Performance Pathologies in Hardware Transactional Memory

1. Performance Pathologies in Hardware Transactional Memory Jayaram Bobba, Kevin E. Moore, Haris Volos, Luke Yen, Mark D. Hill, Michael M. Swift, and David A. Wood Multifacet Project (www.cs.wisc.edu/multifacet) Computer Sciences Dept., University of Wisconsin Madison

2. Executive Summary (1/2) • Multiple Hardware Transactional Memory Designs • Best Performance ? • Depends ! Livelock University of Wisconsin Madison

3. pa●thol●o●gy: any deviation from a normal, healthy or efficient condition Executive Summary (2/2) • Why? • Performance Pathologies • What to do? • Better Conflict Resolution Livelock Performance PathologiesPerformance University of Wisconsin Madison

4. Talk Outline • Motivation • Base HTM Results • Performance Pathologies • Enhanced HTM Systems University of Wisconsin Madison

5. Motivation • Which HTM system performs best? • Different Assumptions • Hardware • Broadcast vs Directory Based • Software: • Continuous Transactions vs Critical Section Transactions • How to compare? University of Wisconsin Madison

6. HTM Design Space Stanford TCC Illinois Bulk No HTMs (yet) LL Herlihy/Moss TM MIT LTM Intel/Brown VTM MIT UTM Wisconsin LogTM EE EL Conflict Resolution: Adhoc “Better By Design” ? University of Wisconsin Madison

7. Experimental Setup • Base HTM Systems • Common Hardware Platform • 32 core CMP • Directory-Based Coherence • Common TM Programming Model • Critical Section Based Transactions • Common Workloads • 5 SPLASH + 2 Microbenchmarks (Btree, LFUCache) LL EL EE University of Wisconsin Madison

8. And the winner is… • Low Contention: Similar • High Contention: Depends Performance Pathologies Livelock University of Wisconsin Madison

9. Talk Outline • Motivation • Base HTM Results • Performance Pathologies • Enhanced HTM Systems University of Wisconsin Madison

10. LL EL EE Performance Pathologies • StarvingElder • SerializedCommit • RestartConvoy • FriendlyFire • DuelingUpgrades • FutileStall • StarvingWriter University of Wisconsin Madison

11. System LL • Conflict Detection: Lazy Detect conflicts at commit time (Validation) • Version Management: Lazy New value elsewhere, Update on commit + Abort - Commit • Conflict Resolution: Committer Wins compare with TCC, Bulk University of Wisconsin Madison

12. T1 T2 T3 load A store A ABORT load A store A ABORT load A store A ABORT COMMITING STALLED ABORTING StarvingElder • CAUSE • Committer Wins • EFFECT(S) • Starvation • Load Imbalance • FIX • Elder gets Priority ? Time University of Wisconsin Madison

13. T3 T2 T1 T4 SerializedCommit • CAUSE • Serial Validation • EFFECT(S) • Unnecessary Stall • FIX • Parallel Validation ? Time University of Wisconsin Madison

14. load A load A load A load A store A ABORT ABORT ABORT load A load A load A load A store A ABORT ABORT ABORT load A load A load A RestartConvoy • CAUSE • Fast Abort + • Immediate Restart • EFFECT(S) • Resource Contention • Serialized Execution • ≈ Lock Convoys • FIX • Stagger Restarts ? T1 T2 T4 T3 University of Wisconsin Madison

15. Pathologies LL • Pathology (% ExecutionTime) • Enhancement • Linear Backoff on Abort • Eliminate RestartConvoys LL LL_B University of Wisconsin Madison

16. Enhanced LL ( ) LL_B • Pathology (% ExecutionTime) • Performance LL_B LL University of Wisconsin Madison

17. Performance Pathologies • StarvingElder • SerializedCommit • RestartConvoy • FriendlyFire • DuelingUpgrades • FutileStall • StarvingWriter LL EL EE University of Wisconsin Madison

18. System EL • Conflict Detection: Eager • Detect conflicts immediately • Version Management: Lazy • New Value elsewhere, Update on commit + Abort - Commit • Conflict Resolution: • Requester Wins • Exponential Backoff on Abort compare with HMTM, LTM University of Wisconsin Madison

19. T1 T2 T3 load A store A ABORT load A ABORT store A load A ABORT store A ABORT load A ABORT store A FriendlyFire • CAUSE • Requester Wins • EFFECT(S) • Unnecessary Aborts • Livelock • FIX • Priority Based Conflict Resolution ? University of Wisconsin Madison

20. Pathologies EL • Pathology (% ExecutionTime) • Enhancement • Timestamp Based Conflict Resolution • Eliminate FriendlyFire EL EL_T University of Wisconsin Madison

21. Enhanced EL ( ) EL_T • Pathology (% ExecutionTime) • Performance EL EL_T University of Wisconsin Madison

22. Performance Pathologies • StarvingElder • SerializedCommit • RestartConvoy • FriendlyFire • DuelingUpgrades • FutileStall • StarvingWriter LL EL EE University of Wisconsin Madison

23. System EE • Conflict Detection: Eager • Detect conflicts immediately • Version Management: Eager • New Value in-place, Restore on Abort - Abort + Commit • Conflict Resolution • Stall Requester • Abort Requester on possible deadlock (Conservative Deadlock Avoidance) compare with LogTM University of Wisconsin Madison

24. T1 T2 load A load A store A STALL store A ABORT load A store A DuelingUpgrades • CAUSE • Conservative Deadlock Avoidance • EFFECT(S) • Unnecessary Stall • Unnecessary Abort • FIX • Acquire Store Permissions Early ? University of Wisconsin Madison

25. FutileStall • CAUSE • Stall Requester • EFFECT(S) • Unnecessary Stall • FIX • No simple fix ? T1 T2 load A store A STALL ABORT University of Wisconsin Madison

26. T1 T2 T3 T4 load A store A load A load A load A load A StarvingWriter • CAUSE • Stall Requester • EFFECT(S) • Starvation • Load Imbalance • ≈ Reader/Writer Locks • FIX • Writer Priority ? University of Wisconsin Madison

27. Pathologies EE • Pathology (% ExecutionTime) • Enhancements • DuelingUpgrades: Store-Set Predictor • StarvingWriter: Writer Aborts Readers immediately EE_HP EE University of Wisconsin Madison

28. Enhanced EE ( ) EE_HP • Pathology (% ExecutionTime) • Performance EE_HP EE University of Wisconsin Madison

29. Caveats / FutureWork LL_B • High Contention Scenarios • Better Conflict Resolution • Unidentified Pathologies • Better TM Workloads LL University of Wisconsin Madison

30. Executive Summary • Performance Pathologies • Enhanced HTM systems Livelock University of Wisconsin Madison

31. FutileStall DuelingUpgrades FriendlyFire RestartConvoy StarvingWriter StarvingElder SerializedCommit