Assignment 1

Assignment 1: Synchronisation

Due Date: 8am, Monday morning, 5th of September

Worth 25 marks (of the 100 available for the class mark component of the course)

The 10% bonus for one week early applies

The extra 5% bonus for a submitted, working assignment within 48 hours of release applies if you submit it before Friday, midnight.

See course intro for exact details.



In this assignment you will solve a number of synchronisation and locking problems. You will also get experience with data structure and resource management issues.

Please complete the reading exercises for your Week 5 tutorial.

Write Readable Code

In your programming assignments, you are expected to write well-documented, readable code. There are a variety of reasons to strive for clear and readable code. Code that is understandable to others is a requirement for any real-world programmer, not to mention the fact that after enough time, you will be in the shoes of one of the others when attempting to understand what you wrote in the past. Finally, clear, concise, well-commented code makes it easier for the assignment marker to award you marks! (This is especially important if you can't get the assignment running. If you can't figure out what is going on, how do you expect us to).

There is no single right way to organise and document your code. It is not our intent to dictate a particular coding style for this class. The best way to learn about writing readable code is to read other people's code, for example OS/161. When you read someone else's code, note what you like and what you don't like. Pay close attention to the lines of comments which most clearly and efficiently explain what is going on. When you write code yourself, keep these observations in mind.

Here are some general tips for writing better code:


By default, we assume both group partners have done similar amount of coding, and we distribute the marks equally between both group members. However, if both of you agree, you can split the work (and the marks) differently (e.g., 40/60 instead of 50/50). In this case, you have to hand in a signed note to your tutor (or both tutors if you attend different tutes) stating how you want to split the marks before the end of the late submission deadline. If you can't agree on a fair split, contact your tutor. We will consult CVS logs to determine the distribution. If CVS hasn't been used, the split will be 50/50 (one more reason to use CVS). Please understand that we can't consider any complaints about group members after the deadline for the late submission has passed.

Setting Up Assignment 1

Some more practice with CVS

At the end of the previous assignment, you would have committed the changes you made to the CVS repository and tagged as asst0-final.

You may have made some changes to the sources since then. To check this you can use the following command

% cd ~/cs3231/asst0-src
% cvs -q -n update
M kern/main/main.c

A short note on the arguments to cvs:

Note that in our example, we have modified (M) the kern/main/main.c file since the last commit. The output above might be different for you. It may be empty, in which case you have not made any changes since asst0 finished.

If you have made changes, decide whether you wish to keep them to play around with later, or whether you simply wish to throw them away. If you wish to keep them, you should commit your changes back to the repository using cvs commit.

Now remove your asst0-src tree to prepare for this assignment (yes really).

% cd ~/cs3231
% rm -rf asst0-src
Note you can get your original tree back at anytime by running cvs checkout asst0-src. Feel free to test this if you wish, but make sure you remove it again before proceeding further. Never delete (or cvs init) your repository in /home/osprj000/cvsroot, as you will not be able to retrieve your past work.

We assume that the remainder of the ~/cs3231 directory is in place from the previous assignment, and that your umask, PATH and CVSROOT are still set appropriately.

Obtaining and setting up ASST1 in CVS

In this section, you will be setting up the cvs repository that will contain your code. Only one of you needs to do the following. We suggest your partner sit in on this part of the assignment. You have now completed setting up a shared repository for both partners. The following instructions are now for both partners.

You are now ready to start the assignment.

Begin Your Assignment

Configure OS/161 for Assignment 1

Before proceeding further, configure your new sources.

% cd ~/cs3231/asst1-src
% ./configure

We have provided you with a framework to run your solutions for ASST1. This framework consists of driver code (found in kern/asst1 ) and menu items you can use to execute your solutions from the OS/161 kernel boot menu.

You have to reconfigure your kernel before you can use this framework. The procedure for configuring a kernel is the same as in ASST0, except you will use the ASST1 configuration file:

% cd ~/cs3231/asst1-src/kern/conf	
% ./config ASST1
You should now see an ASST1 directory in the compile directory.

