Project 1: Threads

Fall 2016

Due: Friday, October 21, at 11:59pm

The baseline Nachos implementation has an incomplete thread system. In this project, your job is to complete it, and then use it to solve synchronization problems.


Properly synchronized code should work no matter what order the scheduler chooses to run the threads on the ready list. In other words, we should be able to put a call to KThread.yield (causing the scheduler to choose another thread to run) anywhere in your code where interrupts are enabled, and your code should still be correct. You will be asked to write properly synchronized code as part of the later assignments, so understanding how to do this is crucial to being able to do the project.

To aid you in this, code linked in with Nachos will cause KThread.yield to be called on your behalf in a repeatable (but sometimes unpredictable) way. Nachos code is repeatable in that if you call it repeatedly with the same arguments, it will do exactly the same thing each time. However, if you invoke nachos -s <number> with a different number each time, calls to KThread.yield will be inserted at different places in the code.

You will be modifying source code files in the threads subdirectory, and compiling in the proj1 subdirectory. You are encouraged to add new classes to your project as you see fit; the code we provide you is not a complete skeleton for the project. As described on the main project page, be careful to add any additional classes to the packages (directories) permitted.

There should be no busy-waiting in any of your solutions to this assignment. (The initial implementation of Alarm.waitUntil is an example of busy-waiting.)


  1. (0%) Browse through the initial thread system implementation, starting with This thread system implements thread fork, thread completion, and semaphores for synchronization. It also provides locks and condition variables built on top of semaphores.

    Trace the execution path (by hand) for the startup test case provided. When you trace the execution path, it is helpful to keep track of the state of each thread and which procedures are on each thread's execution stack. You will notice that when one thread calls TCB.contextSwitch, that thread stops executing, and another thread starts running. The first thing the new thread does is to return from TCB.contextSwitch. We realize this comment will seem cryptic to you at first, but you will understand threads once you understand why the TCB.contextSwitch that gets called is different from the TCB.contextSwitch that returns.

    Compile and run the baseline implementation of Nachos in the proj1 directory:

        % cd nachos/proj1
        % make
        % nachos

    The output should be the same as in project 0.

  2. (20%) Complete the implementation of the Alarm class. A thread calls waitUntil(long x) to suspend its execution until wall-clock time has advanced to at least now + x. This method is useful for threads that operate in real time, such as blinking the cursor once per second. There is no requirement that threads start running immediately after waking up; just put them on the ready queue in the timer interrupt handler after they have waited for at least the right amount of time. Do not fork any additional threads to implement waitUntil; you need only modify waitUntil and the timer interrupt handler. waitUntil itself, though, is not limited to being called by one thread; any number of threads may call it and be suspended at any one time. If the wait parameter x is 0 or negative, return without waiting (do not assert).

    Note that only one instance of Alarm may exist at a time (due to a limitation of Nachos), and Nachos already creates one global alarm that is referenced via ThreadedKernel.alarm.

    Testing: Implement tests that verify that a thread waits (approximately) for its requested duration; if the wait parameter is 0 or negative, the thread does not wait; multiple threads waiting on the alarm are woken up at the proper times, and in the proper order. For examples and strategies for implementing tests, see the Testing section below.

  3. (20%) Implement KThread.join, which synchronizes the calling thread with the completion of the called thread. As an example, if thread B executes the following:
        KThread A = new KThread (...);
        A.join ();
    we say that thread B (the "parent") joins with thread A (the "child"). When B calls join on A, there are two possibilities. If Ahas already finished, then B returns immediately from join without blocking. If A has not finished, then B waits inside of join until A finishes; when A finishes, it resumes B.

    Note that join does not have to be called on a thread. A thread should be able to finish successfully even if no other thread calls join on it.

    A thread cannot join to itself. (The initial implementation already checks for this case and invokes Lib.assert when it happens. Keep this Lib.assert call in your code.)

    Join can be called on a thread at most once. If thread B calls join on A, then it is an error for B or any other thread C to call join on A again. Assert on this error.

