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Concurrent Queues and Stacks Companion slides for The Art of Multiprocessor Programming by Maurice Herlihy & Nir Shavit
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 2 The Five-Fold Path Coarse-grained locking Fine-grained locking Optimistic synchronization Lazy synchronization Lock-free synchronization
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 3 Another Fundamental Problem We told you about –Sets implemented using linked lists Next: queues Next: stacks
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 4 Queues & Stacks Both: pool of items Queue –enq() & deq() –First-in-first-out (FIFO) order Stack –push() & pop() –Last-in-first-out (LIFO) order
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 5 Bounded vs Unbounded Bounded –Fixed capacity –Good when resources an issue Unbounded –Holds any number of objects
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 6 Blocking vs Non-Blocking Problem cases: –Removing from empty pool –Adding to full (bounded) pool Blocking –Caller waits until state changes Non-Blocking –Method throws exception
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 7 This Lecture Bounded, Blocking, Lock-based Queue Unbounded, Non-Blocking, Lock-free Queue Monitors ABA problem Unbounded Non-Blocking Lock-free Stack Elimination-Backoff Stack
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 8 Queue: Concurrency enq(x) y=deq() enq() and deq() work at different ends of the object tailhead
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 9 Concurrency enq(x) Challenge: what if the queue is empty or full? y=deq() tail head
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 10 Bounded Queue Sentinel head tail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 11 Bounded Queue head tail First actual item
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 12 Bounded Queue head tail Lock out other deq() calls deqLock
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 13 Bounded Queue head tail Lock out other enq() calls deqLock enqLock
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 14 Not Done Yet head tail deqLock enqLock Need to tell whether queue is full or empty
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 15 Not Done Yet head tail deqLock enqLock Permission to enqueue 8 items permits 8
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 16 Not Done Yet head tail deqLock enqLock Incremented by deq() Decremented by enq() permits 8
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 17 Enqueuer head tail deqLock enqLock permits 8 Lock enqLock
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 18 Enqueuer head tail deqLock enqLock permits 8 Read permits OK
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 19 Enqueuer head tail deqLock enqLock permits 8 No need to lock tail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 20 Enqueuer head tail deqLock enqLock permits 8 Enqueue Node
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 21 Enqueuer head tail deqLock enqLock permits 8 7 getAndDecrement()
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 22 Enqueuer head tail deqLock enqLock permits 8 Release lock 7
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 23 Enqueuer head tail deqLock enqLock permits 7 If queue was empty, notify/signal waiting dequeuers
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 24 Unsuccesful Enqueuer head tail deqLock enqLock permits 0 Uh-oh Read permits
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 25 Dequeuer head tail deqLock enqLock permits 8 Lock deqLock
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 26 Dequeuer head tail deqLock enqLock permits 7 Read sentinel’s next field OK
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 27 Dequeuer head tail deqLock enqLock permits 7 Read value
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 28 Dequeuer head tail deqLock enqLock permits 7 Make first Node new sentinel
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 29 Dequeuer head tail deqLock enqLock permits 8 Increment permits
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 30 Dequeuer head tail deqLock enqLock permits 7 Release deqLock
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 31 Unsuccesful Dequeuer head tail deqLock enqLock permits 8 Read sentinel’s next field uh-oh
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 32 Bounded Queue public class BoundedQueue { ReentrantLock enqLock, deqLock; Condition notEmptyCondition, notFullCondition; AtomicInteger permits; Node head; Node tail; int capacity; enqLock = new ReentrantLock(); notFullCondition = enqLock.newCondition(); deqLock = new ReentrantLock(); notEmptyCondition = deqLock.newCondition(); }
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 33 Bounded Queue public class BoundedQueue { ReentrantLock enqLock, deqLock; Condition notEmptyCondition, notFullCondition; AtomicInteger permits; Node head; Node tail; int capacity; enqLock = new ReentrantLock(); notFullCondition = enqLock.newCondition(); deqLock = new ReentrantLock(); notEmptyCondition = deqLock.newCondition(); } Enq & deq locks
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 34 Digression: Monitor Locks Java Synchronized objects and Java ReentrantLocks are monitors Allow blocking on a condition rather than spinning Threads: –acquire and release lock –wait on a condition
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 35 public interface Lock { void lock(); void lockInterruptibly() throws InterruptedException; boolean tryLock(); boolean tryLock(long time, TimeUnit unit); Condition newCondition(); void unlock; } The Java Lock Interface Acquire lock
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 36 public interface Lock { void lock(); void lockInterruptibly() throws InterruptedException; boolean tryLock(); boolean tryLock(long time, TimeUnit unit); Condition newCondition(); void unlock; } The Java Lock Interface Release lock
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 37 public interface Lock { void lock(); void lockInterruptibly() throws InterruptedException; boolean tryLock(); boolean tryLock(long time, TimeUnit unit); Condition newCondition(); void unlock; } The Java Lock Interface Try for lock, but not too hard
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 38 public interface Lock { void lock(); void lockInterruptibly() throws InterruptedException; boolean tryLock(); boolean tryLock(long time, TimeUnit unit); Condition newCondition(); void unlock; } The Java Lock Interface Create condition to wait on
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 39 The Java Lock Interface public interface Lock { void lock(); void lockInterruptibly() throws InterruptedException; boolean tryLock(); boolean tryLock(long time, TimeUnit unit); Condition newCondition(); void unlock; } Guess what this method does?
