Concurrent Lock-Free and Wait-Free Algorithms

Medha Vasanth School of Computer Science Carleton University, Ottawa, Canada [email protected] www.dehne.net

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Outline
Introduction
Compare and Swap
Building Blocks • Micheal and Scott Queue (MS Queue) • Kogan and Petrank Queue (KP Queue)







Fast-path Slow-path Approach Correctness Implementation and Results Questions COMP 5704 Project Presentation

Introduction
Parallel Data Structures – FIFO Queue Tail

Head Dummy Node

Value 5 next


Lock Freedom progress

–

3

provides

7 null

global

program


Wait Freedom – ensures progress of every process
Linearizability COMP 5704 Project Presentation

Compare and Swap (CAS) 

Basic primitive CAS(mem_loc,exp_val,new_val) val mem_loc No

CAS Unsuccessful

If val==exp_val

Yes

Set mem_loc to new_val CAS Successful

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Building Blocks Michael and Scott Queue (MS Queue) Enqueue: Tail pointing to the last node Tail

Head Dummy Node

Value 5 next

3 null

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7 null

Building Blocks Michael and Scott Queue (MS Queue) Enqueue: Tail pointing to the last node Tail

Head Dummy Node

Value 5 next

3 null

Tail

Head Dummy Node

7 null

Value 5 next

3

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7 null

Building Blocks Enqueue: Tail pointing to the second last node Head Dummy Node

Tail Value 5 next

3 null

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Building Blocks Enqueue: Tail pointing to the second last node Head Dummy Node

Tail Value 5 next

Head 3 null

Dummy Node

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Tail Value 5 next

3 null

Building Blocks Enqueue: Tail pointing to the second last node Head Dummy Node

Tail Value 5 next

Head 3 null

Dummy Node

Value 5 next

Tail

Head Dummy Node

Tail

Value 5 next

3

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7 null

3 null

Building Blocks Dequeue: Head pointing to the first node Head Dummy Node

Tail Value 5 next

3

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7 null

Building Blocks Dequeue: Head pointing to the first node Head

Tail

Dummy Node

Value 5 next

7 null

3

Head Tail Dummy Node

Value 3 next

7 null

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Building Blocks Kogan and Petrank Queue (KP Queue)
Extends the idea of the MS Queue
Uses priority based helping with phase numbers
Basic Idea • All threads performing the same operation realize that some operation is in progress • Update the state array • Fix the internal structure of the queue COMP 5704 Project Presentation

Building Blocks Enqueue : Step 1 head

tail

value: 200 enqTid: 0 deqTid: -1 next:

500 1 -1

300 4 -1 null

State of thread t5 phase: 9 pending: true enqueue: true next:

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100 5 -1 null

Building Blocks Enqueue: Step 2 head value: 200 enqTid: 0 deqTid: -1 next:

tail 500 1 -1

300 4 -1

State of thread t5 phase: 9 pending: true enqueue: true next:

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100 5 -1 null

Building Blocks Enqueue: Step 3 head value: 200 enqTid: 0 deqTid: -1 next:

tail 500 1 -1

300 4 -1

State of thread t5 phase: 9 pending: false enqueue: true next:

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100 5 -1 null

Building Blocks Enqueue: Step 4 head value: 200 enqTid: 0 deqTid: -1 next:

tail 500 1 -1

300 4 -1

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100 5 -1 null

Building Blocks Dequeue: Step 1 State of thread t1 phase: 9 pending: true enqueue: false next: head value: 200 enqTid: 0 deqTid: -1 next:

tail 500 1 -1

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300 4 -1

100 5 -1 null

Building Blocks Dequeue: Step 2 State of thread t1 phase: 9 pending: true enqueue: false next: head value: 200 enqTid: 0 deqTid: 1 next:

tail 500 1 -1

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300 4 -1

100 5 -1 null

Building Blocks Dequeue: Step 3 State of thread t1 phase: 9 pending: false true enqueue: false next: head value: 200 enqTid: 0 deqTid: 1 next:

tail 500 1 -1

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300 4 -1

100 5 -1 null

Building Blocks Dequeue: Step 4 head value: 200 enqTid: 0 deqTid: 1 next:

500 1 -1

tail 300 4 -1

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100 5 -1 null

Building Blocks Dequeue: Step 4 head value: 200 enqTid: 0 deqTid: 1 next:

tail

500 1 -1

300 4 -1

head

100 5 -1 null

tail

value:500 enqTid:1 deqTid-1 next:

300 4 -1

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100 5 -1 null

Fast-path Slow-path Approach yes

Diagram taken from Kogan, Alex, and Erez Petrank. "A methodology for creating fast wait-free data structures." Proceedings of the 17th ACM SIGPLAN symposium on Principles and Practice of Parallel

Do I need to Help?

Programming. ACM, 2012.

Help slow op

no

Try to apply my op using Fast path (most (most X X times) times)

Success?

yes

return

no Try to apply my op using Slow path (Until successful) COMP 5704 Project Presentation

Fast-path Slow-path Approach Do I need to help? Helping Record of thread ti

State of thread tj

curTid: j lastPhase: 9 nextCheck: 0

phase: 9 pending: true enqueue: true next:

Fast path: MS Queue Slow path: KP Queue

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Correctness Correctness can be achieved by proving the following properties


Linearizability : Execution of every operation has a set of linearization points.




Wait Freedom : Every thread completes its operation in O( F + D.n 2 ) steps.

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Implementation and Results Java implementation with MPJ. Fast-path Slow-path Approach 380.00 340.00 Time in milliseconds



300.00

FH(50,50)

260.00

FH(20,20)

220.00

FH(10,10) FH(0,0)

180.00 140.00 100.00 4

8

12

16

No of Threads

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Implementation and Results 



Performance can be improved by changing constants. Scalable and as efficient as the lock-free MS Queue.

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Questions ?

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Questions  



How does the CAS operation work? What is the difference between lock-freedom and wait-freedom? When does a thread execute on the slowpath?

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