1. Dancing With Uncertainty Saša Misailović Stelios Sidiroglou Martin Rinard MIT CSAIL

2. Example Water: Simulates system of water molecules HH O HH O HH O H H O H H O HH O HH O

3. Example Water: Simulates system of water molecules HH O HH O HH O H H O H H O HH O HH O

4. Example Water: Simulates system of water molecules HH O HH O HH O H H O H H O HH O HH O

5. Example Water: Simulates liquid water molecules HH O HH O HH O H H O H H O HH O HH O

6. Example Water: Simulates system of water molecules HH O HH O HH O H H O H H O HH O HH O

7. Example Water: Simulates system of water molecules HH O HH O HH O H H O H H O HH O HH O

8. Dubstep Explores the effects of selectively removing synchronization

9. Dubstep Highlights 1.Removing locks and opportunistic barriers trade accuracy for performance 2.Automatically explores the tradeoff space induced by candidate transformations 3.Uses statistical analysis to characterize impact of transformations on accuracy

10. Dubstep Workflow Prepare Find Transform Analyze Navigate

11. Dubstep Workflow Prepare Find Transform Analyze Navigate 1.Prepare representative inputs 2.Prepare accuracy model – Output abstraction (important parts of output) – Accuracy bound (amount of tolerable error)

12. Dubstep Workflow Prepare Find Transform Analyze Navigate Loops with parallel constructs Profiling: performance & memory

13. Dubstep Workflow Prepare Find Transform Analyze Navigate Loops with parallel constructs Profiling: performance & memory Interf (56.4%) Poteng (43.4%)

14. Dubstep Workflow Removing synchronization Prepare Find Transform Analyze Navigate void scratchPad::updateForces (double R[3][3]) { mutex_lock(this->lock); this->H1force.vecAdd(R[0]); this->Oforce.vecAdd(R[1]); this->H2force.vecAdd(R[2]); mutex_unlock(this->lock); }

15. Dubstep Workflow Removing synchronization Prepare Find Transform Analyze Navigate void scratchPad::updateForces (double R[3][3]) { mutex_lock(this->lock); this->H1force.vecAdd(R[0]); this->Oforce.vecAdd(R[1]); this->H2force.vecAdd(R[2]); mutex_unlock(this->lock); }

16. Dubstep Workflow Removing synchronization Prepare Find Transform Analyze Navigate void scratchPad::updateForces (double R[3][3]) { this->H1force.vecAdd(R[0]); this->Oforce.vecAdd(R[1]); this->H2force.vecAdd(R[2]); }

17. Dubstep Workflow Opportunistic barriers Prepare Find Transform Analyze Navigate void ensemble::interf(){ parallel_for( interf_body, 0, NumMol-1 ); }

18. Dubstep Workflow Opportunistic barriers Prepare Find Transform Analyze Navigate void ensemble::interf(){ parallel_for( interf_body, 0, NumMol-1 ); }

19. Dubstep Workflow Opportunistic barriers Prepare Find Transform Analyze Navigate void ensemble::interf(){ parallel_for( interf_body, 0, NumMol-1 ); } Schedule threads Execute interf_body in parallel Wait for all threads to complete

20. Dubstep Workflow Opportunistic barriers Prepare Find Transform Analyze Navigate void ensemble::interf(){ parallel_for*( interf_body, 0, NumMol-1 ); } Schedule threads Execute interf_body in parallel Wait for half of threads to complete Instruct remaining threads to stop [Rinard, OOPSLA 2007]

21. Dubstep Workflow Analyze transformed program: Criticality –M–Memory safety, integrity Performance –S–Speedup comparison Accuracy –S–Statistical analysis Prepare Find Transform Analyze Navigate

22. Dubstep Workflow Prepare Find Transform Analyze Navigate c Input Original Program Output Output Abstraction (Application-Specific) Transformed Program Difference Bound δ <

23. Dubstep Workflow Navigate the tradeoff space: Transform and analyze one location at a time – 3 locations in water Transform multiple locations in the same candidate program – Guided by the results of the previous step Prepare Find Transform Analyze Navigate

24. Search Space Exploration LI BI BR LI+BI LI+BP BI+BP LI+BI+BP Relative Speedup Accuracy loss LI – Synchronization Interf BI – Barrier Interf BP – Barrier Poteng Baseline: original parallel program runs 6.2 times faster than sequential on 8 cores

25. Search Space Exploration LI BI BR LI+BI LI+BP BI+BP LI+BI+BP Relative Speedup Accuracy loss LI – Synchronization Interf BI – Barrier Interf BP – Barrier Poteng How confident can we be about these observations? Baseline: original parallel program runs 6.2 times faster than sequential on 8 cores

26. Execution Reliability The probability p that the transformed program on the given input produces the result with error less than bound δ While we cannot model p, we can specify minimum acceptable reliability r

27. Execution Reliability Determine if program’s reliability p > r

28. Execution Reliability Determine if program’s reliability p > r How to pick N?

29. How Many Runs Are Enough? Procedure that determines that p > r : Returns correct result most of the time – Wrong decision rate  – Tolerance region  Quickly determines extreme (very good or bad) transformations

30. Statistical Analysis Sequential Probability Ratio Test

31. Statistical Analysis Sequential Probability Ratio Test

32. Statistical Analysis Sequential Probability Ratio Test Bound ( δ ) Best Transformation 0.01LI 0.05LI 0.10LI+BI+BR 0.15LI+BI+BR

33. Statistical Analysis Sequential Probability Ratio Test Bound ( δ ) Best Transformation 0.01LI 0.05LI 0.10LI+BI+BR 0.15LI+BI+BR

34. Exploring Tradeoff Space Start: Sequential program with for loops Transformations: Parallel loop introduction Synchronization, Replication Quickstep [MIT-TR-2010-38, TECS/PEC 2012] Prepare Find Transform Analyze Navigate

35. Exploring Tradeoff Space Start: Program with for loops Transformations: Skip loop iterations (multiple forms) Loop Perforation [ICSE 2010, ONWARD 2010, SAS 2011, FSE 2011] Prepare Find Transform Analyze Navigate

36. Exploring Tradeoff Space Start: Program with command line parameters Transformations: Alternate function versions activated by CL parameters Dynamic Knobs [ASPLOS 2011] Prepare Find Transform Analyze Navigate

37. Exploring Tradeoff Space Start: Program is a tree of Map-Reduce type tasks Transformations: Function Substitution Reduction Sampling NapRed [POPL 2012] Prepare Find Transform Analyze Navigate

38. Exploring Tradeoff Space Start: Parallel program with for loops Transformations: Removing Locks Opportunistic Barriers Dubstep [Today: RACES 2012] Prepare Find Transform Analyze Navigate

39. Reasoning About Accuracy Exploring levels of accuracy guarantees: Logic-based Probabilistic Statistical Empirical Prepare Find Transform Analyze Navigate