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 40 Lock Conditions public interface Condition { void await(); boolean await(long time, TimeUnit unit); … void signal(); void signalAll(); }
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 41 public interface Condition { void await(); boolean await(long time, TimeUnit unit); … void signal(); void signalAll(); } Lock Conditions Release lock and wait on condition
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 42 public interface Condition { void await(); boolean await(long time, TimeUnit unit); … void signal(); void signalAll(); } Lock Conditions Wake up one waiting thread
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 43 public interface Condition { void await(); boolean await(long time, TimeUnit unit); … void signal(); void signalAll(); } Lock Conditions Wake up all waiting threads
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 44 Await Releases lock associated with q Sleeps (gives up processor) Awakens (resumes running) Reacquires lock & returns q.await()
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 45 Signal Awakens one waiting thread –Which will reacquire lock q.signal();
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 46 Signal All Awakens all waiting threads –Which will each reacquire lock q.signalAll();
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 47 A Monitor Lock Critical Section waiting room lock() unLock()
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 48 Unsuccessful Deq Critical Section waiting room lock() await() deq() Oh no, empty!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 49 Another One Critical Section waiting room lock() await() deq() Oh no, empty!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 50 Enqueuer to the Rescue Critical Section waiting room lock() signalAll() enq( ) unLock() Yawn!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 51 Yawn! Monitor Signalling Critical Section waiting room Yawn! Awakened thread might still lose lock to outside contender…
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 52 Dequeurs Signalled Critical Section waiting room Found it Yawn!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 53 Yawn! Dequeurs Signalled Critical Section waiting room Still empty!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 54 Dollar Short + Day Late Critical Section waiting room
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 55 Lost Wake-Up Critical Section waiting room lock() signal () enq( ) unLock() Yawn!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 56 Lost Wake-Up Critical Section waiting room lock() enq( ) unLock() Yawn!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 57 Lost Wake-Up Critical Section waiting room Yawn!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 58 Lost Wake-Up Critical Section waiting room Found it
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 59 What’s Wrong Here? Critical Section waiting room zzzz….!
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Solution to Lost Wakeup Always use signalAll and notifyAll Not signal and notify Art of Multiprocessor Programming© Herlihy-Shavit 2007 60
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 61 public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods
![Art of Multiprocessor Programming© Herlihy-Shavit public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods](http://images.slideplayer.com./14/4289595/slides/slide_61.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 62 public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Each object has an implicit lock with an implicit condition
![Art of Multiprocessor Programming© Herlihy-Shavit public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Each object has an implicit lock with an implicit condition](http://images.slideplayer.com./14/4289595/slides/slide_62.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Each object has an implicit lock with an implicit condition")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 63 public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Lock on entry, unlock on return
![Art of Multiprocessor Programming© Herlihy-Shavit public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Lock on entry, unlock on return](http://images.slideplayer.com./14/4289595/slides/slide_63.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Lock on entry, unlock on return")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 64 public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Wait on implicit condition
![Art of Multiprocessor Programming© Herlihy-Shavit public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Wait on implicit condition](http://images.slideplayer.com./14/4289595/slides/slide_64.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Wait on implicit condition")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 65 public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Signal all threads waiting on condition
![Art of Multiprocessor Programming© Herlihy-Shavit public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Signal all threads waiting on condition](http://images.slideplayer.com./14/4289595/slides/slide_65.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class Queue { int head = 0, tail = 0; T[QSIZE] items; public synchronized T deq() { while (tail – head == 0) this.wait(); T result = items[head % QSIZE]; head++; this.notifyAll(); return result; } … }} Java Synchronized Methods Signal all threads waiting on condition")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 66 (Pop!) The Bounded Queue public class BoundedQueue { ReentrantLock enqLock, deqLock; Condition notEmptyCondition, notFullCondition; AtomicInteger permits; Node head; Node tail; int capacity; enqLock = new ReentrantLock(); notFullCondition = enqLock.newCondition(); deqLock = new ReentrantLock(); notEmptyCondition = deqLock.newCondition(); }
