Assignment #2: Multi-process Scheduling

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Operating Systems COMP 310 – ECSE 427 McGill University

Assignment #2: Multi-process Scheduling

1. Assignment Description
This is the second of a series of three assignments that build upon each other. In this assignment, you will
extend the simulated OS to support running concurrent processes.
This assignment is significantly longer than the first one, so please plan your time wisely,
and don’t hesitate to ask questions on Ed if you get stuck.
You will use the C programming language in your implementation. The assignment is presented from a
Linux point of view using the SOCS server, which you can reach remotely using
ssh or putty. Please make sure your assignment runs on since these are the
computers the TAs will be using for testing your code.
1.1 Starter files description:
You have three options:
• [Recommended] Use your solution to Assignment 1 as starter code for this assignment. If your
solution passes the public unit tests, it is solid enough to use as a basis for the second assignment.
• Use the official solution to Assignment 1 provided by the OS team as starter code. The solution
will be released on February 16, so you will have to wait for another week to start coding. You
can use this time to go over the assignment instructions carefully and sketch your solution.
• Ask a friend, or a team for their solution to Assignment 1. In this case, you have to give credit to
your peers in the README. Failure to do so will be considered plagiarism.
1.2 Your tasks:
Your tasks for this assignment are as follows:
• Implement the scheduling infrastructure.
• Extend the existing OS Shell syntax to create concurrent processes.
• Implement different scheduling policies for these concurrent processes.
On a high level, in this assignment you will run concurrent processes via the exec command, and you will
explore different scheduling strategies and concurrency control. Exec can take up to three files as
arguments. The files are scripts which will run as concurrent processes. For each exec argument (i.e.,
each script), you will need to load the full script code into your shell memory. For this assignment, you
can assume that the scripts will be short enough to fully fit into the shell memory – this will change in
Assignment 3.
You will also need to implement a few data structures to keep track of the code execution for the scripts,
as the scheduler switches the processes in and out of the “running” state. After this infrastructure is set
Operating Systems COMP 310 – ECSE 427 McGill University
Balmau Assignment #2 Page 2 of 7
up, you will implement the following scheduling policies: FCFS, SJF, and RR. Finally, you will add simple
concurrency control, where the scripts can access and modify a shared variable in the shell memory.
More details on the behavior of your scheduler follow in the rest of this section.
Even though we will make some recommendations, you have full freedom for the implementation. In
• Unless we explicitly mention how to handle a corner case in the assignment, you are free to
handle corner cases as you wish, without getting penalized by the TAs.
• You are free to craft your own error messages (please keep it polite).
• Just make sure that your output is the same as the expected output we provide in the test cases
in Section 2.
• Formatting issues such as tabs instead of spaces, new lines, extra command line prompts, etc. will
not be penalized.
Let’s start coding! J
1.2.1. Implement the scheduling infrastructure
We will start by building the basic scheduling infrastructure. For this intermediate step, you will modify
the run command to use the scheduler and run SCRIPT as a process. Note that, even if this step is
completed successfully, you will see no difference in output compared to the run command in
Assignment 1.
However, this step is crucial, as it sets up the scaffolding for the exec command in the following section.
As a reminder from Assignment 1, the run API is:
run SCRIPT Executes the commands in the file SCRIPT
run assumes that a file exists with the provided file name, in the current directory. It opens that
text file and then sends each line one at a time to the interpreter. The interpreter treats each line
of text as a command. At the end of the script, the file is closed, and the command line prompt is
displayed once more. While the script executes, the command line prompt is not displayed. If an
error occurs while executing the script due a command syntax error, then the error is displayed,
and the script continues executing.
You will need to do the following to run the SCRIPT as a process:
1. Code loading. Instead of loading and executing each line of the SCRIPT one by one, you will load
the entire source code of the SCRIPT file into the OS Shell memory. It is up to you to decide how
to encode each line in the Shell memory.
o Hint1: If you solved Section 1.2.1 in Assignment 1 correctly, it might come in handy for
loading the source code into the Shell memory.
2. PCB. Create a data-structure to hold the SCRIPT PCB. PCB could be a struct. In the PCB, at a
minimum, you need to keep track of:
o The process PID. Make sure each process has a unique PID.
Operating Systems COMP 310 – ECSE 427 McGill University
Balmau Assignment #2 Page 3 of 7
o The spot in the Shell memory where you loaded the SCRIPT instructions. For instance, if
you loaded the instructions contiguously in the Shell memory (highly recommended), you
can keep track of the start position and length of the script.
o The current instruction to execute (i.e., serving the role of a program counter).
