jobs are fixed and known. A schedule of the jobs is computed off-line and is stored for use at run time. The scheduler schedules the jobs according to this schedule at each scheduling decision time. In this way, scheduling overhead during run-time can be minimized.
A frequently adopted choice is to make scheduling decisions at regularly spaced time instants. One way to implement a scheduler that makes scheduling decisions periodically is to use a hardware timer. The timer is set to expire periodically without the intervention of the scheduler. When the system is initialized, the scheduler selects and schedules the job(s) that will execute until the next scheduling decision time and then blocks itself waiting for the expiration of the timer. When the timer expires, the scheduler awakes and repeats these actions.
