CPSC–410/611/613 Machine Problem 5: Kernel-Level Thread Scheduling


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In this machine problem you will add scheduling of multiple kernel-level threads to your code
base. The emphasis is on scheduling. The test code for this MP (file kernel.C) illustrates that
a scheduler is not necessary for multiple threads to happily execute in a system.

For example, the
following code displays two routines (each executed by a different thread) that explicitly hand the
control back and forth, and so achieve a simple form of multithreading:
void fun1 () {
console_puts (“FUN 1 INVOKED BY THREAD 1\n”) ;
for (;;) {
< do something … >
thread_dispatch_to ( thread2 ) ;
void fun2 () {
console_puts (“FUN 2 INVOKED BY THREAD 2\n”) ;
for (;;) {
< do something … >
thread_dispatch_to ( thread1 ) ;

In this piece of code the CPU is passed back and forth between Thread 1 and Thread 2. And
this works fine, until we want to add a third thread. Do we really want to modify the code of
fun2() to dispatch Thread 3 on the CPU? And does the programmer of Thread 3 really want to
know that the CPU needs to be dispatched back to Thread 1? What if she does not want to release
the CPU at all? So, we cannot rely on the threads to play nice, and we need a third party that
allocates the CPU on behalf of the running threads: We need a scheduler.

The interface of the
scheduler is defined in file scheduler.H. It exports the following functionality:
class Scheduler {
public :
Scheduler () ;

/* Setup the scheduler . This sets up the ready queue , for example . If the
scheduler implements some sort of round – robin scheme , then the
end_of_quantum handler is installed here as well . */
virtual void yield () ;

/* Called by the currently running thread in order to give up the CPU. The
scheduler selects the next thread from the ready queue to load onto the
CPU , and calls the dispatcher function defined in class Thread to do the
context switch . */
virtual void resume ( Thread * _thread ) ;

/* Add the given thread to the ready queue of the scheduler . This is called
for threads that were waiting for an event to happen , or that have to
give up the CPU in response to a preemption . */
virtual void add ( Thread * _thread ) ;

/* Make the given thread runnable by the scheduler . This function is
typically called after a thread has been created . Depending on the
implementation , this may not entail more than simply adding the thread to
the ready queue (see Scheduler :: resume ). */

virtual void terminate ( Thread * _thread ) ;
/* Remove the given thread from the scheduler in preparation for destruction
of the thread . */

You are to implement a simple First-In-First-Out (FIFO) scheduler. This is very easy to achieve:
The scheduler maintains a so-called ready queue, which is a list of threads (or, better, their thread
control blocks) that are waiting to get to the CPU. One thread is running on the CPU, typically.

Whenever a running thread calls yield(), the scheduler finds the next thread to run at the head
of the ready queue, and calls the dispatcher to invoke a context switch. Whenever the system
decides that a thread, say Thread t1, should become ready to execute on the CPU again it calls
resume(t1), which adds t1 to the end of the ready queue. Other threads may be waiting for
events to happen, such as for a page fault or some other I/O operation to complete. We call these
threads blocked. We don’t worry about blocked threads for now, as all the threads are either busy
executing or waiting on the ready queue. Later, when threads access devices, we will have to deal
with blocked threads as well.

Implementation of the Scheduler

You realize the scheduler by implementing the exported functions (declared in file scheduler.H)
inside a file called scheduler.C. Implement primarily the constructor and the functions yield(),
add(), and resume(). When you are ready to handle terminating thread functions, you may want
to implement the function terminate() as well.

Depending on how you want to implement the ready queue, you may have to modify the thread
control block in thread.H.

Testing your Scheduler

The file kernel.C is set up to make testing of your scheduler easy. The file creates four threads,
each of which is assigned a different function – called fun1 for Thread 1 to fun4 for Thread 4.
These thread functions call a function pass on CPU() to explicitly dispatch the next thread on the
CPU in the way described above. A macro USES SCHEDULER defines how control is passed among
threads in file kernel.C. If the macro is defined, then the code makes use of the scheduler to hand
control from one thread to the next. For details, look at the source code in kernel.C.

Threads with Terminating Thread Functions

Currently, all thread functions in kernel.C are non-terminating (basically infinite loops). They
only stop executing when they pass control to another thread. The system at this point does not
know what to do when the thread function returns. In other words, there is no support in the lowlevel thread management for threads with thread functions that return and therefore stop executing.

You are to study the thread management code and propose and implement a solution that allows
a thread to cleanly terminate when its thread function returns. You need to worry about releasing
the CPU, releasing memory, giving control to the next thread, etc. Test your solution by making
the thread functions in kernel.C return. You can easily do this by modifying the for loops to have
an upper bound. For example, modify the lines for(int j = 0;; j++) to read for (int j = 0;
j < DEFAULT NB ITERS; j++) or similar. A macro TERMINATING FUNCTIONS defines whether the thread functions for the first two threads terminate after 10 iterations or not. For details, look at
the source code in kernel.C.

Opportunities for Bonus Points
If you want to extend your work beyond the bare minimum, here are a few options.
OPTION 1: Correct handling of interrupts. (This option carries 6 bonus points.) You
will notice that interrupts are disabled after we start the first thread. One symptom is that the
periodic clock update message is missing.

This is caused by the way we create threads: We set
up the context so that the Interrupt Enable Flag (IF) in the EFLAGS status register is zero,
which disables interrupts once the created thread returns from the fake ”exception” when it starts.

