Description
INTRODUCTION
In this project you will implement a multiprocessor operating system simulator using a popular user space
threading library for linux called pthreads. The framework for the multithreaded OS simulator is nearly
complete, but missing one critical component: the CPU scheduler! Your task is to implement the CPU
scheduler, using three different scheduling algorithms.
NOTE: MAKE SURE THAT MULTIPLE CPU CORES ARE TURNED ON IN YOUR
VIRTUAL MACHINE
We have provided you with source files that constitute the framework for your simulator. You will only
need to modify answers.txt and student.c. However, just because you are only modifying two files doesn’t
mean that you should ignore the other ones – there is helpful information in the other files. Information
about using the pthreads library is given in Problem 0. We have provided you these files:
• Makefile – Working one provided to you; Modify at your own peril
• os-sim.c – Code for the operating system simulator which calls your CPU scheduler
• os-sim.h – Header file for the simulator which calls your CPU scheduler
• process.c – Descriptions of the simulated processes
• process.h – Header file for the process data
• student.h – Header file for your code to interface with the OS scheduler
• student.c – This file contains stub functions for your CPU scheduler. You will be modifying this file
• answers.txt – A blank file where you will be answering some questions
Scheduling Algorithms
For your simulator, you will implement the following scheduling algorithms:
• First In, First Out (FIFO) – Runnable processes are kept in a ready queue. FIFO is non-preemptive;
once a process begins running on a CPU, it will continue until it either completes or blocks for I/O
• Round Robin – Similar to FIFO, except preemptive. Each process is assigned a timeslice when it is
scheduled. At the end of the timeslice, if the process is still running, the process is preempted, and
moved to the tail of the ready queue.
• Static Priority – The processes with the highest priorities always get the CPU. Lower-priority processes
may be preempted if a process with higher priority becomes runnable.
Process States
In our OS simulation, there are five possible states for a process, which are in the process state t enum in
the os-sim.h:
• NEW – The process is being created, and has not yet begun executing
• READY – The process is ready to execute, and is waiting to be scheduled on a CPU
• RUNNING – The process is currently executing on a cpu
• WAITING – The process has temporarily stopped executing, and is waiting on an I/O request to
complete
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PROJECT 6 – MULTITHREADING CS 2200 – SYSTEMS AND NETWORKS
• TERMINATED – The process has completed
There is a field name state in the PCB, which must be updated with the current state of the process. The
simulator will use this field to collect statistics.
Figure 1: Process States
The Ready Queue
On most systems there are a large number of processes, but only one or two CPUs on which to execute
them. When there are more processes ready to execute than CPUs, processes must wait in the READY state
until a CPU becomes available. To keep track of the processes waiting to execute, we keep a ready queue of
the processes in the READY state.
Since the ready queue is accessed by multiple processors, which may add and remove processes from it,
the ready queue must be protected by some form of synchronization–for this project, you will use a mutex
lock. The ready queue should use a different mutex than the current mutex.
Scheduling Processes
schedule() is the core function of the CPU scheduler. It is invoked whenever a CPU becomes available
for running a process. schedule() must search the ready queue, select a runnable process, and call the
context switch() function to switch the process onto the CPU.
There is a special process, the idle process, which is scheduled whenever there are no processes in the
READY state.
CPU Scheduler Invocation
There are four events which will cause the simulator to invoke schedule():
1. yield() – A process completes its CPU operations and yields the processor to perform an I/O request
2. wake up() – A process that previously yielded completes its I/O request, and is ready to perform CPU
operations.wake up() wake up() is also called when a process in the NEW state becomes runnable
3. preempt() – When using a Round-Robin or Static Priority scheduling algorithms, a CPU bound process may be preempted before it completes its CPU operations
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PROJECT 6 – MULTITHREADING CS 2200 – SYSTEMS AND NETWORKS
4. terminate() – A process exits or is killed
The CPU scheduler also contains one other important function: idle(); This functions simulates the idle
process. In the real world, the idle process puts the processor in a low power mode and waits. For our OS
simulation, you will make use of the pthread condition variable to block the thread until a process enters
the ready queue again.