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 67 Bounded Queue Fields public class BoundedQueue { ReentrantLock enqLock, deqLock; Condition notEmptyCondition, notFullCondition; AtomicInteger permits; Node head; Node tail; int capacity; enqLock = new ReentrantLock(); notFullCondition = enqLock.newCondition(); deqLock = new ReentrantLock(); notEmptyCondition = deqLock.newCondition(); } Enq & deq locks
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 68 Bounded Queue Fields public class BoundedQueue { ReentrantLock enqLock, deqLock; Condition notEmptyCondition, notFullCondition; AtomicInteger permits; Node head; Node tail; int capacity; enqLock = new ReentrantLock(); notFullCondition = enqLock.newCondition(); deqLock = new ReentrantLock(); notEmptyCondition = deqLock.newCondition(); } Enq lock’s associated condition
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 69 Bounded Queue Fields public class BoundedQueue { ReentrantLock enqLock, deqLock; Condition notEmptyCondition, notFullCondition; AtomicInteger permits; Node head; Node tail; int capacity; enqLock = new ReentrantLock(); notFullCondition = enqLock.newCondition(); deqLock = new ReentrantLock(); notEmptyCondition = deqLock.newCondition(); } Num permits: 0 to capacity
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 70 Bounded Queue Fields public class BoundedQueue { ReentrantLock enqLock, deqLock; Condition notEmptyCondition, notFullCondition; AtomicInteger permits; Node head; Node tail; int capacity; enqLock = new ReentrantLock(); notFullCondition = enqLock.newCondition(); deqLock = new ReentrantLock(); notEmptyCondition = deqLock.newCondition(); } Head and Tail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 71 Enq Method Part One public void enq(T x) { boolean mustWakeDequeuers = false; enqLock.lock(); try { while (permits.get() == 0) notFullCondition.await(); Node e = new Node(x); tail.next = e; tail = e; if (permits.getAndDecrement() == capacity) mustWakeDequeuers = true; } finally { enqLock.unlock(); } … }
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 72 public void enq(T x) { boolean mustWakeDequeuers = false; enqLock.lock(); try { while (permits.get() == 0) notFullCondition.await(); Node e = new Node(x); tail.next = e; tail = e; if (permits.getAndDecrement() == capacity) mustWakeDequeuers = true; } finally { enqLock.unlock(); } … } Enq Method Part One Lock and unlock enq lock
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 73 public void enq(T x) { boolean mustWakeDequeuers = false; enqLock.lock(); try { while (permits.get() == 0) notFullCondition.await(); Node e = new Node(x); tail.next = e; tail = e; if (permits.getAndDecrement() == capacity) mustWakeDequeuers = true; } finally { enqLock.unlock(); } … } Enq Method Part One If queue is full, patiently await further instructions …
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 74 public void enq(T x) { boolean mustWakeDequeuers = false; enqLock.lock(); try { while (permits.get() == 0) notFullCondition.await(); Node e = new Node(x); tail.next = e; tail = e; if (permits.getAndDecrement() == capacity) mustWakeDequeuers = true; } finally { enqLock.unlock(); } … } Be Afraid How do we know the permits field won’t change?
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 75 public void enq(T x) { boolean mustWakeDequeuers = false; enqLock.lock(); try { while (permits.get() == 0) notFullCondition.await(); Node e = new Node(x); tail.next = e; tail = e; if (permits.getAndDecrement() == capacity) mustWakeDequeuers = true; } finally { enqLock.unlock(); } … } Enq Method Part One Add new node
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 76 public void enq(T x) { boolean mustWakeDequeuers = false; enqLock.lock(); try { while (permits.get() == 0) notFullCondition.await(); Node e = new Node(x); tail.next = e; tail = e; if (permits.getAndDecrement() == capacity) mustWakeDequeuers = true; } finally { enqLock.unlock(); } … } Enq Method Part One If queue was empty, wake frustrated dequeuers
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 77 Enq Method Part Deux public void enq(T x) { … if (mustWakeDequeuers) { deqLock.lock(); try { notEmptyCondition.signalAll(); } finally { deqLock.unlock(); }
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 78 Enq Method Part Deux public void enq(T x) { … if (mustWakeDequeuers) { deqLock.lock(); try { notEmptyCondition.signalAll(); } finally { deqLock.unlock(); } Are there dequeuers to be signaled?
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 79 public void enq(T x) { … if (mustWakeDequeuers) { deqLock.lock(); try { notEmptyCondition.signalAll(); } finally { deqLock.unlock(); } Enq Method Part Deux Lock and unlock deq lock
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 80 public void enq(T x) { … if (mustWakeDequeuers) { deqLock.lock(); try { notEmptyCondition.signalAll(); } finally { deqLock.unlock(); } Enq Method Part Deux Signal dequeuers that queue no longer empty
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 81 The Enq() & Deq() Methods Share no locks –That’s good But do share an atomic counter –Accessed on every method call –That’s not so good Can we alleviate this bottleneck?
