3.2 Process Scheduling

Objectives

1. Discuss topics related to scheduling

Notes

• The process scheduler or simply scheduler selects the next process to run from a set of ready processes.
• We know that each CPU can run one process at a time.
• They discuss two different types of processes
  • I/O bound
  • CPU bound
• Figure 3.2 page 113 shows a series of queues.
  • This is called a queuing diagram.
  • ready or runtime queue for tasks that are ready to be executed.
  • wait queue for tasks that are waiting on something.
  • The life of a process
    • It starts and is added to the ready queue.
    • At some point it is removed from the ready queue and allowed to execute
      1. It uses the allotted time
         • It is placed back into the ready queue.
      2. It needs something.
         • It is placed in the appropriate queue for the need
         • The need is fulfilled
         • The process is placed into the ready queue
    • At some time the process is finished executing
      • The PCB and all other data is released.
• The CPU scheduler is responsible for selecting a ready task to run.
  • It must
    • Remove the currently running process from the CPU
      • Save the state
        • registers, stack, heap, ...
      • Move the PCB to the correct queue.
    • Move the new process into the CPU
    • This is called a context switch.
      • See the picture page 114.
      • It is supported at various levels by the hardware
      • And may or may not involve writing to the hard drive.
  • If we write to hard disk, this is known as swapping
    • The old process is swapped out
    • The new process is swapped in.
  • Memory management is involved in this process