1. Chapter 5: Process Scheduling

2. Chapter 5: Process Scheduling
Basic Concepts
Scheduling Criteria
Scheduling Algorithms (6)
Multiple-Processor Scheduling
Thread Scheduling
OS Examples
Algorithm Evaluation

3. Basic Concepts
In a multiprogramming system, multiple processes exist concurrently in main memory.
Each process alternates between using a processor and waiting for some event to occur, such as the completion of an I/O operation.
The processor is kept busy by executing one process while the others wait.
The key to multiprogramming is scheduling.
Process Scheduling is
the basis of multi-programmed operating system
a fundamental function of operating-system.

4. Basic Concepts In a single-processor system Process scheduling is
Only one process can run at a time
Any others must wait until the CPU is free and can be rescheduled.
When the running process goes to the waiting state,
the OS may select another process to assign CPU to improve CPU utilization.
Every time one process has to wait, another process can take over use of the CPU
Process scheduling is
to select a process from the ready queue and assign the CPU
How to calculate CPU utilization?

5. Diagram of Process State from ch.3
It is important to realize that only one process can be running on any processor at any instant.
Many processes may be ready and waiting states.

6. Process Scheduling from ch.3
The process selection is carried out by the short-term scheduler (or CPU scheduler).
The scheduler selects a process from the processes in memory that are ready to execute and allocates the CPU to that process.

7. CPU - I/O burst Cycle Process execution consists of
a cycle of CPU execution (CPU burst) and I/O wait (I/O burst)
Process alternate between these two states
Process execution begins with a CPU burst, which is followed by an I/O burst, and so on.
Eventually, the final CPU burst ends with an system call to terminate execution.
CPU burst distribution of a process
varies greatly from process to process and from computer to computer

8. Alternating Sequence of CPU & I/O Bursts
CPU burst time
I/O burst time

9. Histogram of CPU-burst Times
CPU burst distribution is generally characterized
as exponential or hyper-exponential
with large number of short CPU burst and small number of long CPU burst
I/O bound process has many short CPU bursts
CPU bound process might have a few long CPU bursts.

10. Process Scheduler
selects one of the processes in memory that are ready to execute, and allocates the CPU to the selected process.
CPU scheduling decisions may take place when a process:
1. switches from running to waiting state: I/O request, invocation of wait() for the termination of other process
2. switches from running to ready state: when interrupt occurs
3. switches from waiting to ready: at completion of I/O
4. terminates

11. Process Scheduler(cont.)
Scheduling under 1 and 4 is non-preemptive
Scheduling under 2 and 3 is preemptive

12. Non-preemptive vs. Preemptive
Non-preemptive scheduling
Once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU
either by terminating or by switching to the waiting state.
used by Windows 3.x
Preemptive scheduling
Current running process can be switched with another at any time
because interrupt can occur at any time
Most of modern OS provides this scheme. (Windows XP, Max OS, UNIX)

13. CPU-scheduling function
Dispatcher
Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:
switching context
switching to user mode
jumping to the proper location in the user program to restart that program
Dispatch latency – time it takes for the dispatcher to stop one process and start another running
CPU-scheduling function
? ?

14. Context Switch from ch. 3

15. Scheduling Criteria
Based on the scheduling criteria, the performance of various scheduling algorithm could be evaluated.
Different scheduling algorithms have different properties.
CPU utilization –keep the CPU as busy as possible. i.e., ratio (%) of the amount of time while the CPU was busy per time unit.
Throughput – # of processes that complete their execution per time unit.
Turnaround time – the interval from the time of submission of a process to the time of completion. Sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O
Waiting time – Amount of time a process has been waiting in the ready queue, which is affected by scheduling algorithm
Response time – In an interactive system, amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)

16. Optimization Criteria
It is desirable to maximize:
The CPU utilization
The throughput
It is desirable to minimize:
The turnaround time
The waiting time
The response time
However in some circumstances, it is desirable to optimize the minimum or maximum values rather than the average.
Interactive systems, it is more important to minimize the variance in the response time than minimize the average response time.

17. Process Scheduling Algorithms
First-Come, First-Served Scheduling (FCFS)
Shortest-Job-First Scheduling (SJF)
Priority Scheduling
Round-Robin Scheduling
Our measure of comparison is the average waiting time.

18. First-Come, First-Served (FCFS) Scheduling
The process that request the CPU first is allocated the CPU first.
Process Burst Time(ms)
P1 24
P2 3
P3 3
Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is:
Waiting time for P1 = 0; P2 = 24; P3 = 27
Average waiting time: ( )/3 = 17ms P1 P2 P3 24 27 30
Q: Is it preempted or non-preempted?
Q: CPU utilization, Throughput, waiting time, turnaround time

19. FCFS Scheduling Suppose that the processes arrive in the order
P2 , P3 , P1
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 P1 P3 P2 6 3 30