This is ok, because we may not want to deal with interrupts when we are just starting up our
thread. But at some point we need to re-enable interrupts. And turn them off again when we
need to ensure mutual exclusion. It is up to you to modify the code in the scheduler and the
thread management to ensure correct enabling/disabling of interrupts. The functions to do that
are provided in machine.H.

OPTION 2: Round-Robin Scheduling: (This option carries 6 bonus points.
) Once we
have interrupts set up correctly, we can expand our simple FIFO scheduler into a basic roundrobin scheduler. For this, we want to generate an interrupt at a periodic interval (say 50 msec).

You should preferably do this by deriving a class EOQTimer from the provided SimpleTimer that
implements the end-of-quantum functionality for the scheduler. In kernel.C you then construct
an EOQ timer instead of a simple timer. The interrupt handler of the EOQ timer then triggers the
preemption of the currently running thread. During the preemption the currently running thread
puts itself on the ready queue and then gives up the CPU.

If you think this is just a resume() followed by a yield(), think again. The situation is a
bit more complicated than that. For example, the current thread is giving up the CPU inside
an interrupt handler, and the new thread may or may not be returning from a preemption, and
therefore may or may not be returning from an interrupt handler. In both cases we need to let the
interrupt controller (PIC) know that the interrupt has been handled. In addition, we need to stop
the original thread from informing the PIC when it returns from the interrupt possibly much later.

It is very likely that you will need to modify the low-level exception and interrupt handling code
to get this to work correctly.

OPTION 3: Processes: (This option carries 12 bonus points.
) Until now, all threads share
the same address space. Extend the system to allow for the creation of threads that have different
address spaces; basically, implement processes. If you plan to attack this option, you will have to
handle a number of different aspects, including creation of multiple address spaces (multiple page
tables,) and switching from one address space to another as part of the thread switch. The code
for the thread switch needs to be somewhat extended to handle the switch from one page table to
the other.

We keep the specification of this option intentionally vague in order to have you come
up with your own design. Again: Do not underestimate the difficulty of this option!

1Attack this problem only after you have convinced yourself that you are managing interrupts correctly. Otherwise
you will waste a lot of time debugging!

2This option is very challenging and only for students who feel totally under-challenged. You absolutely have to
contact the instructor before starting on this option! If you do not, we will not consider your submission for this

A Note about the new Main File
The main file for this MP is somewhat similar in nature to the earlier MPs. We have removed page
tables and paging, and we have added code to initialize threading, for scheduling, and for creating
a small number of threads.

The two classes ContFramePool (defined in files cont frame pool.C/H) VMPool (defined in files
vm pool.C/H), which together implement virtual memory pools in previous MPs, have been replaced
by the two very simplified classes FramePool (defined in files frame pool.C/H) and MemPool (defined
in files mem pool.C/H), which implement a very simple physical-memory allocator. You will not
have to worry about memory management in this MP. In particular, there will be no virtual memory,
and paging will not be turned on.3

The Assignment

1. Download the provided source code for this MP.
2. Implement the routines defined in file scheduler.H (and described above) to initialize the
scheduler, to add new threads, and to perform the scheduling operations.
3. You may need to modify file scheduler.H. If so, document how and why.

4. Modify file kernel.C to replace explicit dispatching with calls to the scheduler. This can be
done by uncommenting the definition of macro USES SCHEDULER that enables scheduling.
Details about how to do this are given in file kernel.C.

5. Add support for threads with terminating thread functions. This may require modifications
to file thread.C. If so, document how and why. Test your solution by uncommenting the
definition of the appropriate macro in file kernel.C.

6. If you decide to pursue one or more of the options, proceed as follows.
Option 1 Fix the interrupt management so that interrupts remain enabled outside of critical

Option 2 Modify the scheduler to implement round-robin with a 50 msec time quantum.
(Note: Since the preemption interrupt arrives periodically, this is not a very good roundrobin scheduler. Whenever a thread gives up the CPU voluntarily, the next thread is

Option 3 Contact the instructor to discuss your plan to implement multiple address spaces.
Modify the thread creation, page table initialization, and thread switching to allow for
multiple address spaces. Add page tables back into the code and exercise your new
system. It will be up to you to come up with a design, the implementation, and the test
suite for your solution.

What to Hand In
You are to hand in a ZIP file, with name mp5.zip, containing the following files:
3Students who decide to attack Option 3 will have to add paging and page tables back in to test their approach.

1. A design document, called design.pdf (in PDF format) that describes your design and the
implementation of the FIFO scheduler, and any of the selected options. Clearly identify at
the beginning of your design document and in your submitted code which options
you have selected, if any.

2. All the source files and the makefile needed to compile the code.
3. The assignment should not require modifications to files other than scheduler.H/C and
thread.H/C, in addition to the implementation of the timers. Any modifications to these
files must be documented. Any modification to other files must be clearly motivated and
documented. Be very careful when you modify the public interface provided by
the .H files. We are using our own test code to check the correctness of your implementations,
and we have to ensure that your code is compatible with our tests.

4. Clearly identify and comment changes to existing code.

5. Grading of these MPs is a very tedious chore. These handin instructions are meant to mitigate
the difficulty of grading, and to ensure that the grader does not overlook any of your efforts.

Note: Pay attention to the capitalization in file names. For example, if we request a file called
file.H, we want the file name to end with a capital H, not a lower-case one. While Windows does
not care about capitalization in file names, other operating systems do. This then causes all kinds
of problems when the TA grades the submission.
Failure to follow the handin instructions will result in lost points.