    Testing: Implement tests that verify if a parent calls join on a child and the child is still executing, the parent waits; if a parent calls join on a child and the child has finished executing, the parent does not block; if a thread calls join on itself, Nachos asserts; if join is called more than once on a thread, Nachos asserts; one parent thread can join with multiple child threads in succession; independent pairs of parent/child threads can join with each other without interference.

  4. (30%) Implement condition variables using interrupt enable and disable to provide atomicity. The class Condition is a sample implementation that uses semaphores, and your job is to provide an equivalent implementation in class Condition2 by manipulating interrupts instead of using semaphores. Once you are done, you will have two alternative implementations that provide the exact same functionality. Examine the existing implementation of class Semaphore to guide you on how to manipulate interrupts for when you implement the methods of Condition2.

    A thread must have acquired the lock associated with the condition variable when it invokes methods on the CV. The underlying implementation of the Lock class already has code to assert in these cases, but we recommend writing a test program that causes such an error so that you can see what happens.

    Testing: Implement tests that verify that sleep blocks the calling thread; wake wakes up at most one thread, even if multiple threads are waiting; wakeAll wakes up all waiting threads; if a thread calls any of the synchronization methods without holding the lock, Nachos asserts; wake and wakeAll with no waiting threads have no effect, yet future threads that sleep will still block (i.e., the wake/wakeAll is "lost", which is in contrast to the semantics of semaphores).

  5. (30%) Implement the Communicator class to perform synchronous send and receive of one word messages (also known as Ada-style rendezvous). Synchronize threads calling the methods void speak(int word) and int listen() using condition variables. A thread calling speak atomically waits until a different thread calls listen on the same Communicator object, facilitating the word parameter to be transfered between threads. Once the transfer is made—and only once it is made—both threads can return. Similarly, a thread calling listen waits until a different thread calls speak, at which point the transfer is made, and both can return. In both cases, listen returns the word passed to speak. After the transfer is made, it does not matter the order in which either thread returns.

    Your implementation should work correctly even if there are multiple speakers and listeners for the same Communicator object. In this case, a message is still passed between one speaker and one listener—the other threads are queued. Listener and speaker threads pair up in first-come-first-served order. (Your implementation cannot use a queue or list data structure to keep track of the multiple threads or the multiple words passed in; condition variables have all the queues needed.)

    To summarize:

    Testing: Implement tests that verify the semantics above: a speaker blocks until a listener has received the value (and vice-versa); if there are multiple speakers and listeners, they exchange values in FIFO order; if there are more speakers than listeners, the "extra" speakers block (and vice-versa); when there are multiple Communicator objects, the speakers and listeners for one object do not interact with the speakers and listeners for other objects.


It is your responsibility to implement your own tests to thoroughly exercise your code to ensure that it meets the requirements specified for each part of the project. Testing is an important skill to develop, and the Nachos projects will help you to continue to develop that skill. You can add calls to testing code in ThreadedKernel.selfTest, and add class-specific code in selfTest methods of each class.

As a testing strategy, first start with simple tests and then implement more complicated tests. When something goes wrong with a simple test, it is easier to pinpoint what aspect of your implementation has a bug. When something goes wrong with a more complicated test, it is more difficult to determine where the bug may be unless you can rule out all the causes that your simple tests have shown to already be correct. We also strongly recommend implementing tests as separate methods, rather than making changes to just one or a few methods. Rather than making a change to an existing test to evaluate new functionality, clone the test into a new method and make the change. That way, your earlier tests are always there in case you need to use them again. You can comment out calls to previous tests so that you can concentrate on one test at a time.

To help you get jumpstarted on testing, here are a handful of example test programs across the various problems:

Code Submission

You do not have to do anything special to submit your project. We will use a snapshot of your Nachos implementation in your github repository as it exists at the deadline, and grade that version. (Even if you have made changes to your repo after the deadline, that's ok, we will use a snapshot of your code at the deadline.)


You can discuss concepts with students in other groups, but do not cheat when implementing your project. Cheating includes copying code from someone else's implementation, or copying code from an implementation found on the Internet. See the main project page for more information.

We will manually check and also run code plagiarism tools on submissions and multiple Internet distributions (if you can find it, so can we).