20. FCFS Scheduling FCFS scheduling algorithm is non-preemptive
Once the CPU has been allocated to a process, that process keeps the CPU until it releases the CPU, either by terminating or by requesting I/O.
is particularly troublesome for time-sharing systems (response time ).
Convoy effect (short process behind long process)occurs:
When one CPU-bound process with long CPU burst and many I/O-bound process which short CPU burst.
All I/O bound process waits for the CPU-bound process to get off the CPU while I/O is idle
All I/O- and CPU- bound processes executes I/O operation while CPU is idle.
results in low CPU and device utilization

21. Shortest-Job-First (SJF) Scheduling
SJF associates with each process the length of its next CPU burst.
use these lengths to schedule the process with the shortest time
Two schemes:
non-preemptive – 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

22. Example of Non-Preemptive SJF
Process Burst Time
P1 6
P2 8
P3 7
P4 0 3
SJF scheduling chart (non-preemptive)
Average waiting time = ( ) / 4 = 7 P4 P3 P1 3 16 9 P2 24

23. Example of Preemptive SJF (SRTF)
Process Arrival Time Burst Time P P P P
SJF (preemptive)
Average waiting time = ( )/4 = 3 P1 P3 P2 4 2 11 P4 5 7 16
Q: CPU utilization, Throughput, waiting time, turnaround time

24. How do we know the length of the next CPU burst?
by computing an approximation of the length of the next CPU burst (estimate the length of the next CPU burst)
can be done by using the length of previous CPU bursts, using exponential averaging
The value of tn contains our most recent information.
n+1 stores the past history
The parameter  controls the relative weight of recent and past history in our prediction.
Recent history is a good indicator of the near future

25. Prediction of the Length of the Next CPU Burst
In this example, 0 = 10,  = ½ , t1=6
1 =  x t0 + (1- ) x 0 = ½ x 6 + ½ x 10 = 8
2 =  x t2 + (1- ) x 1 = ½ x 4 + ½ x 8 = 6

26. Examples of Exponential Averaging
 = 0
n+1 = n = n-1 = n-2 . … = 0
Recent history does not count
 = 1
n+1 = tn
Only the actual last CPU burst counts i.e., the most recent CPU burst
If we expand the formula, we get:
n+1 =  tn + (1 - ) tn …
+ (1 -  )j  tn -j + …
+ (1 -  )n +1 0
Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor

27. Priority Scheduling
A priority number (integer) is associated with each process
The CPU is allocated to the process with the highest priority (smallest integer  highest priority)
Preemptive
Non-preemptive

28. Example 1 of Non-Preemptive Priority
Process Burst Time Priority arrival time P P P P P
Priority Scheduling (non-preemptive)
Average waiting time = ( )/5 = 8.2 P2 P5 P1 P3 P4
Turnaround time = terminate time – acceptable time (starting time)
waiting time = Start executing in CPU - Ready queue arrive time 16 1 6 18 19

29. Example 2 of Non-Preemptive Priority
Process
Ready queue arrive time
CPU burst time
Priority P1
3 ms 5 P2 1
7 ms 3 P3 4
2 ms P4 2 P5 6
4 ms
scheduling chart (non-preemptive) P1 P5 P2 P3 P4 3 17 14 10 19
Turnaround time = terminate time – acceptable time (starting time)
waiting time = Start executing in CPU - Ready queue arrive time
Total Waiting time = (0-0) + (3-1) +(17-4) + (14-2) + (10-6) = =31 ms
Average waiting time= 31/5 = 6.2 ms
Total Turnaround time= (3-0) + (10-1) + (19-4) + (17-2) + (14- 6) = 50 ms
Average Turnaround time= 50/5 = 10ms

30. Example of Preemptive Priority
Process
Ready queue arrive time
Cpu burst time
Priority P1
3 ms 5 P2 1
7 ms 3 P3 4
2 ms P4 2 P5 6
4 ms
scheduling chart (preemptive)
Process P1 preempted by p2 because p2 has higher priority P3 P5 P2 P1 P4 6 13 1 17 10 15 19
Total Waiting time = ( ) + ( ) +(15-4) + (10-2) + (6-6) =42ms
Average waiting time= 42/5= 8.4 ms
Total Turnaround time= (19-0) + (15-1) + (17-4) + (13-2) + (10-6) = 61ms
Average Turnaround time= 61/5 = 12.2ms

31. Priority Scheduling
SJF is a priority scheduling where priority is the predicted next CPU burst time
Problem  Starvation – low priority processes may never execute
Solution  Aging – as time progresses, increase the priority of the process
127

32. Round Robin (RR) Scheduling
It is similar to FCFS scheduling, but preemption is added to enable the system to switch between process.
Each process gets a small unit of CPU time (time quantum), usually milliseconds.
After this time has elapsed, the process is preempted and added to the end of the ready queue.
Interactive
Ensures CPU equally shared among active processes

33. If CPU cycle (CPU burst time ) > time quantum
Job is preempted and put at the end of the READY Q
If CPU cycle < time quantum
If job finished
resources released
If interrupted by I/O request
Info saved in PCB
Linked at end of appropriate I/O queue
When I/O complete, job returns to READY Q