Building for ASST1

When you built OS/161 for ASST0, you ran make from compile/ASST0 . In ASST1, you run make from (you guessed it) compile/ASST1 .
% cd ../compile/ASST1
% make depend
% make
% make install
If you are told that the compile/ASST1 directory does not exist, make sure you ran config for ASST1. Run the resulting kernel:
% cd ~/cs3231/root
% sys161 kernel 
sys161: System/161 release 1.1, compiled Jul 28 2003 17:28:51

OS/161 base system version 1.08
Copyright (c) 2000, 2001, 2002, 2003
   President and Fellows of Harvard College.  All rights reserved.

Put-your-group-name-here's system version 0 (ASST1 #6)

Cpu is MIPS r2000/r3000
1876k physical memory available
Device probe...
lamebus0 (system main bus)
emu0 at lamebus0
ltrace0 at lamebus0
ltimer0 at lamebus0
hardclock on ltimer0 (10000 hz)
beep0 at ltimer0
rtclock0 at ltimer0
lrandom0 at lamebus0
random0 at lrandom0
lser0 at lamebus0
con0 at lser0
pseudorand0 (virtual)

OS/161 kernel [? for menu]: 

Command Line Arguments to OS/161

Your solutions to ASST1 will be tested by running OS/161 with command line arguments that correspond to the menu options in the OS/161 boot menu.

IMPORTANT: Please DO NOT change these menu option strings!

Here are some examples of using command line args to select OS/161 menu items:

sys161 kernel "at;bt;q"
This is the same as starting up with sys161 kernel, then running "at" at the menu prompt (invoking the array test), then when that finishes running "bt" (bitmap test), then quitting by typing "q".
sys161 kernel "q"
This is the simplest example. This will start the kernel up, then quit as soon as it's finished booting. Try it yourself with other menu commands. Remember that the commands must be separated by semicolons (";").

"Physical" Memory

HEADS UP!!!! Make sure you do the following Failing to do so will potentially lead to subtle problems that will be very difficult to diagnose.

In order to execute the tests in this assignment, you will need more than the 512 KB of memory configured into System/161 by default. We suggest that you allocate at least 2MB of RAM to System/161. This configuration option is passed to the busctl device with the ramsize parameter in your ~/cs3231/root/sys161.conf file. Make sure the busctl device line looks like the following:

31 busctl ramsize=2097152 
Note: 2097152 bytes is 2MB.

Concurrent Programming with OS/161

If your code is properly synchronised, the timing of context switches and the order in which threads run should not change the behaviour of your solution. Of course, your threads may print messages in different orders, but you should be able to easily verify that they follow all of the constraints applied to them and that they do not deadlock.

Built-in thread tests

When you booted OS/161 in ASST0, you may have seen the options to run the thread tests. The thread test code uses the semaphore synchronisation primitive. You should trace the execution of one of these thread tests in GDB to see how the scheduler acts, how threads are created, and what exactly happens in a context switch. You should be able to step through a call to mi_switch() and see exactly where the current thread changes.

Thread test 1 ( " tt1 " at the prompt or tt1 on the kernel command line) prints the numbers 0 through 7 each time each thread loops. Thread test 2 (" tt2 ") prints only when each thread starts and exits. The latter is intended to show that the scheduler doesn't cause starvation--the threads should all start together, spin for awhile, and then end together.

Debugging concurrent programs

thread_yield() is automatically called for you at intervals that vary randomly. While this randomness is fairly close to reality, it complicates the process of debugging your concurrent programs.

The random number generator used to vary the time between these thread_yield() calls uses the same seed as the random device in System/161. This means that you can reproduce a specific execution sequence by using a fixed seed for the random number generator. You can pass an explicit seed into random device by editing the "random" line in your sys161.conf file. For example, to set the seed to 1, you would edit the line to look like:

	28 random seed=1 
We recommend that while you are writing and debugging your solutions you pick a seed and use it consistently. Once you are confident that your threads do what they are supposed to do, set the random device to autoseed. This should allow you to test your solutions under varying conditions and may expose scenarios that you had not anticipated. To reproduce your test cases, you additionally need to run your tests via command line args to sys161 as described above.