The Simulator
We will use pthreads to simulate an operating system on a multiprocessor computer. We will use one
thread per CPU and one thread as a “supervisor“ for our simulation. The CPU threads will simulate the
currently-running processes on each CPU, and the supervisor thread will print output and dispatch events
to the CPU threads.
Since the code you write will be called from multiple threads, the CPU scheduler you write must be threadsafe! This means that all data structures you use, including your ready queue, must be protected using
mutexes.
The number of CPUs is specified as a command-line parameter to the simulator. For this project, you will
be performing experiments with 1, 2, and 4 CPU simulations.
Also, for demonstration purposes, the simulator executes much slower than a real system would. In the
real world, a CPU burst might range from one to a few hundred milliseconds, whereas in this simulator,
they range from 0.2 to 2.0 seconds.
Figure 2: Simulator Function Calls
Sample Output
Compile and run the simulator with ./os-sim 2. After a few seconds, hit Control-C to exit. You will see
the output below:
Time Ru Re Wa CPU0 CPU1 I/O Queue
==== == == == ==== ==== =========
0.0 0 0 0 (IDLE) (IDLE) <<
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PROJECT 6 – MULTITHREADING CS 2200 – SYSTEMS AND NETWORKS
0.1 0 0 0 (IDLE) (IDLE) <<
0.2 0 0 0 (IDLE) (IDLE) <<
0.3 0 0 0 (IDLE) (IDLE) <<
0.4 0 0 0 (IDLE) (IDLE) <<
0.5 0 0 0 (IDLE) (IDLE) <<
0.6 0 0 0 (IDLE) (IDLE) <<
0.7 0 0 0 (IDLE) (IDLE) <<
0.8 0 0 0 (IDLE) (IDLE) <<
0.9 0 0 0 (IDLE) (IDLE) <<
1.0 0 0 0 (IDLE) (IDLE) <<
……
The simulator generates a Gantt Chart, showing the current state of the OS at every 100ms interval. The
leftmost column shows the current time, in seconds. The next three columns show the number of Running,
Ready, and Waiting processes, respectively. The next two columns show the process currently running on
each CPU. The rightmost column shows the processes which are currently in the I/O queue, with the head
of the queue on the left and the tail of the queue on the right.
As you can see, nothing is executing. This is because we have no CPU scheduler to select processes to
execute! Once you complete Problem 1 and implement a basic FIFO scheduler, you will see the processes
executing on the CPUs.
Test Processes
For this simulation, we will use a series of eight test processes, five CPU-bound and three I/O bound. For
simplicity, we have labeled each process starting with a “C“ or “I“ to indicate CPU or I/O bound respectively. The table below shows a detailed breakdown of the processes.
For this project, priorities range from 0 to 10, with 10 being the highest priority. Note that the I/O-bound
processes have been given higher priorities than the CPU-bound processes.
Table 2: Process Descriptions
PID Process Name CPU or I/O bound Priority Start Time
0 Iapache I/O-bound 8 0.0 s
1 Ibash I/O-bound 7 1.0 s
2 Imozilla I/O-bound 7 2.0 s
3 Ccpu CPU-bound 5 3.0 s
4 Cgcc CPU-bound 1 4.0 s
5 Cspice CPU-bound 2 5.0 s
6 Cmysql CPU-bound 3 6.0 s
7 Csim CPU-bound 4 7.0 s
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PROJECT 6 – MULTITHREADING CS 2200 – SYSTEMS AND NETWORKS
Problem 0: pthreads Review
[0 points]
Spend some time and take a look at the pthreads documentation. Make a small multi-threaded program
where two threads print the numbers 1-1000. This will help you understand the lifecycle of threads.
