1. Lecture 18 Syed Mansoor Sarwar
Operating Systems
Lecture 18
Syed Mansoor Sarwar

2. © Copyright Virtual University of Pakistan
Agenda for Today
Review of previous lecture
UNIX System V scheduling
Algorithm evaluation
Process synchronization
Recap of lecture
27 February 2019
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Review of Lecture 17
Multi-level queues scheduling
Multi-level feedback queues scheduling
UNIX System V scheduling algorithm
27 February 2019
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4. UNIX System V Scheduling Algorithm
Every second, the priority number of all those processes that are in the main memory and ready to run is updated by using the following formula:
Priority# = (Recent CPU Usage)/2 + Thr. Pri.+ nice
Threshold priority and nice values are always positive to prevent a user from migrating out of its assigned group
27 February 2019
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UNIX System V Example PA PB PC
CPU Count 1 … 60 30 15 7 8 67 33 16
CPU Count 1 … 60 30 15 7 8 67 33
CPU Count 1 … 60 30 15 7
Time
Priority 60 75 67 63 76 68
Priority 60 75 67 63 76
Priority 60 75 67 63 1 2 3 4 5
27 February 2019
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6. Round Robin Scheduling and Process Priorities60 B
Higher Priority B A A A 1 2 3 B
A B B
A runs first A
27 February 2019
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Algorithm Evaluation
Analytic Evaluation
The algorithm and some system workload are used to produce a formula or number which gives the performance of the algorithm for that workload.
Deterministic modeling
Queuing models
Implementation
27 February 2019
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8. Deterministic Modeling
Predetermined workload and performance of each algorithm for that workload. Use of Gantt charts.
Simple and fast
Exact numbers for comparison
Requires exact input
Performance figures may not be true in general
27 February 2019
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9. Deterministic Modeling
Process Arrival Time Burst Time P P P P
Gantt chart
Average waiting time = ( )/4 = 3 P3 P2 4 2 11 P4 5 7 P1 16
27 February 2019
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Queuing Modeling
Computer system viewed as a network of queues and servers: ready queue, I/O queue, event queues, CPUs, I/O device controllers, etc.
Input: Arrival and service rates
Output: CPU utilization, average
queue length, average waiting time, …
27 February 2019
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Queuing Modeling
Little’s Formula:
n = λ* W
where
n = average queue length
λ = average arrival rate
W = average waiting time in a
queue
27 February 2019
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12. Queuing Modeling Let the average job arrival rate be 0.5 Algorithm
Average Wait Time
W=tw
Average Queue Length(n)
FCFS
4.6
2.3
SJF
3.6
1.8
SRTF
3.2
1.6
RR (q=1)
7.0
3.5
RR (q=4)
6.0
3.0
27 February 2019
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Queuing Modeling
Complicated mathematics
Distributions (Poisson, uniform, exponential, etc) for the arrival and departure rates can be difficult to work with
Assumptions may not be accurate
Approximation of the real system
27 February 2019
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Simulation
Programming model for the computer system
Workload generated by assuming some distribution and a random number generator, or by collecting data from the actual system.
27 February 2019
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Simulation
Characteristics
Expensive: hours of programming and execution time
May be erroneous because of the assumptions about distributions
27 February 2019
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Simulation
27 February 2019
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Implementation
Best
Most expensive
Good option due to Open Source kernels such as Linux
27 February 2019
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18. Process Synchronization
Concurrent access to shared data may result in data inconsistency.
Maintaining data consistency requires mechanisms to ensure that cooperating processes access shared data sequentially.
27 February 2019
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19. Bounded-Buffer Problem
Shared data #define BUFFER_SIZE 10
typedef struct {
. . .
} item;
item buffer[BUFFER_SIZE];
int in = 0, out = 0;
int counter = 0;
27 February 2019
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20. Bounded-Buffer Problem
Producer process
item nextProduced; …
while (1) {
while (counter == BUFFER_SIZE) ;
buffer[in] = nextProduced;
in = (in + 1) % BUFFER_SIZE;
counter++; }
27 February 2019
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21. Bounded-Buffer Problem
Consumer process
item nextConsumed;
while (1) {
while (counter == 0) ;
nextConsumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
counter--; }
27 February 2019
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22. Bounded-Buffer Problem
“counter++” in assembly language
MOV R1, counter
INC R1 MOV counter, R1
“counter--” in assembly language MOV R2, counter
DEC R2 MOV counter, R2
27 February 2019
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23. Bounded-Buffer Problem
If both the producer and consumer attempt to update the buffer concurrently, the machine language statements may get interleaved.
Interleaving depends upon how the producer and consumer processes are scheduled.
27 February 2019
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24. Bounded-Buffer Problem
Assume counter is initially 5. One interleaving of statements is: producer: MOV R1, counter (R1 = 5) INC R (R1 = 6) consumer: MOV R2, counter (R2 = 5) DEC R (R2 = 4) producer: MOV counter, R1 (counter = 6) consumer: MOV counter, R2 (counter = 4)
The value of count may be either 4 or 6, where the correct result should be 5.
27 February 2019
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25. Process Synchronization
Race Condition: The situation where several processes access and manipulate shared data concurrently, the final value of the data depends on which process finishes last.
27 February 2019
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26. Process Synchronization
Critical Section: A piece of code in a cooperating process in which the process may updates shared data (variable, file, database, etc.).
Critical Section Problem: Serialize executions of critical sections in cooperating processes
27 February 2019
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27. Solution of the Critical Problem
Software based solutions
Hardware based solutions
Operating system based solution
27 February 2019
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Structure of Solution
do {
critical section
reminder section
} while (1);
entry section
exit section
27 February 2019
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29. © Copyright Virtual University of Pakistan
Recap of Lecture
UNIX System V scheduling
Algorithm evaluation
Process synchronization
Recap of lecture
27 February 2019
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30. Lecture 18 Syed Mansoor Sarwar
Operating Systems
Lecture 18
Syed Mansoor Sarwar