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A Java Implementation of a LockFree Concurrent Priority Queue
Bart Verzijlenberg
November 15, 2007
Agenda
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Algorithm Review
Implementation
Java Package used
Testing
Conclusion
Questions
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Algorithm Review
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Priority Queue
Lock-free
Relies on atomic Compare and Swap
operations
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2
Algorithm Review
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The algorithm uses the Skip-List data structure
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Extends the Skip-List for concurrent use
The Skip-List is sorted on the priority of the nodes
The algorithm is lock-free
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No blocking
Prevent dead locks
Always progress by at least one operation
Risk of starvation
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Algorithm Review
Skip-List
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Multi-layer linked list
Probabilistic alternative to balanced trees
H forward pointer for a node of height h
Each pointer i points to the next node with
a height of at least i
Probabilistic time complexity log(N) for N
nodes
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Inserting in a Skip-List
Inserting 17 in the list
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Implementation
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Two classes
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Node
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Represents an entry in the queue
SkipQueue
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Contains the algorithm logic
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Node Class
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SkipQueue Class
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Java Package Used
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AtomicMarkableReference
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Stores
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In Java.util.concurrent.atomic
A reference to an object
A boolean flag
Provides ability to
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Atomically set both the flag and reference
Compare and Swap (CAS) the flag and reference
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Testing
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Multi-threaded
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10 insert threads
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Each inserting 100,000 objects with random
integer keys
10 delete threads
Each deleting while the queue is not empty
 If the queue is empty, sleep for a bit and try again
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Testing
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Maintain a counter
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Stores the number of nodes dequeued
When 1,000,000 nodes removed
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Stop the delete threads
Check that
the queue is empty
 removed exactly 1,000,000 nodes
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November 15, 2007
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Testing
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10 insert threads
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Each inserting 100,000 random integer
numbers
When all numbers have been inserted
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Remove them one at a time.
Making sure the nodes come out sorted
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Removing Atomicity
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Do atomic references make a difference
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Replace each AtomicMarkableReference with a
similar class
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i.e. does concurrency come into play, or are we just
lucky
Same functions
But they are not atomic
The queue becomes locked after a small number
of concurrent deletes.
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Conclusion
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Further testing is needed to verify the
correctness of the implementation
Tests so far are positive
But cannot be certain there are no
problems
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Questions
November 15, 2007