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Chien-Hua Shann, Ting-Lu Huang and Cheng Chen National Chiao Tung University

Nastaran Shafiei

IMPLEMENTATION OF A NON-BLOCKING QUEUE ALGORITHM

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Algorithm Description Class Diagram Algorithm Implementation Results

Outline

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

The Algorithm

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NonblockingQueue

L: int Queue: AtomicLongArray REAR: AtomicLong FRONT: AtomicLong

Enqueue

val: int Enqueue(int value) void run()

Dequeue

void run()

Thread

…

…

getValPart(long item)getRefPart(long item) getQueueItem(int val, int ref)main(String[] args)

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Shared Variablespublic 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()

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Implementation of Array ElementsAn Array Element:

getCounter(long element) ;

val ref ref ref<< >>

int ref int val

32 bits

64 bits

32 bits

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Implementation of Array Elements getQueueValue(long element)

getQueueEntryType(int val, int ref)

val ref val>>

val ref<< + val = val ref

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1. Make private copies using get(): rear = REAR.get(); front = FRONT.get(); x = Queue.get ((int)rear % L);

2. Check conditions: rear == REAR.get()

rear != FRONT.get() + L

Implementation of Enqueue.run()

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3. 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)

REAR.compareAndSet( rear , rear+1);

Implementation of Enqueue.run()

1010

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Algorithm Behaviours

Point contentionNumber of simultaneously active processes

Number of threads: 2Number of total operations: 200

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Thread1

# op: 100

Thread2

# op: 100

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Algorithm Behaviours

Point contentionNumber of simultaneously active processes

Number of threads: 4Number of total operations: 200

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Thread1

# op: 50

Thread3

# op: 50

Thread2

# op: 50

Thread4

# op: 50

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Algorithm Behaviours

Point contentionNumber of simultaneously active processes

Number of threads: 8Number of total operations: 200

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Thread1

# op: 25

Thread3

# op: 25

Thread2

# op: 25

Thread4

# op: 25

Thread5

# op: 25

Thread7

# op: 25

Thread6

# op: 25

Thread8

# op: 25

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Questions?