Process Scheduling

REPRESENTATION OF PROCESS SCHEDULING

Scheduling Queues

• Job queue – set of all processes in the system.
• Ready queue – set of all processes residing in main memory, ready and waiting to execute.
• Device queues – set of processes waiting for an I/O device.
• Processes migrate between the various queues.
  
  
  

Schedulers

• Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue.
• Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.
• Short-term scheduler is invoked very frequently (milliseconds) fi (must be fast).
• Long-term scheduler is invoked very infrequently (seconds, minutes) fi (may be slow).
• The long-term scheduler controls the degree of multiprogramming.
  
• Processes can be described as either:
  -I/O-bound process – spends more time doing I/O than computations, man many short CPU bursts.
  -CPU-bound process – spends more time doing computations; few very long CPU bursts.
  
  
  

Context Switch

• When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process.
• Context-switch time is overhead; the system does no useful work while switching.
• Time dependent on hardware support.