3. Ready Queue. Create a data structure for the ready queue. The ready queue contains the PCBs of
all the processes currently executing (in this case, there will be a single process). One way to
implement the ready queue is to add a next pointer in the PCB (which points to the next PCB in
the ready queue), and a pointer that tracks the head of the ready queue.
4. Scheduler logic. If steps 1—3 were done correctly, we are now in good shape to execute SCRIPT
through the scheduler.
o The PCB for SCRIPT is added at the tail of the ready queue. Note that since the run
command only executes one script at a time, SCRIPT is the only process in the ready
queue (i.e., it is both the tail and the head of the queue). This will change in Section 1.2.2
for the exec command.
o The scheduler runs the process at the head of the ready queue, by sending the process’
current instruction to the interpreter.
o The scheduler switches processes in and out of the ready queue, according to the
scheduling policy. For now, the scheduling policy is FCFS, as seen in class.
o When a process is done executing, it is cleaned up (see step 5 below) and the next process
in the ready queue starts executing.
5. Clean-up. Finally, after the SCRIPT terminates, you need to remove the SCRIPT source code
from the Shell memory.
• The shell memory is large enough to hold three scripts and still have a bit of extra space. In our
reference solution, the size of the Shell memory is 1000 lines, thus each script will have at most
300 lines of source code. If you implemented your shell from scratch, please use the same limits.
• You can also assume that each command (i.e., line) in the scripts will not be larger than 100
If everything is correct so far, your run command should have the same behavior as in Assignment 1. You
can use the existing unit tests from Assignment 1 to make sure your code works correctly.
1.2.2. Extend the OS Shell with the exec command
We are now ready to add concurrent process execution in our shell. In this section, we will extend the OS
Shell interface with the exec command:
exec prog1 prog2 prog3 POLICY Executes up to 3 concurrent programs, according to a
given scheduling policy
• exec takes up to four arguments. The two calls below are also valid calls of exec:
o exec prog1 POLICY
o exec prog1 prog2 POLICY
• POLICY is always the last parameter of exec.
• POLICY can take the following three values: FCFS, SJF, RR, or AGING. If other arguments are given,
the shell outputs an error message, and exec terminates, returning the command prompt to the user.
Operating Systems COMP 310 – ECSE 427 McGill University
Balmau Assignment #2 Page 4 of 7
Exec behavior for single-process. The behavior of exec prog1 POLICY is the same as the behavior of
run prog1, regardless of the policy value. Use this comparison as a sanity check.
Exec behavior for multi-process. Exec runs multiple processes concurrently as follows:
• The entire source code of each process is loaded into the Shell memory.
• PCBs are created for each process.
• PCBs are added to the ready queue, according to the scheduling policy. For now, implement only
the FCFS policy.
• When processes finish executing, they are removed from the ready queue and their code is
cleaned up from the shell memory.
• For simplicity, we are simulating a single core CPU.
• We do not support threading.
• We will not be testing recursive exec calls.
• Each exec argument is the name of a different script filename. If two exec arguments are
identical, the shell displays an error (of your choice) and exec terminates, returning the
command prompt to the user (or keeps running the remaining instructions, if in batch mode).
• If there is a code loading error (e.g., running out of space in the shell memory), then no programs
run. The shell displays an error, the command prompt is returned, and the user will have to input
the exec command again.
Example execution
prog1 code prog2 code prog3 code
echo helloP1
set x 10
echo $x
echo byeP1
echo helloP2
set y 20
echo $y
print y
echo byeP2
echo helloP3
set z 30
echo byeP3
$ exec prog1 prog2 prog3 FCFS
$ //exec ends and returns command prompt to user
1.2.3. Adding Scheduling Policies
Extend the scheduler to support the Shortest Job First (SJF) and Round Robin (RR) policies, as seen in class.
• For SJF, use the number of lines of code in each program to estimate the job length.
Operating Systems COMP 310 – ECSE 427 McGill University
Balmau Assignment #2 Page 5 of 7
• For RR, schedulers typically use a timer to determine when the turn of a process ended. In this
assignment, we will use a fixed number of instructions as a time slice. Each process gets to run 2
instructions before getting switched out.
Example execution (prog1, prog2, prog3 code is the same as in Section 1.2.2)
Example SJF Example RR
$ exec prog1 prog2 prog3 SJF
$ exec prog1 prog2 prog3 RR
1.2.4. SJF with job Aging
One of the important issues with SJF is that short jobs continuously preempt long jobs, leading to
starvation. Aging is a common technique that addresses this issue. In this final exercise, you will
implement a simple aging mechanism to promote longer running jobs to the head of the ready queue.
The aging mechanism works as follows:
• Instead of sorting jobs by estimated job length, we will sort them by a “job length score”. You can
keep track of the job length score in the PCB.
• In the beginning of the exec command, the “job length score” of each job is equal to their job
length (i.e., the number of lines of code in the script) like in Section 1.2.3.
