# Assignment 4 CPU Scheduling

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CPSC 457- Principle of Operating Systems
Assignment 4
CPU Scheduling

1 The Problem
The Simply Irie restaurant thieves are still out there (at least at the time of this writing)
and their next target is our own Dining Center. So, one night they break in and steal most
of the chairs and tables (no one knows why they didn’t take the Nescafe machines instead).
The university does not have enough time to add tables in the DC so they have to do
with the few tables left for at least two days. That’s when you realize that the scheduling
policies you learned in your CPSC 457 course can be used to reduce some of frustration of
scarcity of seats in the DC. Let’s say there are N tables left in the Dining Center. Students
come in with some random inter-arrival time and each student sits at one table. If all
tables are full, they have to wait in line. Note that the line might build up indefinitely. A
student that sits at a table will eat for a specific amount of time and then leave. Note that
the time it takes each students is pre-determined. So each student knows before joining
the line how much they are going to take eating. You want to try out different scheduling
algorithms for sitting students at the tables in the first day, and compare them and stick
with the best one for the second day. The algorithms are compared by these criteria:
1. Turnaround Time: The time it takes a student from first sitting at a table until they
finish and leave DC. The smaller this number the better.
2. Waiting time: The total amount of time a student spent outside of dining area
(waiting in line). The smaller this number the better.
Here are the scheduling algorithms:
1. FIFO: First student in, is the first to be served. Seems pretty fair right?
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2. Round-Robin: Each student can’t take more than a specific time quantum at the
table. If they surpass that time slot, they are going to have to get up and wait
outside even if they are in the middle of eating (however rude that is).
You need to implement the above algorithms in two separate files: One named roundrobin.c or .cpp and the other fifo.c or fifo.cpp, depending on whether you use C, or C++,
respectively.
2 The Input
The input of the program is read from a file. The name of the file is going to be taken
from user input. The file1
is structured as follows:
1. The first line of the file is a single integer specifying the time quantum size of the
round-robin algorithm.
2. After that, an arbitrary number of lines follow each containing two numbers separated
by one space character. The first number is an integer that denotes the relative
timestamp of the student arrival. The first timestamp is always zero and timestamps
increase afterwards. The value of each timestamp, therefore, indicates how long it
has been since the previous arrival. The second number is an integer specifying the
amount of time it takes that specific student to eat.
3 The Implementation
You need to solve this problem for number of tables N = 4. Each table has a server which
is represented by a thread. You should have at least 5 threads:
1. One thread reads information about students from the input file and adds them to
the Queue. This is the producer.
2. Four threads for scheduling students and serving them accordingly.
The scheduling, itself, is done using threads (the pthread library). You should have one
thread for each dining table, as we have one dining area server responsible for each table.
You can implement the Queue using whatever data structure you see fit (at this point you
must have seen enough examples to be able to make a decent choice (we suggest a doubly
linked list). Just be aware of two things:
1. The data structure should work with sharing, communication and synchronization
mechanisms.
2. The data structure should support infinite (theoretically) queue size.
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the file shall be used for both scheduling algorithms although the quantum is not needed in the FIFO
algorithm.
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4 How to do It
The following are suggested implementation steps:
1. Start with the student producing thread, that is going to give you an idea of what
data structure is suitable for the assignment and how you should implement it. There
are two general steps here:
(a) Creating a student, which requires reading arrival time and eating duration of
the student from the input file, and adding the info to a proper data structure
that represents a student.
(b) Adding the newly created student to the Queue. At this stage you will probably
encounter the problem of exceeding the size of the allocated queue data structure
as you continually add students. There are two solutions for this problem. The
simple one is allocating enough memory in the beginning so that you are sure
you won’t encounter this issue. The second solution is dynamically allocating
new memory regions on the fly. Mention in your report if you have used
this approach and also mention how and where, to get some bonus
points.
2. After you are finished with the producer, write a very simple scheduler that reads the
items added to the Queue by the producer to make sure the communication between
the two works correctly. Add the synchronization mechanisms for the Queue as well.
3. Then you can start implementing the scheduling algorithms one by one. At this point
you might start to think about the details of representing each student in the Queue.
We suggest you include these details at least:
(a) A unique ID. This is can be generated automatically using a counter initialized
to 0, and incremented before reading each line; i.e. the first student will have
ID = 1, the second 2, and so on.
(b) How much of eating the student has left.
4. You might want to start with the FIFO scheduling algorithm since its the easiest
one. The scheduling takes place in three general steps:
(a) Take a student out of the Queue according to the scheduling policy. In the case
of FIFO, the Queue acts like a FIFO queue and you need to take the head of
the Queue for serving.
(b) Start the dining process and do busy waiting ( a while loop maybe) until time
is up. In the FIFO scheduling algorithm which is non-preemptive, the student
will sit at the table and eat until s/he is done, then the student will get out of
the dining centre.
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(c) In the case of Round-Robin, it might as well be the case that the student’s time
slot is up and s/he needs to be preempted. You need to add the student to the
Queue again and sit another student at the table. Be aware that any access to
the Queue must be synchronized using mutex locks or semaphores.
We advise against using the clock() function for timestamps unless you find a way
around its limitations. It is better to use the chrono library API calls. Also, after reading
from the file has finished, your program needs to join with the threads and exit gracefully,
deallocating resources.
5 The output
You output will be processed using an interpreter for correction, so make sure you strictly
follow the format specified. The program is expected to produce lines of output at different
times:
1. Whenever a student arrives and joins the Queue, this statement must be printed:
Arrive <student ID>
2. Whenever a student is selected by a scheduling thread to sit at a table, this statement
must be printed:
Sit <student ID>
3. Whenever a student is preempted and put in the line, this statement must be printed:
Preempt <student ID>
4. Whenever a student is finished and leaves the Dining Center, this must be printed:
Leave <student ID> Turnaround <turnaround time measured> Wait <waiting time
measured>
Note that the times specified here are in seconds. Just replace whatever is enclosed
by <> with the required value, no <> should be in the output. If your timings are off by
a couple of milliseconds, you need to round to the closest integer. e.g. 1.99 s should be
rounded to 2 seconds.
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6 Grading
This assignment will carry 25%. In addition to code checking, your program will be
tested against some prepared and well designed test files. You can also provide your own
test file. Be reminded about the University strict rules on plagiarism and other related
details mentioned in the course outline.
7 To Submit
You need to submit source files, executables, along with a Makefile that will be used to
compile and link your shell. These files should be combined into a tar file that will unpack
the files into a directory whose name is your user-id, followed by your surname. The tar
file will be submitted on the D2L page of the course.
8 Deadline
April 12, 2022 before 11:55 P.M. Late submission is subject to the regular penalty of 20%
for every day or part of the day.
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