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 82 Split the Counter The enq() method –Decrements only –Cares only if value is zero The deq() method –Increments only –Cares only if value is capacity
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 83 Split Counter Enqueuer decrements enqSidePermits Dequeuer increments deqSidePermits When enqueuer runs out –Locks deqLock –Transfers permits Intermittent synchronization –Not with each method call –Need both locks! (careful …)
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 84 A Lock-Free Queue Sentinel head tail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 85 Compare and Set CAS
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 86 Enqueue head tail Enq( )
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 87 Enqueue head tail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 88 Logical Enqueue head tail CAS
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 89 Physical Enqueue head tail Enqueue Node CAS
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 90 Enqueue These two steps are not atomic The tail field refers to either –Actual last Node (good) –Penultimate Node (not so good) Be prepared!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 91 Enqueue What do you do if you find –A trailing tail? Stop and help fix it –If tail node has non-null next field –CAS the queue’s tail field to tail.next As in the universal construction
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 92 When CASs Fail During logical enqueue –Abandon hope, restart –Still lock-free (why?) During physical enqueue –Ignore it (why?)
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 93 Dequeuer head tail Read value
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 94 Dequeuer head tail Make first Node new sentinel CAS
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 95 Memory Reuse? What do we do with nodes after we dequeue them? Java: let garbage collector deal? Suppose there is no GC, or we prefer not to use it?
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 96 Dequeuer head tail CAS Can recycle
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 97 Simple Solution Each thread has a free list of unused queue nodes Allocate node: pop from list Free node: push onto list Deal with underflow somehow …
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 98 Why Recycling is Hard Free pool headtail Want to redirect tail from grey to red zzz…
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 99 Both Nodes Reclaimed Free pool zzz headtail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 100 One Node Recycled Free pool Yawn! headtail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 101 Why Recycling is Hard Free pool CAS headtail OK, here I go!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 102 Final State Free pool zOMG what went wrong? headtail Bad news
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 103 The Dreaded ABA Problem Head pointer has value A Thread reads value A headtail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 104 Dreaded ABA continued zzz Head pointer has value B Node A freed headtail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 105 Dreaded ABA continued Yawn! Head pointer has value A again Node A recycled & reinitialized headtail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 106 Dreaded ABA continued CAS succeeds because pointer matches even though pointer’s meaning has changed CAS headtail
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 107 The Dreaded ABA Problem Is a result of CAS() semantics –I blame Sun, Intel, AMD, … Not with Load-Locked/Store- Conditional –Good for IBM?
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 108 Dreaded ABA – A Solution Tag each pointer with a counter Unique over lifetime of node Pointer size vs word size issues Overflow? –Don’t worry be happy? –Bounded tags? AtomicStampedReference class
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 109 Atomic Stamped Reference AtomicStampedReference class –Java.util.concurrent.atomic package address S Stamp Reference Can get reference and stamp atomically
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 110 Concurrent Stack Methods –push(x) –pop() Last-in, First-out (LIFO) order Lock-Free!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 111 Empty Stack Top
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 112 Push Top
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 113 Push Top CAS
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 114 Push Top
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 115 Push Top
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 116 Push Top
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 117 Push Top CAS
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 118 Push Top
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 119 Pop Top
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 120 Pop Top CAS
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 121 Pop Top CAS mine!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 122 Pop Top CAS
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 123 Pop Top
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 124 public class LockFreeStack { private AtomicReference top = new AtomicReference(null); public boolean tryPush(Node node){ Node oldTop = top.get(); node.next = oldTop; return(top.compareAndSet(oldTop, node)) } public void push(T value) { Node node = new Node(value); while (true) { if (tryPush(node)) { return; } else backoff.backoff(); }} Lock-free Stack
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 125 public class LockFreeStack { private AtomicReference top = new AtomicReference(null); public Boolean tryPush(Node node){ Node oldTop = top.get(); node.next = oldTop; return(top.compareAndSet(oldTop, node)) } public void push(T value) { Node node = new Node(value); while (true) { if (tryPush(node)) { return; } else backoff.backoff() }} Lock-free Stack tryPush attempts to push a node
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 126 public class LockFreeStack { private AtomicReference top = new AtomicReference(null); public boolean tryPush(Node node){ Node oldTop = top.get(); node.next = oldTop; return(top.compareAndSet(oldTop, node)) } public void push(T value) { Node node = new Node(value); while (true) { if (tryPush(node)) { return; } else backoff.backoff() }} Lock-free Stack Read top value
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 127 public class LockFreeStack { private AtomicReference top = new AtomicReference(null); public boolean tryPush(Node node){ Node oldTop = top.get(); node.next = oldTop; return(top.compareAndSet(oldTop, node)) } public void push(T value) { Node node = new Node(value); while (true) { if (tryPush(node)) { return; } else backoff.backoff() }} Lock-free Stack current top will be new node’s successor