34. Example of RR with Time Quantum = 4
Process Burst Time Arrival Time P P
P3 3 0
The Gantt chart is: P1 P2 P3 4 7 10 14 18 22 26 30
Note that p1 enter the ready queue again with arrival time 4ms
Did we consider context switching in this example ?
Notice that:
suppose that there are ten processes ready to run, Q = 100 msec, and C = 5 msec. Process 0 (at the head of the ready queue) gets to run immediately. Process 1 can run only after Process 0's quantum expires (100 msec) and the context switch takes place (5 msec), so it starts to run at 105 msec these calculations without context switching time
Total Waiting time = ( ) + (4-0) +(7-0) =17ms
Average waiting time= 17/3= 5.66 ms
Context switch ???
Total Turnaround time= (30-0) + (7-0) + (10-0) = 47ms
Average Turnaround time= 47/5= 9.4 ms 34

35. Example of RR with Time Quantum = 10
The Gantt chart is:
Process
Ready queue arrive time
CPU burst time P1
7 ms P2 1
13 ms P3 5
25 ms P4 6
22 ms P5 8
40 ms 17 74 7 27 37 47 50 60 64 P2 P1 P3 P4 P5 79 89 99
Note that p2 reenter the ready queue again with arrival time 17
Total Waiting time = (0-0) + ( )+( )+( )+
( ) = ms
Average waiting time= 180/5= 36 ms
Total Turnaround time= (7-0) + (50-1) + (79-5)+(64-6)+(99-8) = 279ms
Average Turnaround time= 279/5= 55.8 ms

36. Round Robin (RR) Scheduling
Performance depends on the size of the time quantum.
If the time quantum is extremely large, the RR policy is the same as the same as the FCFS policy
If the time quantum is extremely small (say 1 millisec) , the RR approach is called processor sharing i.e., provides high concurrency: each of n processes has its own processor running at 1/n the speed of the real processor

37. Time Quantum and Context Switch Timeb c
In (a) the job finishes before the time quantum expires.
In (b) and (c), the time quantum expires first, interrupting the job
The effect of context switching on the performance of RR scheduling, for example one process of 10 time quantum.
quantum = 12 time units, finished in less than 1 time quantum
quantum = 6 time units, requires 2 quanta, 1 context switch
quantum = 1 time units, requires 10 quanta, 9 context switch

38. Round Robin (RR) Scheduling
The time quantum q must be large with respect to context switch, otherwise overhead is too high
If the context switching time is 10% of the time quantum, then about 10% of the CPU time will be spent in context switching
Most modern OS have time quanta ranging from 10 to 100 milliseconds,
The time required for a context switch is typically less than 10 microseconds; thus the context-switch time is a small fraction of the time quantum.
The overhead associated with a context switch can be expressed as:
context switch overhead = C / (Q+C)
where Q is the length of the time-slice and C is the context switch time. An increase in Q increases efficiency but reduces average response time. As an example, suppose that there are ten processes ready to run, Q = 100 msec, and C = 5 msec. Process 0 (at the head of the ready queue) gets to run immediately. Process 1 can run only after Process 0's quantum expires (100 msec) and the context switch takes place (5 msec), so it starts to run at 105 msec. Likewise, process 2 can run only after another 105 msec.

39. Turnaround Time varies with the Time Quantum
The turnaround time depends on the size of the time quantum
The average turnaround time of a set of processes
dose not necessarily improve
as the time quantum size
Increased.
The average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum.

40. Scheduling Algorithm with multi-Queues
Multi-level Queue Scheduling
Multi-level Feedback Queue Scheduling

41. Multilevel Queue
Ready queue is partitioned into separate queues: foreground (interactive) background (batch)
The processes are permanently assigned to one queue, generally based on some property, or process type.
Each queue has its own scheduling algorithm
foreground – RR
background – FCFS
Scheduling must be done between the queues
Fixed priority scheduling - serve all from foreground then from background, Possibility of starvation.
Time slice scheduling – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR, 20% to background in FCFS

42. Multilevel Queue Scheduling
No process in the batch queue could run unless the queues with high priority were all empty.
If an interactive editing process entered the ready queue while a batch process was running, the batch process would be preempted.

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

44. 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 RR. 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 RR and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.
The job is serverd based on FCFS in queue Q2

45. Multilevel Feedback Queues

46. Summary
CPU scheduling is the task of selecting a waiting process from the ready queue and allocating the CPU to it.
The CPU is allocated to the selected process by the dispatcher.
FCFS scheduling is simple, cause short processes to wait for long time
SJF scheduling is provably optimal, providing the shortest averaging waiting time. But predicting the length of the next CPU bursts is difficult.
Priority scheduling allocates the CPU to the heights priority process.
Both priority and SJF may suffer from starvation. Aging is a technique to prevent starvation.
RR scheduling is more appropriate for a time-shared system.
Major problem of RR scheduling is the selection of the time quantum.
FCFS is non-preemptive, RR is preemptive, SJF and Priority may be preemptive and non-preemptive.
Multilevel queue allows different scheduling algorithm for each queue.
Multilevel feedback queue allow process to move from one queue to another.

47. End of Chapter 5