• The scheduler will re-assess the ready queue every time slice. For this exercise, we will use a time
slice of 1 instruction.
o After a given time-slice, the scheduler “ages” all the jobs that are in the ready queue,
apart from the current head of the queue.
o The aging process decreases a job’s “job length score” by 1. The job length score cannot
be lower than 0.
o If after the aging procedure there is a job in the queue with a score that is lower than the
current running job, the following happens:
§ The current running job is preempted
§ the job with the new lowest job length score is placed at the head of the running
queue. In case of a tie, the process closer to the head of the running queue has
§ The scheduler runs the new process in the head of the ready queue.
o If after the aging procedure the current head of the ready queue is still the job with the
lowest “job length score”, then the current job continues to run for the next time slice.
Operating Systems COMP 310 – ECSE 427 McGill University
Balmau Assignment #2 Page 6 of 7
prog1 code prog2 code prog3 code
echo helloP1
set x 10
echo $x
echo byeP1
echo helloP2
set y 20
echo $y
print y
echo byeP2
echo helloP3
set z 30
echo byeP3
Execution of SJF with aging and a time slice of 1 instruction; the state of the ready queue shown in comments:
$ exec prog1 prog2 prog3 AGING
helloP3 // (P3, 3), (P1, 4), (P2, 5) à aging (P3, 3), (P1, 3), (P2, 4) à no promotion
//Nothing printed for set z 30 // (P3, 3), (P1, 3), (P2, 4) àaging (P3, 3), (P1, 2), (P2, 3) àpromote P1
helloP1 // (P1, 2), (P2, 3), (P3, 3) àaging (P1, 2), (P2, 2), (P3, 2) àno promotion
//Nothing printed for set x 10 // (P1, 2), (P2, 2), (P3, 2) àaging (P1, 2), (P2, 1), (P3, 1) àpromote P2
helloP2 // (P2, 1), (P3, 1), (P1, 2) àaging (P2, 1), (P3, 0), (P1, 1) àpromote P3
byeP3 // (P3, 0), (P1, 1), (P2, 1) àaging (P3, 0), (P1, 0), (P2, 0), àpromote P1
10 // (P1, 0), (P2, 0), no more aging possible
byeP1 // (P1, 0), (P2, 0), no more aging possible
//Nothing printed for set y 20 // (P2, 0), no more aging possible
20 // (P2, 0), no more aging possible
20 // (P2, 0), no more aging possible
byeP2 // (P2, 0), no more aging possible
IMPORTANT: The TAs will use batch mode when testing your code, so make sure that your program
produces the expected outputs when testcases run in batch mode. You can assume that the TAs will run
one test at a time in batch mode.
We provide you with 5 testcases and expected outputs for your code. Please run the testcases to ensure
your code runs as expected.
The assignment is due on March 7, 2022 at 23:59, no extensions. You need to submit the following:
• All the files in your project, including the testcases and a README, in one zipped file, named as
• If you are working in a team, the zipped file name is
If the name is too long, only include the last names and McGill IDs.
• The README should mention the author name(s) and McGill ID(s), any comments the author(s)
would like the TA to see, and mention whether the code uses the starter code provided by the OS
team or give credit to starter code borrowed from your peers.
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Balmau Assignment #2 Page 7 of 7
• The project must compile on mimi by running make clean; make mysh
• The project must run in batch mode, i.e. ./mysh < testfile.txt
Note: You must submit your own work. You can speak to each other for help but copied code will be
handled as to McGill regulations. Submissions are automatically checked via plagiarism detection tools.
Your assignment is graded out of 20 points.
• IMPORTANT: Your program must compile and run on mimi to be graded. If the code does not
compile/run using the commands in Section 3, you will receive 0 pointsfor the entire assignment.
• Known testcases: 10 points. You were provided 5 testcases, with expected outputs. If your code
matches the expected output, you will receive 2 points for each testcase. You will receive 0 points
for each testcase where your output does not match the expected output. Formatting issues such
as tabs instead of spaces, new lines, extra command line prompts, etc. will not be penalized.
• Surprise testcases: 10 points. The OS team also has 5 surprise testcases, similar to the ones
already provided to you. The surprise testcases are graded the same as the known testcases.
• The TA will look at your source code only if the program runs (correctly or not). The TA looks at
your code to verify that you implemented the requirement as requested. Specifically:
o Hardcoded solutions will receive 0 points for the hardcoded testcase, even if the output
is correct.
o Programming expectations. Your code needs to meet the programming style posted on
myCourses. If not, your TA may remove up to 5 points, as they see fit.
o You must write this assignment in the C Programming language, otherwise the
assignment will receive 0 points.


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