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 128 public class LockFreeStack { private AtomicReference top = new AtomicReference(null); public boolean tryPush(Node node){ Node oldTop = top.get(); node.next = oldTop; return(top.compareAndSet(oldTop, node)) } public void push(T value) { Node node = new Node(value); while (true) { if (tryPush(node)) { return; } else backoff.backoff() }} Lock-free Stack Try to swing top, return success or failure
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 129 public class LockFreeStack { private AtomicReference top = new AtomicReference(null); public boolean tryPush(Node node){ Node oldTop = top.get(); node.next = oldTop; return(top.compareAndSet(oldTop, node)) } public void push(T value) { Node node = new Node(value); while (true) { if (tryPush(node)) { return; } else backoff.backoff() }} Lock-free Stack Push calls tryPush
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 130 public class LockFreeStack { private AtomicReference top = new AtomicReference(null); public boolean tryPush(Node node){ Node oldTop = top.get(); node.next = oldTop; return(top.compareAndSet(oldTop, node)) } public void push(T value) { Node node = new Node(value); while (true) { if (tryPush(node)) { return; } else backoff.backoff() }} Lock-free Stack Create new node
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 131 public class LockFreeStack { private AtomicReference top = new AtomicReference(null); public boolean tryPush(Node node){ Node oldTop = top.get(); node.next = oldTop; return(top.compareAndSet(oldTop, node)) } public void push(T value) { Node node = new Node(value); while (true) { if (tryPush(node)) { return; } else backoff.backoff() }} Lock-free Stack If tryPush() fails, back off before retrying Makes scheduling benevolent so Method is effectively wait- free
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 132 Lock-free Stack Good –No locking Bad –Without GC, fear ABA –Without backoff, huge contention at top –In any case, no parallelism
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 133 Big Question Are stacks inherently sequential? Reasons why –Every pop() call fights for top item Reasons why not –Stay tuned …
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 134 Elimination-Backoff Stack How to –“turn contention into parallelism” Replace familiar –exponential backoff With alternative –elimination-backoff
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 135 Observation Push( ) Pop() linearizable stack After an equal number of pushes and pops, stack stays the same Yes!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 136 Idea: Elimination Array Push( ) Pop() stack Pick at random Pick at random Elimination Array
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 137 Push Collides With Pop Push( ) Pop() stack continue No need to access stack Yes!
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 138 No Collision Push( ) Pop() stack If no collision, access stack If pushes collide or pops collide access stack
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 139 Elimination-Backoff Stack Lock-free stack + elimination array Access Lock-free stack, –If uncontended, apply operation –if contended, back off to elimination array and attempt elimination
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 140 Elimination-Backoff Stack Push( ) Pop() Top CAS If CAS fails, back off
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 141 Dynamic Range and Delay Push( ) Pick random range and max time to wait for collision based on level of contention encountered
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 142 Linearizability Un-eliminated calls –linearized as before Eliminated calls: –linearize pop() immediately after matching push() Combination is a linearizable stack
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 143 Un-Eliminated Linearizability push(v 1 ) time linearizable push(v 1 ) pop(v 1 )
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 144 Eliminated Linearizability pop(v 2 )push(v 1 ) push(v 2 ) time push(v 2 ) linearizable pop(v 2 ) push(v 1 ) pop(v 1 ) Collision Point Red calls are eliminated
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 145 Backoff Has Dual Effect Elimination introduces parallelism Backoff onto array cuts contention on lock-free stack Elimination in array cuts down total number of threads ever accessing lock-free stack
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 146 public class EliminationArray { private static final int duration =...; private static final int timeUnit =...; Exchanger [] exchanger; public EliminationArray(int capacity) { exchanger = new Exchanger[capacity]; for (int i = 0; i < capacity; i++) exchanger[i] = new Exchanger (); … } … } Elimination Array
![Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { private static final int duration =...; private static final int timeUnit =...; Exchanger [] exchanger; public EliminationArray(int capacity) { exchanger = new Exchanger[capacity]; for (int i = 0; i < capacity; i++) exchanger[i] = new Exchanger (); … } … } Elimination Array](http://images.slideplayer.com./14/4289595/slides/slide_146.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { private static final int duration =...; private static final int timeUnit =...; Exchanger [] exchanger; public EliminationArray(int capacity) { exchanger = new Exchanger[capacity]; for (int i = 0; i < capacity; i++) exchanger[i] = new Exchanger (); … } … } Elimination Array")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 147 public class EliminationArray { private static final int duration =...; private static final int timeUnit =...; Exchanger [] exchanger; public EliminationArray(int capacity) { exchanger = new Exchanger[capacity]; for (int i = 0; i < capacity; i++) exchanger[i] = new Exchanger (); … } … } Elimination Array An array of exchangers
![Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { private static final int duration =...; private static final int timeUnit =...; Exchanger [] exchanger; public EliminationArray(int capacity) { exchanger = new Exchanger[capacity]; for (int i = 0; i < capacity; i++) exchanger[i] = new Exchanger (); … } … } Elimination Array An array of exchangers](http://images.slideplayer.com./14/4289595/slides/slide_147.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { private static final int duration =...; private static final int timeUnit =...; Exchanger [] exchanger; public EliminationArray(int capacity) { exchanger = new Exchanger[capacity]; for (int i = 0; i < capacity; i++) exchanger[i] = new Exchanger (); … } … } Elimination Array An array of exchangers")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 148 Lock-free Exchanger EMPTY
