Lecture 21: Linearizability II

COSC 273: Parallel and Distributed Computing

Spring 2023

Announcements

1. Quiz this Friday
   • sequential consistency
   • linearizability
   • recall stack operations
     • push(x), pop()

Last Time: Linearizability

An execution of a shared object is linearizable if:

• exists a linearization point in each method call such that execution is consistent with sequential execution where method calls occur in order of corresponding linearization points

An implementation of an object is linearizable if every execution is linearizable.

Linearizable TwoCounter

public class TwoCounter {
    int[] counts = new int[2];	
    public void increment (int amt) {
        int i = ThreadID.get(); // thread IDs are 0 and 1
        int count = counts[i];
        counts[i] = count + amt;		
    }
    public int read () {
        int count = counts[0];
        count = count + counts[1];
        return count;
    }}

ThreeCounter Example

public class ThreeCounter {
    int[] counts = new int[3];
	
    public void increment (int amt) {
        int i = ThreadID.get(); // thread IDs are 0, 1, and 2
        int count = counts[i];
        counts[i] = count + amt;		
    }	
}

A read Method

    public int read () {
        int count = counts[0];
        count = count + counts[1];
        count = count + counts[2];		
        return count;
    }

Is ThreeCounter Linearizable?

Writing Between the Lines

    public int read () {
        int count = counts[0];
        count = count + counts[1];
        count = count + counts[2];		
        return count;
    }

Sequentially Consistency

Questions.

1. Is the previous execution sequentially consistent?
2. Is ThreeCounter sequentially consistent?

A Queue Again

Question. How to implement a (non-concurrent) queue with a linked list?

A Concurrent Queue

• Use linked list implementation of queue
• Store:
  • Node head sentinal
    • deq returns head.next value (if any), updates head
  • Node tail
    • enq updates tail.next, updates tail
• Locks:
  • enqLock locks enq operation
  • deqLock locks deq operation
  • individual Nodes are not locked

Unbounded Queue in Pictures

Dequeue 1: Aquire deqLock

Dequeue 2: Get Element (or Exception)

Dequeue 3: Update head

Dequeue 4: Release Lock

Enqueue 1: Make Node

Enqueue 2: Acquire enqLock

Enqueue 3: Update tail.next

Enqueue 4: Update tail

Enqueue 5: Release Lock

Question

Why do we need the sentinel node?

UnboundedQueue in Code

public class UnboundedQueue<T> implements SimpleQueue<T> {
    final ReentrantLock enqLock;
    final ReentrantLock deqLock;
    volatile Node head;
    volatile Node tail;

    public UnboundedQueue() {
	head = new Node(null); tail = head;
	enqLock = new ReentrantLock();
	deqLock = new ReentrantLock(); }
    ...
}

Node Class

    class Node {
	final T value;
	volatile Node next;

	public Node (T value) {
	    this.value = value;
	}
    }

enq Method

    public void enq (T value) {
	enqLock.lock();
	try {
	    Node nd = new Node(value);
	    tail.next = nd;
	    tail = nd;
	} finally {
	    enqLock.unlock();
	}
    }

deq Method

    public T deq() throws EmptyException {
	T value;
	deqLock.lock();
	try {
	    if (head.next == null){throw new EmptyException();}
	    value = head.next.value;
	    head = head.next;
            return value;
	} finally {
	    deqLock.unlock();
	}
    }

Is UnboundedQueue Linearizable?

1. What concurrent operations do we need to consider?
2. What internal states do we need to consider?
3. What are the linearization points (if any)?

Pertinent Lines

    public void enq (T value) {
	    Node nd = new Node(value);
	    tail.next = nd;
	    tail = nd;
    }
    public T deq() throws EmptyException {
	    if (head.next == null){throw new EmptyException();}
	    value = head.next.value;
	    head = head.next;
            return value;
    }

Next Time

Concurrent queues without locks?!?!