1. Process Scheduling
Chapter 4

2. A schematic of scheduling

3. Scheduling-related Concepts and Terms

4. Kinds of Scheduling
Non-preemptive scheduling: A process runs to completion when scheduled
Preemptive scheduling: A process may be preempted by the kernel, and another process may be scheduled. Hence a set of processes are processed in an overlapped manner.

5. Processes for scheduling

6. Non-preemptive scheduling
Performance criteria:
Throughput
Turnaround times of processes
Weighted turnaround
Change the order in which processes are serviced
Non-preemptive scheduling policies:
- First come first served (FCFS)
- Shortest request first
Q: Which policy will provide better throughput under what conditions?

7. Scheduling using FCFS and SRN policies

8. Performance of FCFS and SRN scheduling

9. Preemptive scheduling policies
Performance criteria:
Throughput
Response time to a subrequest
Scheduling policies
Round-robin with time slicing (RR)
Least completed next (LCN)
Shortest time to go (STG): shortest remaining time
Highest response ratio (HRN): defined later

10. Round-robin with time slicing (RR)

11. Least completed next (LCN)

12. Shortest time to go (STG)

13. Scheduling using preemptive scheduling policies

14. Operation of preemptive scheduling policies

15. Performance of preemptive scheduling policies

16. Variation of average response time with time slice

17. Highest response ratio next (HRN)
Two ways of defining response ratio of a process
Response ratio = Time in the system / service time recd
Response ratio = (Time in the system + execution time)/
service time received
We use the first definition in the next slide
HRN eliminates the drawbacks of LCN and STG policies.
Q: How?

18. Performance of HRN scheduling policy

19. Long, medium and short-term scheduling

20. Event handling and scheduling

21. Long, medium and short-term scheduling in a time sharing system

22. Mechanism and policy modules of process scheduler

23. Simple priority-based scheduling

24. Priority-based scheduling

25. Priority-based scheduling
What should the kernel do if there aren’t any ready processes?
Schedule a dummy process that does nothing
Put the processor to sleep (conserves power!)

26. Round-robin scheduling

27. Ready lists in a time sharing system

28. Practical scheduling policies
Provide a good balance of response time and overhead
Vary the time slice!
Vary the priority
Provide fair service to processes (Q: How defined?)
In round-robin, all process receive approx similar service. What if one application has 5 processes, while another application has only 1 process?

29. Multilevel scheduling

30. Ready lists in a multilevel scheduler

31. Multi-level adaptive scheduling
Vary the priority of a process depending on its recent behaviour
If the process uses up its time slice, it must be (more) CPU bound than assumed, so reduce its priority
If a process is starved of CPU attention for some time, increase its priority

32. Example of multi-level adaptive scheduling

33. Fair share scheduling

34. Scheduling in Unix
Processes are assigned priorities that are integer values
A larger value implies a lower priority
Priority of a process is varied based on its CPU usage
So as not to have an LCN-like effect (starvation!), the effect of CPU time used is decays with time, hence only recent CPU usage affects priority of a process
Priority of a process
= Base priority + nice value
+ f(CPU time used by the process

35. Operation of a Unix-like scheduling policy when process perform I/O

36. Fair-share scheduling using a Unix-like policy
Groups of processes are defined for ensuring fair share of CPU time
Priority of a process depends on the CPU time used by all processes its group
Priority of a process
= Base priority + nice value
+ f(CPU time used by the process)
+ f(CPU time used by all processes in the same group)

37. Operation of a fair share scheduling

38. Real time scheduling
Perform scheduling such that each process can meet its deadline
Procedure:
Consider precedences between processes, i.e. which process should complete before which other process
Find the deadline of each application
Use a scheduling policy which ensures that deadlines are met (in a soft real time system, it is adequate to try to meet the deadlines, and succeed most of the time)

39. Real time scheduling

40. Process precedence graph (PPG) for a simple real time system

41. Deadlines for processes of previous slide
Total execution time = 25 seconds
If the deadline for the application as a whole is 35 seconds
process P6 has a deadline of 35 seconds
processes P4 and P5 have deadlines of 30 seconds
Q: What are the deadlines of other processes?

42. Feasible schedule
An application has a feasible schedule if there exists at least one sequence in which its processes can be scheduled so that deadlines of all processes are met
Justify the following statement:
“If a feasible schedule exists for an application, then earliest deadline first (EDF) scheduling can meet all deadlines.”

43. Performance of Earliest Deadline First Scheduling

44. Rate Monotonic Scheduling (RMS)
Priority of a process is inversely proportional to its periodicity
Priority-based scheduling is now used
Q: Does RMS guarantee that deadlines will be met if an application has a feasible schedule?

45. Rate Monotonic Scheduling (RMS)
Consider three processes:
P P P3
Time period
Execution time
What are their priorities?
Does a feasible schedule exist?
Consider summation of (execution time/time period)
Feasible schedule exists if summation is < 1.
Will RMS find a feasible schedule?

46. Summary of performance analysis