1. 1/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control BranchTap Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Patrick Akl and Andreas Moshovos AENAO Research Group Department of Electrical and Computer Engineering University of Toronto

2. 2/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control We wish to make the recovery fast What Happens on a Branch Misprediction? Execution Timeline Misprediction Discovered Recover Processor State Redirect Fetch Resume Execution Predict a Branch Outcome Predicted PathCorrect Path

3. 3/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Existing mechanisms –Reorder buffer based: slow –Instantaneous checkpoints: faster Problem: can’t have enough checkpoints State-of-the-art solution: checkpoint prediction –Allocate the few checkpoints judiciously Another degree of freedom: speculation control –Sometimes deeper speculation = higher recovery cost Can hurt performance –Throttle speculation State-of-the-art recovery

4. 4/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control No additional checkpoints are needed Dynamically adapts to application behavior Improves performance for most programs –Misprediction performance penalty reduced by 28% on AVG BranchTap comes “for free” –Very simple to implement –Better than more accurate checkpoint predictors BranchTap Results / Benefits

5. 5/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Outline Background BranchTap Methodology and Results Summary

6. 6/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control State Recovery Example: Register Alias Table RAT Architectural Register Physical Register # arch. regs Lg(# arch. regs) A add r1, r2, 100 B breq r1, E Csub r1, r2, r2 Original Code A add p4, p2, 100 B breq p4, E Csub r5, p2, p2 Renamed Code p1 p2 p3 p4p5 p4

7. 7/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control ROB: Slow, Fine-Grain Recovery Too slow: recovery latency proportional to number of instructions to squash Reorder Buffer BBBBB 1.Misprediction discovered 2. Locate newest instruction 3. Undo RAT updates in reverse order Program Order RAT INVALID Each entry contains 1.Architectural destination register 2.Its previous RAT map

8. 8/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Global Checkpoints: Fast, Coarse-Grain Recovery Branch w/ GC: Recovery is “Instantaneous” Reorder Buffer BBBBB 1.Misprediction discovered Program Order RAT INVALID checkpoint

9. 9/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Impact of More Checkpoints More checkpoints ? –Power hungry structure –Increased delay Only a few checkpoints can practically be implemented –Cannot always cover all branches architectural register physical register Actual Implementation Working Copy checkpoints RAT Concept

10. 10/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Intelligent Checkpointing State of the art solution –Checkpoint allocation: Allocate checkpoints at hard-to- predict branches –Checkpoint management: Release checkpoints as soon as they are no longer needed Use few checkpoints efficiently

11. 11/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Mispeculation on a branch w/ a GC: Direct recovery Mispeculation on a branch w/o a GC: Indirect recovery With intelligent checkpointing: 30% Indirect recoveries  75% of performance loss Conventional Mechanisms: Recovery Scenarios BBB ROB BBB checkpoint Fast Recovery Slow Recovery checkpoint

12. 12/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Outline Background BranchTap Methodology and Results Summary

13. 13/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control BranchTap Motivation ROB No Wait Scenario Misprediction discovered ~ Recovery Cost checkpoint Low confidence branch checkpoint ROB Sometimes, it is better to wait if no checkpoint is available Wait Scenario BBB BBB

14. 14/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control BranchTap Concept Key idea: stall when speculation is likely to deteriorate performance –Count the number of low confidence branches w/o a checkpoint –If it exceeds a threshold, stall Threshold selection –Fixed Varies greatly across programs Can deteriorate performance significantly –Adaptive Robust performance Minimize recovery cost while conserving good speculation opportunities

15. 15/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Threshold Adaptation Policy BranchTap adapts across and within applications

16. 16/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Outline Background BranchTap Methodology and Results Summary

17. 17/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Results Overview Performance w/o Checkpoints –BranchTap improves even with just an ROB Performance w/ 4 Checkpoints –BranchTap improves over conventional recovery methods Performance w/ Larger Checkpoint Predictors –BranchTap offers better performance than a 64x larger predictor

18. 18/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Methodology Simulator based on Simplescalar 24 SPEC CPU 2000 benchmarks Reference Inputs Processor configurations –8-way OoO core –Up to 1K in-flight instructions –1K-entry confidence table for low confidence branch identification 1B committed instructions after skipping 100B

19. 19/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control “Perfect Checkpointing” Configuration A checkpoint is auto-magically taken at all mispredicted branches –All recoveries are fast We report the “deterioration relative to perfect checkpointing”

20. 20/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Performance with No Checkpoints Deterioration relative to “perfect checkpointing” -39% deterioration BranchTap improves over conventional mechanisms Adaptation leads to robust performance improvements better

21. 21/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Deterioration relative to “perfect checkpointing” BranchTap with 4 checkpoints is better than 6 checkpoints alone Performance Evaluation with 4 Checkpoints -28% deterioration better

22. 22/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control BranchTap with a 1K-entry confidence table and 4 GCs: –Higher performance than a 64K-entry confidence table with 4 GCs –Lower complexity, virtually comes “for free” BranchTap vs. Larger Checkpoint Predictors BranchTap deterioration confidence table size better

23. 23/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Outline Background BranchTap Methodology and Results Summary

24. 24/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Summary Performance with 4 (no) checkpoints –~28 (39) % of misprediction penalty removed –BranchTap is robust: Up to 6 (13) % better and max 1.2 (0.1) % worse than conventional mechanisms BranchTap is very simple to implement –Few counters and comparators BranchTap is better than other alternatives –BT + 1K predictor better than a 64K predictor alone –BT + 4 GCs better than 6 GCs alone

25. 25/25 June 28 th, 2006 BranchTap: Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control BranchTap Improving Performance With Very Few Checkpoints Through Adaptive Speculation Control Patrick Akl and Andreas Moshovos AENAO Research Group Department of Electrical and Computer Engineering University of Toronto {pakl, moshovos}@eecg.toronto.edu