1. Scheduling Introduction to Scheduling
Bursts of CPU usage alternate with periods of I/O wait
a CPU-bound process
an I/O bound process

2. When to Schedule 1. At process creation 2. At process termination
3. At blocking system calls
4. At I/O interrupt
Preemtive and non-preemtive scheduling algorithms

3. Goals of Scheduling

4. Scheduling in Batch Systems (1)
(a) First-come-first-served (FCFS)
(b) Shortest-job-first (SJF) - provably optimal if all jobs are known in advance
(c) Shortest-remaining-time-next (SRTN)

5. Scheduling in Batch Systems (2)
Three level scheduling

6. Scheduling in Interactive Systems (1)
Round Robin Scheduling
(a) list of runnable processes
(b) list of runnable processes after B uses up its quantum
Length of quantum:
too short => waste of CPU time
too long => poor response

7. Scheduling in Interactive Systems (2)
A scheduling algorithm with four priority classes
Static and dynamic priority allocations, e.g. 1/f.

8. Scheduling in Interactive Systems (3)
Multiple queues
Shortest process next; aging (weighted average)
Guaranteed scheduling (1/n)
Lottery scheduling - biased randomness, passing tickets
Fair share scheduling

9. Scheduling in Real-Time Systems
Schedulable real-time system
Given
m periodic events
event i occurs within period Pi and requires Ci seconds
Then the load can only be handled if

10. Policy versus Mechanism
Separate what is allowed to be done with how it is done
a process knows which of its children threads are important and need priority
Scheduling algorithm parameterized
mechanism in the kernel
Parameters filled in by user processes
policy set by user process

11. Thread Scheduling (1) Possible scheduling of user-level threads
50-msec process quantum, threads run 5 msec/CPU burst
Application-specific thread schedulers

12. Thread Scheduling (2) Possible scheduling of kernel-level threads
50-msec process quantum
threads run 5 msec/CPU burst

13. The UNIX scheduler is based on a multilevel queue structure

14. Scheduling in Windows (1)
Mapping of Win32 priorities to Windows 2000 priorities

15. Scheduling in Windows (2)
Windows 2000 supports 32 priorities for threads