1. CMPT 300: Operating Systems I Ch 5: CPU Scheduling Dr. Mohamed Hefeeda
School of Computing Science
Simon Fraser University
CMPT 300: Operating Systems I
Ch 5: CPU Scheduling
Dr. Mohamed Hefeeda

2. Chapter 5: Objectives Understand Scheduling Criteria
Scheduling Algorithms
Multiple-Processor Scheduling

3. Basic Concepts Maximum CPU utilization obtained with multiprogramming
CPU–I/O Burst Cycle
Process execution consists of a cycle of CPU execution and I/O wait
How long is the CPU burst?

4. CPU Burst Distribution
CPU bursts vary greatly from process to process and from computer to computer
But, in general, they tend to have the following distribution (exponential)
Few long bursts
Many short bursts

5. CPU Scheduler Selects one process from ready queue to run on CPU
Scheduling can be
Nonpreemptive
Once a process is allocated the CPU, it does not leave unless:
it has to wait, e.g., for I/O request or for a child to terminate
it terminates
Preemptive
OS can force (preempt) a process from CPU at anytime
Say, to allocate CPU to another higher-priority process
Which is harder to implement? and why?
Preemptive is harder: Need to maintain consistency of
data shared between processes, and more importantly, kernel data structures (e.g., I/O queues)
Think of a preemption while kernel is executing a sys call on behalf of a process (many OSs, wait for sys call to finish)

6. Dispatcher Scheduler: selects one process to run on CPU
Dispatcher: allocates CPU to the selected process, which involves:
switching context
switching to user mode
jumping to the proper location (in the selected process) and restarting it
Dispatch latency – time it takes for the dispatcher to stop one process and start another
How would scheduler select a process to run?

7. Scheduling Criteria CPU utilization – keep the CPU as busy as possible
Maximize
Throughput – # of processes that complete their execution per time unit
Turnaround time – amount of time to execute a particular process (time from submission to termination)
Minimize
Waiting time – amount of time a process has been waiting in ready queue
Response time – amount of time it takes from when a request is submitted until the first response is produced

8. Scheduling Algorithms
First Come, First Served
Shortest Job First
Priority
Round Robin
Multilevel queues
Note: A process may have many CPU bursts, but in the following examples we show only one for simplicity

9. First-Come, First-Served (FCFS) Scheduling
Process Burst Time
P1 24
P2 3
P3 3
Suppose processes arrive in order: P1 , P2 , P3 The Gantt Chart for the schedule is:
Waiting time for P1 = 0; P2 = 24; P3 = 27
Average waiting time: ( )/3 = 17 P1 P2 P3 24 27 30

10. FCFS Scheduling (cont’d)
Suppose processes arrive in the order
P2 , P3 , P1
3, 3, 24
The Gantt chart for the schedule is:
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: ( )/3 = 3
Much better than previous case
Convoy effect: short process behind long process P1 P3 P2 6 3 30

11. Shortest-Job-First (SJF) Scheduling
Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time
Two schemes:
nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst
preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is known as the Shortest-Remaining-Time-First (SRTF)
SJF is optimal – gives minimum average waiting time for a given set of processes

12. Example: Non-Preemptive SJF
Process Arrival Time Burst Time P P P P
SJF (non-preemptive)
Average waiting time = ( )/4 = 4 P1 P3 P2 7 3 16 P4 8 12

13. Example: Preemptive SJF
Process Arrival Time Burst Time P P P P
SJF (preemptive, SRJF)
Average waiting time = ( )/4 = 3 P1 P3 P2 4 2 11 P4 5 7 16

14. Estimating Length of Next CPU Burst
Can only estimate the length
Can be done by using the length of previous CPU bursts, using exponential averaging

15. Exponential Averaging
If we expand the formula, we get:
n+1 =  tn +  (1 - ) tn -1 +  (1 -  )2 tn … + (1 -  )n +1 0
Examples:
 = 0 ==> n+1 = n ==> Last CPU burst does not count (transient value)
 =1 ==> n+1 =  tn ==> Only last CPU burst counts (history is stale)

16. Prediction of CPU Burst Lengths: Expo Average
Assume  = 0.5, 0 = 10

17. Priority Scheduling
A priority number (integer) is associated with each process
CPU is allocated to the process with the highest priority (smallest integer  highest priority)
Preemptive
nonpreemptive
SJF is a priority scheduling where priority is the predicted next CPU burst time
Problem: Starvation – low priority processes may never execute
Solution?
Aging: Increase the priority of a process as it waits in the system

18. Round Robin (RR)
Each process gets a small unit of CPU time (time quantum), usually milliseconds
After this time elapses, the process is preempted and added to end of ready queue
If there are n processes in ready queue and time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once
No process waits more than (n-1)q time units
Performance
q large  FCFS
q small  too much overhead, because many context switchess

19. Example of RR with Time Quantum = 20
Process Burst Time
P1 53
P2 17
P3 68
P4 24
The Gantt chart is:
Typically, higher average turnaround than SJF, but better response P1 P2 P3 P4 20 37 57 77 97
117
121
134
154
162

