1. IMPLEMENTATION OF A NON-BLOCKING QUEUE ALGORITHM
Chien-Hua Shann, Ting-Lu Huang and Cheng Chen National Chiao Tung University
Nastaran Shafiei 1

2. Outline Algorithm Description Class Diagram Algorithm Implementation
Results 2

3. The Algorithm Finite array, Q Counters, FRONT and REAR
Operations: enqueue, dequeue
Elements of the queue 3

4. NonblockingQueue L: int Enqueue Dequeue val: int Thread
Queue: AtomicLongArray
REAR: AtomicLong
FRONT: AtomicLong
getValPart(long item)
getRefPart(long item)
getQueueItem(int val, int ref)
main(String[] args)
Enqueue
Dequeue
val: int
Enqueue(int value)
void run()
void run()
Thread … …

5. Shared Variables Atomic variables java.util.concurrent.atomic
public static AtomicLongArray Queue;
public static AtomicLong RAER;
public static AtomicLong FRONT;
Atomic variables
java.util.concurrent.atomic
get() and set()
Extend the concept of volatile variables
compareAndSet() 5

6. Implementation of Array Elements
An Array Element:
getCounter(long element) ;
32 bits
32 bits
int val
int ref
64 bits
val
ref
<<
ref
>>
ref

7. Implementation of Array Elements
getQueueValue(long element)
getQueueEntryType(int val, int ref)
val
ref
>>
val
val
<<
val +
ref =
val
ref

8. Implementation of Enqueue.run()
Make private copies using get():
rear = REAR.get();
front = FRONT.get();
x = Queue.get ((int)rear % L);
Check conditions:
rear == REAR.get()
rear != FRONT.get() + L 8

9. Implementation of Enqueue.run()
Check the content of the array element:
Empty - Attempt to store an item and increment REAR using CAS
if (Queue.compareAndSet((int) (rear % L) , x , newValue ))
REAR.compareAndSet( rear , rear+1);
Full - Help the other process
if( getQueueValue(Queue.get((int)rear % L)) != 0) 9

10. 10

11. Algorithm Behaviours Point contention
Number of simultaneously active processes
Number of threads: 2
Number of total operations: 200
Thread1
Thread2
# op: 100
# op: 100 11

12. Algorithm Behaviours Point contention
Number of simultaneously active processes
Number of threads: 4
Number of total operations: 200
Thread1
Thread2
Thread3
Thread4
# op: 50
# op: 50
# op: 50
# op: 50 12

13. Algorithm Behaviours Point contention
Number of simultaneously active processes
Number of threads: 8
Number of total operations: 200
Thread1
Thread2
Thread3
Thread4
Thread5
Thread6
Thread7
Thread8
# op: 25
# op: 25
# op: 25
# op: 25
# op: 25
# op: 25
# op: 25
# op: 25 13

14. 14

15. Questions? 15