You can use these excellent resources to get a better idea of pthreads:
• man pages for all the relevant pthread library calls. In particular, look at pthread mutex init,
pthread mutex lock(), pthread cond init, pthread cond broadcast, and pthread cond wait.
• http://www.llnl.gov/computing/tutorials/pthreads/
Note: When you get to using pthread cond wait(), use a while loop instead of an if statement to enclose
the call to the function. If you look carefully, the pthread documentation says that pthread cond wait
may return even without having acquired the lock. The while makes sure that the condition is checked
before continuing with the execution, ensuring that we acquire the lock. Using an if may cause completely
untraceable bugs in your programs.
Problem 1: FIFO Scheduler
A. [50 points]
Implement the CPU scheduler using the FIFO algorithm. You may do this however you like, however, we
suggest the following:
• Implement a thread-safe ready queue using a linked list. A linked list will allow you to reuse this
ready queue for the Round-Robin and Static Priority scheduling algorithms. (Hint: Look at the pcb t
struct in os-sim.h.)
• Implement the yield(), wake up(), and terminate() functions in student.c. preempt() is not necessary for this stage of the project. See the introduction and comments in the code for the proper
behavior of these events.
• Implement idle(). This function must wait on a condition variable that is signaled whenever a
process is added to the ready queue.
• Implement schedule(). This function should extract the first process in the ready queue, then call
context switch() to select the process to execute. If there are no runnable processes, schedule()
should call context switch with a NULL pointer as parameter to execute the idle process.
Before you begin writing code, look at the contents of the file os-sim.h for a list of
function prototypes and descriptions of the currently used data structures.
Once you successfully complete this portion of the project, make and test your code with ./os-sim 1. You
should see an output similar to the following:
Time Ru Re Wa CPU0 I/O Queue
==== == == == ==== =========
0.0 0 0 0 (IDLE) <<
0.1 1 0 0 Iapache <<
0.2 1 0 0 Iapache <<
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PROJECT 6 – MULTITHREADING CS 2200 – SYSTEMS AND NETWORKS
0.3 1 0 0 Iapache <<
0.4 0 0 1 (IDLE) 0.5 0 0 1 (IDLE) 0.6 1 0 0 Iapache <<
0.7 1 0 0 Iapache <<
0.8 1 0 0 Iapache <<
0.9 1 0 0 Iapache <<
1.0 0 0 1 (IDLE) 1.1 1 0 1 Ibash 1.2 1 0 1 Ibash 1.3 1 0 1 Ibash 1.4 1 0 1 Ibash 1.5 1 0 1 Iapache 1.6 1 0 1 Iapache 1.7 0 0 2 (IDLE) 1.8 0 0 2 (IDLE) 1.9 0 0 2 (IDLE) 2.0 0 0 0 Ibash ……
66.9 1 1 0 Ibash <<
67.0 1 1 0 Ibash <<
67.1 1 1 0 Ibash <<
67.2 1 0 0 Imozilla <<
67.3 1 0 0 Imozilla <<
67.4 1 0 0 Imozilla <<
67.5 1 0 0 Imozilla <<
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PROJECT 6 – MULTITHREADING CS 2200 – SYSTEMS AND NETWORKS
# of Context Switches: 97
Total execution time: 67.6 s
Total time spent in READY state: 389.9 s
(These numbers may be slightly different for you)
Important Information:
• Be sure to update the state field of the PCB. The library will read this field to generate the Running,
Ready, and Waiting columns of the output. These are also used to generate the statistics and the end
of the simulation.
• Four of the five entry points in the scheduler (idle(), yield(), terminate(), and preempt()) should
cause a new process to be scheduled on the CPU. In your handlers, be sure to call schedule(), which
will select a runnable process, and then call context switch(). When these four functions return, the
library will simulate the process selected by context switch().
• context switch() takes a timeslice parameter, which is used for preemptive scheduling algorithms.
