Types Of Scheduling

In mono-tasking operating systems the issue of scheduling is trivial: after the system has set up the execution environment of a process, CPU control is given to it until the process itself exits. The system is not operating at all during the program's execution, save for providing services through subroutine calls. It's only with multi-tasking operating systems that scheduling becomes a top entry in a designer's agenda.

In many multitasking systems the processor scheduling subsystem operates on three levels, differentiated by the time scale at which they perform their operations. 

In this sense we differentiate among:

• Long term scheduling: which determines which programs are admitted to the system for execution and when, and which ones should be exited.
• Medium term scheduling: which determines when processes are to be suspended and resumed.
• Short term scheduling (or dispatching): which determines which of the ready processes can have CPU resources, and for how long.

Taking into account the states of a process, and the time scale at which state transition occur, we can immediately recognise that

• dispatching affects processes
  • running
  • ready
  • blocked
• the medium term scheduling affects processes
  • ready-suspended
  • blocked-suspended
• the long term scheduling affects processes
• new
• exited
Disediakan Oleh: Che Nurlailatul Ashikin