Composite Abortable Locks (CALs)

1. Composite Abortable Locks (CALs) Virendra J Marathe University of Rochester Mark Moir, Nir Shavit Sun Microsystems Labs

2. Mutual Exclusion Locks • Used in Multiprocessor synchronization • Lock owner gets to execute the critical section • Enforces waiting; leads to • Waiting for arbitrary time (real-time systems) • Deadlocks

3. Timeout Capability – Try locks • Timeout and retry • Obeys real-time constraints • Helps avoid deadlocks (common solution in databases) • CAL is a try-lock

4. Talk Outline • Motivation • Overview of CALs • A CLH-based CAL • Experimental Results • Conclusion

5. Motivation: Issues in Try-lock Implementations • Test-and-Set based Backoff locks (BLs) • Fast; easy to implement timeout capability • Don’t scale • Queue based locks (QLs) • Scale very well • Not trivial to implement timeouts • Need separate memory management • High-to-unbounded worst case space overheads

6. Our Approach • Combine the best features of BLs and QLs • BLs: Low space overhead and no memory management • QLs: Scalability • Key Insights • Need only front part of a QL for scaling • Distribute BL across multiple locations • Key Difficulty • The two algorithms are quite different in structure

7. Overview of CAL • Fixed-sized Backoff array • Nodes allotted to threads from this array • Threads insert nodes into wait-queue Logical Queue Ptr 1 2 3 4 Backoff Array Tail

8. Lock Acquisition • Node States: Free (F), Waiting (W), Released (R), Aborted (A) • Node Acquisition (F  W) • Thread selects a node (maybe randomly) • Acquire exclusive ownership using a BL • Enqueue and Wait as per queue mechanism (e.g. CLH, MCS)

9. Node Release • Write R into node to release lock (W  R) • Write A on timeout (W  A) • Someone else will “reclaim” the node (R  F or A  F) • Write F on timeout (W  F) only if the node was not enqueued

10. CLH Queue Lock • Node States: Free (F), Waiting (W), Released (R) Lock Owner Ptr W W W R F Dummy CAS Spin on previous Tail W New Thrd

11. CLH-based CAL Lock Owner CAS Timeout Ver# Ptr F W F A W F W W R F Backoff CAS Tail Thrd B Thrd A Acquire Release Aborts Cleanup

12. Low Contention Case Performance Problem • Need 2 CASes in the critical path • Acquire Node • Enqueue in wait-queue

13. Optimization • Eliminate extra CAS in no contention case • Use lsb of Tail to indicate if lock acquired • If not, flip the bit • Otherwise follow the CLH-based CAL algorithm • Switch back to no contention case • If the lock releaser is the Tail, flip (using CAS) Tail to uncontended mode

14. Experimentation • Aimed at • Scalability, Preemption tolerance, Degradation • Framework • Backoff array of size 4 • 30-processor Sun Enterprise 6000 cache coherent machine with 366MHz UltraSPARC II processors • Critical:Non-critical work  300:300 (nanosecs) • Implementation in C, with Sun’s cc –O5 • Comparison with Scott’s state-of-the-art nonblocking CLH try lock

15. Scalability

16. Effect of Preemption

17. Degradation with lower Timeouts (30 threads)

18. Conclusion • Introduced the new CAL approach to implement try-locks • Scalable • Constant Space Overhead • No extra memory management needed • Preemption tolerant • No fairness guarantees

19. Future Work • Scalability: Stress test for huge number of threads • Address fairness problem • Other variants of CALs (e.g. MCS-lock)

20. Thanks!