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 149 EMPTY Lock-free Exchanger CAS
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 150 WAITING Lock-free Exchanger
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 151 Lock-free Exchanger In search of partner … WAITING
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 152 WAITING Lock-free Exchanger Slot Still waiting … Try to exchange item and set state to BUSY CAS
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 153 BUSY Lock-free Exchanger Slot Partner showed up, take item and reset to EMPTY item stamp/state
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 154 EMPTYBUSY Lock-free Exchanger Slot item stamp/state Partner showed up, take item and reset to EMPTY
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 155 public class Exchanger { AtomicStampedReference slot = new AtomicStampedReference (null, 0); A Lock-Free Exchanger
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 156 public class Exchanger { AtomicStampedReference slot = new AtomicStampedReference (null, 0); A Lock-Free Exchanger Atomically modifiable reference + time stamp
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 157 Atomic Stamped Reference AtomicStampedReference class –Java.util.concurrent.atomic package In C or C++: address S Stamp Reference
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 158 Extracting Reference & Stamp Public T get(int[] stampHolder);
![Art of Multiprocessor Programming© Herlihy-Shavit Extracting Reference & Stamp Public T get(int[] stampHolder);](http://images.slideplayer.com./14/4289595/slides/slide_158.jpg "Art of Multiprocessor Programming© Herlihy-Shavit Extracting Reference & Stamp Public T get(int[] stampHolder);")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 159 Extracting Reference & Stamp Public T get(int[] stampHolder); Returns reference to object of type T Returns stamp at array index 0!
![Art of Multiprocessor Programming© Herlihy-Shavit Extracting Reference & Stamp Public T get(int[] stampHolder); Returns reference to object of type T Returns stamp at array index 0!](http://images.slideplayer.com./14/4289595/slides/slide_159.jpg "Art of Multiprocessor Programming© Herlihy-Shavit Extracting Reference & Stamp Public T get(int[] stampHolder); Returns reference to object of type T Returns stamp at array index 0!")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 160 Exchanger Status enum Status {EMPTY, WAITING, BUSY};
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 161 Exchanger Status enum Status {EMPTY, WAITING, BUSY}; Nothing yet
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 162 Exchange Status enum Status {EMPTY, WAITING, BUSY}; Nothing yet One thread is waiting for rendez-vous
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 163 Exchange Status enum Status {EMPTY, WAITING, BUSY}; Nothing yet One thread is waiting for rendez-vous Other threads busy with rendez-vous
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 164 public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {EMPTY}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } The Exchange
![Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {EMPTY}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } The Exchange](http://images.slideplayer.com./14/4289595/slides/slide_164.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {EMPTY}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } The Exchange")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 165 public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {EMPTY}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } The Exchange Item & timeout
![Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {EMPTY}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } The Exchange Item & timeout](http://images.slideplayer.com./14/4289595/slides/slide_165.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {EMPTY}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } The Exchange Item & timeout")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 166 public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {EMPTY}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } The Exchange Array to hold timestamp
![Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {EMPTY}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } The Exchange Array to hold timestamp](http://images.slideplayer.com./14/4289595/slides/slide_166.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {EMPTY}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } The Exchange Array to hold timestamp")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 167 public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {0}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: // slot is free case WAITING: // someone waiting for me case BUSY: // others exchanging } }} The Exchange Loop until timeout
![Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {0}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: // slot is free case WAITING: // someone waiting for me case BUSY: // others exchanging } }} The Exchange Loop until timeout](http://images.slideplayer.com./14/4289595/slides/slide_167.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {0}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: // slot is free case WAITING: // someone waiting for me case BUSY: // others exchanging } }} The Exchange Loop until timeout")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 168 public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {0}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: // slot is free case WAITING: // someone waiting for me case BUSY: // others exchanging } }} The Exchange Get other’s item and timestamp
![Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {0}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: // slot is free case WAITING: // someone waiting for me case BUSY: // others exchanging } }} The Exchange Get other’s item and timestamp](http://images.slideplayer.com./14/4289595/slides/slide_168.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {0}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: // slot is free case WAITING: // someone waiting for me case BUSY: // others exchanging } }} The Exchange Get other’s item and timestamp")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 169 public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {0}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } }} The Exchange Exchanger slot has three states
![Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {0}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } }} The Exchange Exchanger slot has three states](http://images.slideplayer.com./14/4289595/slides/slide_169.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public T Exchange(T myItem, long nanos) throws TimeoutException { long timeBound = System.nanoTime() + nanos; int[] stampHolder = {0}; while (true) { if (System.nanoTime() > timeBound) throw new TimeoutException(); T herItem = slot.get(stampHolder); int stamp = stampHolder[0]; switch(stamp) { case EMPTY: … // slot is free case WAITING: … // someone waiting for me case BUSY: … // others exchanging } }} The Exchange Exchanger slot has three states")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 170 case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY
![Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY](http://images.slideplayer.com./14/4289595/slides/slide_170.jpg "Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 171 case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Slot is free, try to insert myItem and change state to WAITING
![Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Slot is free, try to insert myItem and change state to WAITING](http://images.slideplayer.com./14/4289595/slides/slide_171.jpg "Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Slot is free, try to insert myItem and change state to WAITING")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 172 case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Loop while still time left to attempt exchange
![Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Loop while still time left to attempt exchange](http://images.slideplayer.com./14/4289595/slides/slide_172.jpg "Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Loop while still time left to attempt exchange")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 173 case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, WAITING, BUSY)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Get item and stamp in slot and check if state changed to BUSY
![Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, WAITING, BUSY)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Get item and stamp in slot and check if state changed to BUSY](http://images.slideplayer.com./14/4289595/slides/slide_173.jpg "Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, WAITING, BUSY)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Get item and stamp in slot and check if state changed to BUSY")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 174 case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY If successful reset slot state to EMPTY
![Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY If successful reset slot state to EMPTY](http://images.slideplayer.com./14/4289595/slides/slide_174.jpg "Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY If successful reset slot state to EMPTY")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 175 case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, WAITING, BUSY)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY and return item found in slot
![Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, WAITING, BUSY)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY and return item found in slot](http://images.slideplayer.com./14/4289595/slides/slide_175.jpg "Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, WAITING, BUSY)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY and return item found in slot")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 176 case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Otherwise we ran out of time, try to reset state to EMPTY, if successful time out
![Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Otherwise we ran out of time, try to reset state to EMPTY, if successful time out](http://images.slideplayer.com./14/4289595/slides/slide_176.jpg "Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Otherwise we ran out of time, try to reset state to EMPTY, if successful time out")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 177 case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, WAITING, BUSY)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY If reset failed, someone showed up after all, take that item
![Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, WAITING, BUSY)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY If reset failed, someone showed up after all, take that item](http://images.slideplayer.com./14/4289595/slides/slide_177.jpg "Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, WAITING, BUSY)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY If reset failed, someone showed up after all, take that item")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 178 case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Set slot to EMPTY with new timestamp and return item found
![Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Set slot to EMPTY with new timestamp and return item found](http://images.slideplayer.com./14/4289595/slides/slide_178.jpg "Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY Set slot to EMPTY with new timestamp and return item found")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 179 case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY If initial CAS failed then someone else changed state from EMPTY to WAITING so retry from start
![Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY If initial CAS failed then someone else changed state from EMPTY to WAITING so retry from start](http://images.slideplayer.com./14/4289595/slides/slide_179.jpg "Art of Multiprocessor Programming© Herlihy-Shavit case EMPTY: // slot is free if (slot.compareAndSet(herItem, myItem, EMPTY, WAITING)) { while (System.nanoTime() < timeBound){ herItem = slot.get(stampHolder); if (stampHolder[0] == BUSY) { slot.set(null, EMPTY); return herItem; }} if (slot.compareAndSet(myItem, null, WAITING, EMPTY)){throw new TimeoutException(); } else { herItem = slot.get(stampHolder); slot.set(null, EMPTY); return herItem; } } break; Exchanger State EMPTY If initial CAS failed then someone else changed state from EMPTY to WAITING so retry from start")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 180 case WAITING: // someone waiting for me if (slot.CAS(herItem, myItem, WAITING, BUSY)) return herItem; break; case BUSY: // others in middle of exchanging break; default: // impossible break; } States WAITING and BUSY
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 181 case WAITING: // someone waiting for me if (slot.CAS(herItem, myItem, WAITING, BUSY)) return herItem; break; case BUSY: // others in middle of exchanging break; default: // impossible break; } States WAITING and BUSY Someone is waiting for an exchange, so try to CAS in my item in & change state to BUSY
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 182 case WAITING: // someone waiting for me if (slot.CAS(herItem, myItem, WAITING, BUSY)) return herItem; break; case BUSY: // others in middle of exchanging break; default: // impossible break; } States WAITING and BUSY If successful, return that item. Otherwise another thread got it.