20. Time Quantum and Context Switch Time
Smaller q  more responsive but more context switches (overhead)

21. Turnaround Time Varies With Time Quantum
Turnaround time varies with quantum, then stabilizes
Rule of thumb for good performance:
80% of CPU bursts should be shorter than time quantum

22. Multilevel Queue Ready queue is partitioned into separate queues:
foreground (interactive)
background (batch)
Each queue has its own scheduling algorithm
foreground – RR
background – FCFS
Scheduling must be done between queues
Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation
Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; e.g.,
80% to foreground in RR
20% to background in FCFS

23. Multilevel Queue Scheduling

24. Multilevel Feedback Queue
A process can move between various queues
aging can be implemented this way
Multilevel-feedback-queue scheduler defined by the following parameters:
number of queues
scheduling algorithms for each queue
method to determine when to upgrade a process
method to determine when to demote a process
method to determine which queue a process will enter when that process needs service

25. Example of Multilevel Feedback Queue
Three queues:
Q0 – RR with time quantum 8 milliseconds
Q1 – RR time quantum 16 milliseconds
Q2 – FCFS
Scheduling
A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.
At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2

26. Multilevel Feedback Queues
Notes:
Short processes get served faster (higher prio)  more responsive
Long processes (CPU bound) sink to bottom  served FCFS  more throughput

27. Multiple-Processor Scheduling
Multiple processors ==> divide load among them
More complex than single CPU scheduling
How to divide load?
Asymmetric multiprocessor
One master processor does the scheduling for others
Symmetric multiprocessor (SMP)
Each processor runs its own scheduler
One common ready queue for all processors, or one ready queue for each
Win XP, Linux, Solaris, Mac OS X support SMP

28. SMP Issues Processor affinity Load balancing
When a process runs on a processor, some data is cached in that processor’s cache
Process migrates to another processor ==>
Cache of new processor has to be re-populated
Cache of old processor has to be invalidated ==>
Performance penalty
Load balancing
BAD: One processor has too much load and another is idle
Balance load using
Push migration: A specific task periodically checks load on all processors and evenly distributes it by moving (pushing) tasks
Pull migration: Idle processor pulls a waiting task from a busy processor
Some systems (e.g., Linux) implement both

29. SMP Issues (cont’d)
Tradeoff between load balancing and processor affinity: what would you do?
May be, invoke load balancer when imbalance exceeds threshold

30. Real-time Scheduling Hard-real time systems Soft-real time systems
A task must be finished within a deadline
Ex: Control of spacecraft
Soft-real time systems
A task is given higher priority over others
Ex: Multimedia systems

31. Operating System Examples
Windows XP scheduling
Linux scheduling

32. Windows XP Scheduler Priority-based, preemptive scheduler
The highest-priority thread will always run
32 levels of priorities, each has a separate queue
Scheduler traverses queues from highest to lowest until it finds a thread that is ready to run
Priorities are divided into classes:
Real-time class (fixed): Levels 16 to 31
Other classes (variable): Levels 1 to 15
Priority may change (decrease or increase)

33. Windows XP Scheduler (cont’d)
Processes are typically created as members of NORMAL_PRIORITY_CLASS
It gets base (NORMAL) priority of that class
Priority decreases
After thread’s quantum time runs out
but never goes below the base (normal) value of its class
Limit CPU consumption of CPU-bound threads
Priority increases
After a thread is released from a wait operation
Bigger increase if thread was waiting for mouse or keyboard
Moderate increase if it was waiting for disk
Also, active window gets a priority boost
Yield good response time

34. Windows XP Scheduler (cont’d)
Win 32 API defines
several priority classes, and
within each class several relative priorities
Priority class

35. Linux Scheduler
Priority-based, preemptive scheduler with two separate ranges
Real-time: 0 to 99
Nice: 100 to 140
Higher priority tasks get larger quanta (unlike Win XP, Solaris)

36. Linux Scheduler (cont’d)
A task is initially assigned a time slice (quantum)
Runqueue has two arrays: active and expired
A runnable task is eligible for CPU if it has time left in its time slice
If time slice runs out, the task is moved to the expired array
Priority increase/decrease may occur before adding to expired array
When there are no tasks in the active array, the expired array becomes the active array and vice versa (change of pointers)

37. Algorithm Evaluation Deterministic modeling Queuing models Simulation
Take a particular predetermined workload and define the performance of each algorithm for that workload
Not general
Queuing models
Use queuing theory to analyze algorithms
Many (unrealistic) assumptions to facilitate analysis
Simulation
Build a simulator and test
synthetic workload (e.g., generated randomly), or
Traces collected from running systems
Implementation
Code it up and test!

38. Summary Process execution: cycle of CPU bursts and I/O bursts
CPU bursts lengths: many short bursts, and few long ones
Scheduler selects one process from ready queue
Dispatcher performs the switching
Scheduling criteria (usually conflicting)
CPU utilization, waiting time, response time, throughput, …
Scheduling Algorithms
FCFS, SJF, Priority, RR, Multilevel Queues, …
Multiprocessor Scheduling
Processor affinity vs. load balancing
Evaluation of Algorithms
Modeling, simulation, implementation
Examples: Win XP, Linux