Homework 03

linearizability and progress

You can download a PDF of this assignment here

Instructions

You may work in groups of up to 4 and submit a single assignment for the group. For computational problems, please show your work; for conceptual questions, please explain your reasoning. Solutions may be neatly hand-written and scanned or typeset. Please submit your solution to Moodle in PDF format.

Due: Friday, April 2, 23:59 AoE


The following exercises refer to the queue implementation, IQueue. For simplicity, assume that the array items is unbounded. You can read about the compareAndSet method for the AtomicInteger class here.

public class IQueue {
    AtomicInteger head = new AtomicInteger(0);
    AtomicInteger tail = new AtomicInteger(0);
    int[] items = new int[Integer.MAX_VALUE];

    public void enq (int x) {
	int slot;
	do {
	    slot = tail.get();
	} while (!tail.compareAndSet(slot, slot+1));
	items[slot] = x;
    }

    public int deq () throws EmptyException {
	int value;
	int slot;
	do {
	    slot = head.get();
	    value = items[slot];
	    if (value == null)
		throw new EmptyException();
	} while (!head.compareAndSet(slot, slot+1));
	return value;
    }
}

Exercises

Exercise 1. Give an execution demonstrating that IQueue is not linearizable.

Exercise 2. Is IQueue wait-free? Lock-free? Why or why not?