Tutorial Exercises

Please answer the following questions and bring them to your tutorial in week 5.

Code reading

To implement synchronisation primitives, you will have to understand the operation of the threading system in OS/161. It may also help you to look at the provided implementation of semaphores. When you are writing solution code for the synchronisation problems it will help if you also understand exactly what the OS/161 scheduler does when it dispatches among threads.

Thread Questions

1. What happens to a thread when it exits (i.e., calls thread_exit() )? What about when it sleeps?
2. What function(s) handle(s) a context switch?
3. How many thread states are there? What are they?
4. What does it mean to turn interrupts off? How is this accomplished? Why is it important to turn off interrupts in the thread subsystem code?
5. What happens when a thread wakes up another thread? How does a sleeping thread get to run again?

Scheduler Questions

6. What function is responsible for choosing the next thread to run?
7. How does that function pick the next thread?
8. What role does the hardware timer play in scheduling? What hardware independent function is called on a timer interrupt?

Synchronisation Questions

9. Describe how thread_sleep() and thread_wakeup() are used to implement semaphores. What is the purpose of the argument passed to thread_sleep() ?
10. Why does the lock API in OS/161 provide lock_do_i_hold() , but not lock_get_holder() ?

Synchronisation Problems

The following problems are designed to familiarise you with some of the problems that arise in concurrent programming and help you learn to identify and solve them.

Identify Deadlocks

11. Here are code samples for two threads that use binary semaphores. Give a sequence of execution and context switches in which these two threads can deadlock.
12. Propose a change to one or both of them that makes deadlock impossible. What general principle do the original threads violate that causes them to deadlock?
semaphore *mutex, *data;
void me() {
	/* do something */
	/* do something else */
	/* clean up */
void you() {
	/* do something */

More Deadlock Identification

13. Here are two more threads. Can they deadlock? If so, give a concurrent execution in which they do and propose a change to one or both that makes them deadlock free.
lock *file1, *file2, *mutex;
void laurel() {
	/* do something */
    	/* write to file 1 */
	/* write to file 2 */
	/* do something */
	/* read from file 1 */
	/* write to file 2 */
void hardy() {
    	/* do stuff */
	/* read from file 1 */
	/* write to file 2 */
	/* do something */
	/* write to file 1 */

Synchronised Queues

14. The thread subsystem in OS/161 uses a queue structure to manage some of its state. This queue structure is not synchronised. Why not? Under what circumstances should you use a synchronised queue structure?

Describe (and give pseudocode for) a synchronised queue data structure based on the queue structure included in the OS/161 codebase. You may use semaphores, locks, and condition variables as you see fit. You must describe (a proof is not necessary) why your algorithm will not deadlock.

Make sure you clearly state your assumptions about the constraints on access to such a structure and how you ensure that these constraints are respected.

Coding Assignment

We know: you've been itching to get to the coding. Well, you've finally arrived!

This is the assessable component of this assignment. It is worth 25 marks of the 100 available for the class mark component of the course.

Solving Synchronisation Problems

The following problems will give you the opportunity to write two fairly straightforward concurrent programs and get a more detailed understanding of how to use concurrency mechanisms to solve problems.

We have provided you with basic driver code that starts a predefined number of threads that execute a predefined activity (in the form of calling functions that you must implement). You are responsible for implementing the functions called.

Remember to specify a seed to use in the random number generator by editing your sys161.conf file, and run your tests using sys161 command line args. It is much easier to debug initial problems when the sequence of execution and context switches is reproducible.

When you configure your kernel for ASST1, the driver code and extra menu options for executing your solutions are automatically compiled in.

Implementing Pipes

Your task is to implement thread communication channels inspired by (but not the same as) Unix pipes as discussed in the lecture. You have to implement the following five functions: