Condition Variables and Transactional Memory: Problem or Opportunity?

1. Polina Dudnik and Michael Swift University of Wisconsin, Madison Condition Variables and Transactional Memory: Problem or Opportunity?

2. Executive Summary • Problem: thread synchronization in TM • Goal: robust synchronization primitive • State of the art: Retry/orelse • Main point: Condition Variables [CVs] still relevant • Our contributions: • TM-compatible condition variables • Implementation independent • Performance comparable to original CVs

3. Outline • Lessons from the past • New ideas for TM • Transaction/Condition Variable Interactions • Design of TM-safe Condition Variables • Evaluation

4. What is the problem? • How should threads coordinate? • How should threads wait for an event or state change? • How does this problem change with transactions?

5. On the Road to CVs • Semaphores [Dijkstra ‘65] • Condition Critical Regions (CCRs) [Hoare ‘72]

6. Hoare on CCR • I feel this proposal [condition critical regions] is not suitable for operating system implementation. My proposed method encourages the programmer to ignore the question of which of several outstanding requests for a resource should be granted. • Sir Anthony Hoare [Belfast ‘71]

7. Limitations of CCR • Atomicity • Excessive Context Switching • Restrictive Scheduling

8. Solution: Condition Variables • Condition variable = queue of waiters • Associated lock maintains mutual exclusion • Signaling a CV = hint that state has changed [Mesa semantics: Lampson ‘79] • Multiple CVs provide prioritized wakeup

9. Problem Solved • CCR • Atomicity Concerns • Performance Issues • Restrictive Scheduling • Condition Variables • Monitors • Precise Wakeup • Explicit Signaling/Multiple CVs

10. CVs Limitations • Nested monitor problem • How about nested function calls? • Or waiting on two event queues? • Retry/orelse solves these problems!

11. TM Synchronization Today • Retry/orelse [Harris et al. 2005]

12. Why not retry? • Nesting • Composability • Restrictive scheduling • Potentially poor scalability • Other proposals have the same problems: • atomic CCRs [Harris 2003], atomic Wait [Smaragdakis 2007], X10 conditional atomic blocks [Charles 2005] • It is too early to give up on condition variables!

13. Outline • Lessons from the past • New ideas • Transaction/Condition Variable Interactions • Design of TM-safe Condition Variables • Evaluation

14. TM Condition Variables • Original condition variables • int get() { • lock(l); • getters++; • while (!available) { • wait(cv, l); • } • getters--; • available = false; • unlock(l); • return contents; • }

15. TM Condition Variables • Convert locks to transactions • int get() { • begin_tx; • getters++; • while (!available) { • wait(cv); • } • getters--; • available = false; • end_tx; • return contents; • }

16. TM Condition Variables • Wait outside the transaction • int get() { • begin_tx; • getters++; • while (!available) { • end_tx; • wait(cv); • begin_tx; • } • getters--; • available = false; • end_tx; • return contents; • }

17. Lost Wakeup • Waiting Thread • begin_tx; • read_state; • prepare_wait(); • end_tx; • wait(); • Signaling Thread • begin_tx; • update_state; • signal(); • end_tx; TIME

18. TM Condition Variables • Split wait • prepare wait within transaction • commit • complete wait • restart transaction • int get() { • begin_tx; • getters++; • while (!available) { • prepare_wait(cv); • end_tx; • complete_wait(cv); • begin_tx; • } • getters--; • available = false; • end_tx; • return contents; • }

19. TM Condition Variables • What happens with concurrent signalers? • int get() { • begin_tx; • getters++; • while (!available) { • prepare_wait(cv); • end_tx; • complete_wait(cv); • begin_tx; • } • getters--; • available = false; • end_tx; • return contents; • }

20. Update-Signal Order • Waiting Thread • begin_tx; • while (!state) { • prepare_wait(); • end_tx; • wait(); • begin_tx; • } • end_tx; • Signaling Thread • begin_tx; • update_state; • signal(); • end_tx; TIME

21. TM-CV Implementations

22. Outline • Lessons from the past • New ideas for TM • Transaction/Condition Variable Interactions • Design of TM-safe Condition Variables • Evaluation

23. Workloads • libMicro • Stress-test of conditional synchronization • FluidAnimate PARSEC • Many critical sections, few condition variables ops • StreamCluster PARSEC • Few critical sections, few condition variable ops • Platform: Solaris + GEMS/LogTM-SE, 16 threads

24. Evaluation • Questions to answer: • Does it work? • How two versions compare in performance? • How does performance compare to locks?

25. Results

26. Evaluation • Answers to questions: • Does it work? • YES • How two versions compare in performance? • Differ under stress • How does performance compare to locks? • Comparable performace

27. Conclusions • Condition Variables are still relevant with TM • Two implementations of TM-CV • Different requirements on TM system • Performance difference subject to potential overlap • Read the paper for: • Implementation independence

28. QUESTIONS?

29. Backup Slides

30. Different Colors

31. Example CV usage • void BeginWrite() { • pthread_mutex_lock(mutex); • while (NWriters == 1 || NReaders > 0){ • ++WaitingWriters; • pthread_cond_wait(CanWrite,mutex); • --WaitingWriters; • } • NWriters = 1; • pthread_mutex_unlock(mutex}; • }

32. Lost Wakeup • Waiting Thread • BEGIN_TX • while (!state) { • prepare_wait() • COMMIT_TX • wait() • BEGIN_TX • } • COMMIT_TX • Signaling Thread • BEGIN_TX • update_state • BEGIN_ESCAPE • signal() • END_ESCAPE • COMMIT_TX