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 183 case WAITING: // someone waiting for me if (slot.CAS(herItem, myItem, WAITING, BUSY)) return herItem; break; case BUSY: // others in middle of exchanging break; default: // impossible break; } States WAITING and BUSY Other threads using slot, so start over.
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 184 The Exchanger Slot Exchanger is lock-free Because the only way an exchange can fail is if others repeatedly succeeded or no-one showed up The slot we need does not require symmetric exchange
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 185 public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array
![Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array](http://images.slideplayer.com./14/4289595/slides/slide_185.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 186 public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array Visit elimination array with value and range
![Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array Visit elimination array with value and range](http://images.slideplayer.com./14/4289595/slides/slide_186.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array Visit elimination array with value and range")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 187 public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array Pick a random array entry
![Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array Pick a random array entry](http://images.slideplayer.com./14/4289595/slides/slide_187.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array Pick a random array entry")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 188 public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array Exchange value or time out
![Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array Exchange value or time out](http://images.slideplayer.com./14/4289595/slides/slide_188.jpg "Art of Multiprocessor Programming© Herlihy-Shavit public class EliminationArray { … public T visit(T value, int Range) throws TimeoutException { int slot = random.nextInt(Range); int nanodur = convertToNanos(duration, timeUnit)); return (exchanger[slot].exchange(value, nanodur) }} Elimination Array Exchange value or time out")
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 189 public void push(T value) {... while (true) { if (tryPush(node)) { return; } else try { T otherValue = eliminationArray.visit(value,policy.Range); if (otherValue == null) { return; } Elimination Stack Push
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 190 public void push(T value) {... while (true) { if (tryPush(node)) { return; } else try { T otherValue = eliminationArray.visit(value,policy.Range); if (otherValue == null) { return; } Elimination Stack Push First try to push
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 191 public void push(T value) {... while (true) { if (tryPush(node)) { return; } else try { T otherValue = eliminationArray.visit(value,policy.Range); if (otherValue == null) { return; } Elimination Stack Push If failed back-off to try to eliminate
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 192 public void push(T value) {... while (true) { if (tryPush(node)) { return; } else try { T otherValue = eliminationArray.visit(value,policy.Range); if (otherValue == null) { return; } Elimination Stack Push Value being pushed and range to try
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 193 public void push(T value) {... while (true) { if (tryPush(node)) { return; } else try { T otherValue = eliminationArray.visit(value,policy.Range); if (otherValue == null) { return; } Elimination Stack Push Only a pop inserts null value so elimination was successful
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 194 public void push(T value) {... while (true) { if (tryPush(node)) { return; } else try { T otherValue = eliminationArray.visit(value,policy.Range); if (otherValue == null) { return; } Elimination Stack Push Else retry push on lock-free stack
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 195 public T pop() {... while (true) { if (tryPop()) { return returnNode.value; } else try { T otherValue = eliminationArray.visit(null,policy.Range; if otherValue != null) { return otherValue; } }} Elimination Stack Pop
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 196 public T pop() {... while (true) { if (tryPop()) { return returnNode.value; } else try { T otherValue = eliminationArray.visit(null,policy.Range; if ( otherValue != null) { return otherValue; } }} Elimination Stack Pop Like push, non-null value pushed by other thread, so elimination succeeds
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 197 Summary We saw both lock-based and lock- free implementations of queues and stacks Not every data structure that looks sequential is sequential. –Linearizable stack not inherently sequential ABA is a real problem, fear it.
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Art of Multiprocessor Programming© Herlihy-Shavit 2007 198 This work is licensed under a Creative Commons Attribution- ShareAlike 2.5 License.Creative Commons Attribution- ShareAlike 2.5 License You are free: –to Share — to copy, distribute and transmit the work –to Remix — to adapt the work Under the following conditions: –Attribution. You must attribute the work to “The Art of Multiprocessor Programming” (but not in any way that suggests that the authors endorse you or your use of the work). –Share Alike. If you alter, transform, or build upon this work, you may distribute the resulting work only under the same, similar or a compatible license. For any reuse or distribution, you must make clear to others the license terms of this work. The best way to do this is with a link to –http://creativecommons.org/licenses/by-sa/3.0/. Any of the above conditions can be waived if you get permission from the copyright holder. Nothing in this license impairs or restricts the author's moral rights.
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