1. Discovering and Exploiting Program Phases Timothy Sherwood, Erez Perelman, Greg Hamerly, Suleyman Sair, Brad Calder CSE 231 Presentation by Justin Ma

2. 400 Million Instructions New Compiler Non-Existent ProcessorNew Processor Simulator Benchmark Spec2000

3. 400 Million Instructions Suppose you have a time budget… Less than half second of execution time What would you simulate? –Beginning? –Middle? –End?

4. 400 Million Instructions gzip gcc Programs exhibit diverse modes of behavior

5. 400 Million Instructions Suppose you have a time budget… Less than half second of execution time What would you simulate? –Beginning? –Middle? –End? –Samples of different modes of behavior

6. Program Phases Observation: programs exhibit various modes of periodic behavior These modes are program phases Challenge: Extract these automatically

7. Phase Basics Intervals – slices in times Phases – intervals with similar behavior Time (Instruction Count) IPC

8. Phase Basics Intervals – slices in times Phases – intervals with similar behavior Time (Instruction Count) IPC

9. Defining “Similar Behavior” Metric for comparing intervals? –Cache misses? –IPC? –Branch misprediction rates? Problem: Performance alone is too architecture dependent

10. Defining “Similar Behavior” Code path traversal –Directly affects time-varying behavior –Execute same code, same performance –Architecture independent Metrics for code path traversal –Frequency of branches –Frequency of function calls –Frequency of basic block calls

11. Basic Block Vector B1 B2B3 B4 0000 B1B2B3B4 Time t

12. Basic Block Vector B1 B2B3 B4 1101 B1B2B3B4 Time t

13. Basic Block Vector B1 B2B3 B4 2112 B1B2B3B4 Time t

14. Basic Block Vector B1 B2B3 B4 2112 B1B2B3B4 Time t 0000 B1B2B3B4 Time t + 1

15. Basic Block Vector B1 B2B3 B4 2112 B1B2B3B4 Time t 1101 B1B2B3B4 Time t + 1

16. Basic Block Vector B1 B2B3 B4 2112 B1B2B3B4 Time t 2202 B1B2B3B4 Time t + 1 Manhattan Distance = |1 – 2| + |1 – 0| = 2 Euclidian Distance = sqrt((1 – 2) 2 + (1 – 0) 2 ) = sqrt(2)

17. Basic Block Similarity Matrix gzip

18. Basic Block Similarity Matrix gcc BBV similarity between intervals reflects performance similarity

19. Automatic Phase Classification Classify intervals into phases –We do not know which BBVs correspond to particular phases a priori k-means clustering –Iterative clustering algorithm –Dimension Reduction Random Linear Projection –Try different k values Use BIC to choose best

20. Automatic Phase Classification

21. Clustering accurately distinguishes phases automatically

22. SimPoint Simulate large programs on a budget Perform detailed simulation on representative code snippets –Choose centroid interval from each phase (10 million instructions) Extrapolate large program performance –Weighted by frequency of phase

23. Simulate 400 million instructions total SimPoint Accurate estimate despite instruction budget

24. Why SimPoint Succeeds Program behavior varies over time SimPoint intelligently chooses which intervals to simulate Regularity within program phases allows accurate extrapolation

25. Online Classification Detect phases as program is running Applications –Thread scheduling –Power management –Predicting future phases Challenges –One pass of input –Limited storage

26. Online Classification

27. High variance in metrics across full trace Low variance shows online classification succeeds in finding phases

28. Conclusions Phases are a vital abstraction –Performance varies greatly w/in program –Attributable to different modes of behavior Can discover phases automatically –Offline: k-means clustering –Online Code path characterization –Strong correlation with actual performance –SimPoint exploits this with great success

30. Outline Introduction (motivate) Basics (definitions, BBV, BBMatrix) Offline Phase Classification –SimPoints Online Phase Classification Conclusions

31. Limitations of Clustering

32. Bayesian Information Criterion Fit to Gaussians

33. Self-Modifying Code Self-modifying code Program Phases 85 o

34. Learning Phases