1. Outline Announcement Process Scheduling– continued
Preemptive scheduling algorithms
Round robin
Multiple-level queue
Please turn in your lab 1 report in hardcopy with
source code and running results attached
I will let you know next Tuesday how to assign up
for demonstration
Please pick up your first quiz if you have not done so

2. Announcement Dr. Andy Wang will give a guest lecture this Thursday
He is going to talk about his research on operating systems
There will be no office hours this Thursday
I will be attending a symposium in DC area
We will have recitation session tomorrow
I will go over the code I have for Lab 2
I will go over two examples on how to synchronize threads
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3. Process Scheduler - review
Ready
List
Scheduler
CPU
Resource
Manager
Resources
Preemption or voluntary yield
Allocate
Request
Done
New
Process
job
“Ready”
“Running”
“Blocked”
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4. The Scheduler Organization
Ready Process
Enqueuer
Ready
List
Dispatcher
Context
Switcher
Process
Descriptor
CPU
From Other
States
Running Process
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5. Scheduling Criteria CPU utilization – keep the CPU as busy as possible
Throughput – # of processes that complete their execution per time unit
Turnaround time – amount of time to execute a particular process
Waiting time – amount of time a process has been waiting in the ready queue
Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)
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6. First-Come-First-Served
Assigns priority to processes in the order in which they request the processor p0 p1 p2 p3 p4
1275
1200
900
475
350
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7. Shortest-Job-Next Scheduling
Associate with each process the length of its next CPU burst.
Use these lengths to schedule the process with the shortest time.
SJN is optimal
gives minimum average waiting time for a given set of processes.
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8. Example of Preemptive SJF
Process Arrival Time Burst Time P P P P
Average time spent in ready queue = ( )/4 = 3 P1 P3 P2 4 2 11 P4 5 7 16
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9. Priority Scheduling
In priority scheduling, processes/threads are allocated to the CPU based on the basis of an externally assigned priority
A commonly used convention is that lower numbers have higher priority
SJN is a priority scheduling where priority is the predicted next CPU burst time.
FCFS is a priority scheduling where priority is the arrival time
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10. Round Robin (RR)
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.
If there are n processes in the ready queue and the 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.
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11. Round Robin (TQ=50) – cont.
i t(pi) p0
TTRnd(p0) = 1100
TTRnd(p1) = 550
TTRnd(p2) = 1275
TTRnd(p3) = 950
TTRnd(p4) = 475
W(p0) = 0
W(p1) = 50
W(p2) = 100
W(p3) = 150
W(p4) = 200
Wavg = ( )/5 = 500/5 = 100
475
400
300
200
100
Equitable
Most widely-used
Fits naturally with interval timer p4 p1 p3 p2
550
650
750
850
950
1050
1150
1250
1275
TTRnd_avg = ( )/5 = 4350/5 = 870
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12. Round-robin Time Quantum
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13. Process/Thread ContextRn
. . .
Status
Registers
Functional Unit
Left Operand
Right Operand
Result
ALU PC IR
Ctl Unit
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14. Context Switching - review
CPU
New Thread Descriptor
Old Thread
Descriptor
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15. Cost of Context Switching
Suppose there are n general registers and m control registers
Each register needs b store operations, where each operation requires K time units
(n+m)b x K time units
Note that at least two pairs of context switches occur when application programs are multiplexed each time
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16. Round Robin (TQ=50) – cont.
Overhead must be considered
i t(pi) p0
TTRnd(p0) = 1320
TTRnd(p1) = 660
TTRnd(p2) = 1535
TTRnd(p3) = 1140
TTRnd(p4) = 565
W(p0) = 0
W(p1) = 60
W(p2) = 120
W(p3) = 180
W(p4) = 240
Wavg = ( )/5 = 600/5 = 120
540
480
360
240
120 p4 p1 p3 p2
575
790
910
1030
1150
1270
1390
1510
1535
TTRnd_avg = ( )/5 = 5220/5 = 1044
635
670
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17. Multi-Level Queues All processes at level i run before
Preemption or voluntary yield
Ready List0
New
Process
Scheduler
Ready List1
CPU
Done
Ready List2
All processes at level i run before
any process at level j
At a level, use another policy, e.g. RR
Ready List3
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18. Multilevel Queues Ready queue is partitioned into separate queues
foreground (interactive)
background (batch)
Each queue has its own scheduling algorithm
foreground – RR
background – FCFS
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19. Multilevel Queues – cont.
Scheduling must be done between the 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; i.e.,
80% to foreground in RR
20% to background in FCFS
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20. Multilevel Queue Scheduling
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21. Multilevel Feedback Queue
A process can move between the 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 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
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22. Multilevel Feedback Queues – cont.
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23. Example of Multilevel Feedback Queue
Three queues:
Q0 – time quantum 8 milliseconds
Q1 – 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.
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24. Two-queue scheduling
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25. Three-queue scheduling
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26. Multiple-Processor Scheduling
CPU scheduling more complex when multiple CPUs are available.
Homogeneous processors within a multiprocessor.
Load sharing
Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing.
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27. Algorithm Evaluation
Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload.
Queuing models
Implementation
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28. Evaluation of CPU Schedulers by Simulation
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29. Contemporary Scheduling
Involuntary CPU sharing -- timer interrupts
Time quantum determined by interval timer -- usually fixed for every process using the system
Sometimes called the time slice length
Priority-based process (job) selection
Select the highest priority process
Priority reflects policy
With preemption
Usually a variant of Multi-Level Queues
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30. Scheduling in real OSs All use a multiple-queue system
UNIX SVR4: 160 levels, three classes
time-sharing, system, real-time
Solaris: 170 levels, four classes
time-sharing, system, real-time, interrupt
OS/2 2.0: 128 level, four classes
background, normal, server, real-time
Windows NT 3.51: 32 levels, two classes
regular and real-time
Mach uses “hand-off scheduling”
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31. BSD 4.4 Scheduling Involuntary CPU Sharing Preemptive algorithms
32 Multi-Level Queues
Queues 0-7 are reserved for system functions
Queues 8-31 are for user space functions
nice influences (but does not dictate) queue level
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32. Windows NT/2K Scheduling
Involuntary CPU Sharing across threads
Preemptive algorithms
32 Multi-Level Queues
Highest 16 levels are “real-time”
Next lower 15 are for system/user threads
Range determined by process base priority
Lowest level is for the idle thread
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33. Scheduler in Linux In file kernel/sched.c
The policy is a variant of RR scheduling
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34. Summary
The scheduler is responsible for multiplexing the CPU among a set of ready processes / threads
It is invoked periodically by a timer interrupt, by a system call, other device interrupts, any time that the running process terminates
It selects from the ready list according to its scheduling policy
Which includes non-preemptive and preemptive algorithms
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