Since FIFO is non-preemptive, use -1 for this parameter to give the process an infinite timeslice.
B. [10 points]
Run your OS simulation with 1, 2, and 4 CPUs. Compare the total execution time of each. Is there a linear
relationship between the number of CPUs and total execution time? Why or why not?
Problem 2: Round-Robin Scheduler
A. [10 points]
Add Round-Robin scheduling functionality to your code. You should modify main() to add a command
line option, -r, which selects the Round-Robin scheduling algorithm, and accepts a parameter, the length of
the timeslice. For this project, timeslices are measured in tenths of seconds. E.g.:
./os-sim <# of CPUs> -r 5
Should run a Round-Robin scheduler with timeslices of 500 ms. While:
./os-sim <# of CPUs>
Should continue to run the FIFO scheduler.
Make sure that you also implement the preempt() function To specify a timeslice when scheduling a
process, use the timeslice parameter of context switch(). The simulator will automatically preempt the
process and call your preempt() handler when a process finishes executing on the CPU for the length of
the timeslice without terminating or yielding for I/O.
B. [10 points]
Run your Round-Robin scheduler with timeslices of 800ms, 600ms, 400ms, and 200ms. Use only one CPU
for your tests. Compare the statistics at the end of the simulation. You will see that the total waiting time
decreases with shorter timeslices. However, in a real OS, the shortest timeslice may not be the best choice.
Explain why that is the case?
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PROJECT 6 – MULTITHREADING CS 2200 – SYSTEMS AND NETWORKS
Static Priority Scheduler
A. [10 points]
Add Static-Priority scheduling support to your code. Modify main() to accept the “-p“ parameter to select
the Static Priority algorithm. The “-r“ and default FIFO scheduler should continue to work.
The scheduler should use the priority specified in the static priority field of the PCB. This priority is a
value from 0 to 10, with 0 being the lowest priority and 10 being the highest priority.
For Static Priority scheduling, you will need to make use of the current[] array and force preempt()
function. The current[] array should be used to keep track of the process currently executing on each
CPU. Since this array is accessed by multiple CPU threads, it must be protected by a mutex. current mutex
has been provided to you for this purpose.
The force preempt() function preempts a running process before its timeslice expires. Your wake up()
handler should make use of this function to preempt a lower priority process when a higher priority process needs a CPU.
B. [10 points]
The Shortest-Job First (SJF) scheduling algorithm is proven to have the optimal average waiting time. However, it is not feasible to implement in a typical scheduler, since the scheduler does not have advanced
knowledge of the length of each CPU burst.
Run each of your three scheduling algorithms (using one CPU), and compare the total waiting times. Which
algorithm is the closest approximation of SJF? Why?
Assignment Submission
Note: Each problem has two parts (labeled A and B). The first is the actual implementation, and the second
is a conceptual question that is to be answered after running some tests. Make sure you complete both.
We have provided you with a make submit command in the Makefile. Use it to generate a tarball that can
be submitted.
Please Note:
• Make sure that the tarball contains the following item:
– answers.txt – Short answers for part B of all the problems.
– Makefile – Working one has been provided to you;
– os-sim.c – Code for the operating system simulator
– os-sim.h – Header file for the simulator
– process.c – Descriptions of the simulated processes
– process.h – Header file for the process data
– student.c – Your code for the scheduler
– student.h – Header file for your scheduler code
We suggest untarring the tarball to make sure that the above contents are all present
• If you code does not compile, you will get a zero!
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PROJECT 6 – MULTITHREADING CS 2200 – SYSTEMS AND NETWORKS
• Keep your answers for part B of all the problems detailed enough to cover the question, including
support from simulator results if appropriate. Don’t write a book; however when in doubt, err on the
side of giving us too much information.
This is the last project that you will have to demo. We will announce
when demos are available. Failure to demo will result in a zero!
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