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TurboROB A Low Cost Checkpoint/Restore Accelerator

TurboROB A Low Cost Checkpoint/Restore Accelerator. Patrick Akl and Andreas Moshovos AENAO Research Group Department of Electrical and Computer Engineering University of Toronto { pakl , moshovos}@eecg.toronto.edu. Recovering From Control Flow Mispredictions. Execution Timeline.

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TurboROB A Low Cost Checkpoint/Restore Accelerator

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  1. TurboROBA Low Cost Checkpoint/Restore Accelerator Patrick Akl and Andreas Moshovos AENAO Research Group Department of Electrical and Computer Engineering University of Toronto {pakl, moshovos}@eecg.toronto.edu

  2. Recovering From Control Flow Mispredictions Execution Timeline Predict a Branch Outcome Misprediction Discovered Recover Processor State Redirect Fetch Correct Path Predicted Path Resume Execution • Accelerate Recovery – Improve Performance

  3. State-of-the-Art Recovery • Scalability and/or Performance Issues Log of Changes State Snapshot Predict a Branch Outcome ROB Misprediction Discovered what old value

  4. Turbo-ROB Log of Changes Predict a Branch Outcome ROB Misprediction Discovered Partial Log of Changes • Make common case fast: • Recover only at branches • Store only as much as needed: • Partial Log

  5. Outline • Control Flow Mispeculation Recovery • TurboROB • Methodology and Results • Summary

  6. State Recovery Example: Register Alias Table Lg(# arch. regs) Original Code RAT A add r1, r2, 100 B breq r1, E C sub r1, r2, r2 p1 p4 p5 p5 p4 Architectural Register p2 p3 # arch. regs Renamed Code A add p4, p2, 100 B breq p4, E C sub r5, p2, p2 Physical Register

  7. B B B B B ROB: Slow, Fine-Grain Recovery Each entry contains • Architectural destination register • Its previous RAT map Program Order 3. Undo RAT updates in reverse order Reorder Buffer • Misprediction discovered 2. Locate newest instruction INVALID RAT • Too slow: recovery latency proportional to number of instructions to squash

  8. B B B B B Global Checkpoints: Fast, Coarse-Grain Recovery Program Order checkpoint checkpoint checkpoint checkpoint Reorder Buffer • Misprediction discovered INVALID RAT • Branch w/ GC: Recovery is “Instantaneous”

  9. RAT checkpoints Working Copy Impact of More Checkpoints Concept ActualImplementation architectural register physical register • More checkpoints ? • Power hungry structure • Increased delay • Only a few checkpoints can practically be implemented • Cannot always cover all branches

  10. B B B B B Intelligent Checkpointing & BranchTap checkpoint checkpoint checkpoint checkpoint • Use Few Checkpoints Effectively • BranchTap: • Throttle Speculation

  11. Conventional Mechanisms: Recovery Scenarios B B B checkpoint B B B checkpoint Re-Execution B B B checkpoint

  12. Outline • Background • Turbo-ROB • Methodology and Results • Summary

  13. Turbo-ROB ~ Recovery Cost B R2 R1 R1 R2 R1 ROB Recovery useful redundant We only need to reverse the first subsequent change for every RAT entry

  14. Turbo-ROB Replacing the ROB B B B TROB Re-Execution B B B TROB

  15. Selective Turbo-ROB w/ ROB B B B TROB Selective Turbo-ROB w/ GCs B B B TROB checkpoint

  16. Outline • Background • TurboROB • Methodology and Results • Summary

  17. Results Overview • TROB as an ROB replacement • BranchTap offers better performance than ROB • Fewer resources • Even for smaller windows • Selective TROB as a GC reduction mechanism • TROB reduces pressure for GCs • Offload a critical structure: RAT • In the paper: • Selective TROB as an ROB accelerator • Even the smallest TROB accelerates recovery

  18. Methodology • Simulator based on Simplescalar • Alpha/OSF • 24 SPEC CPU 2000 benchmarks • Reference Inputs • Processor configurations • 4-way OoO core • 128/256/512 in-flight instructions • 1K-entry confidence table for low confidence branch identification / similar results with Anyweak • 1B committed instructions after skipping 2B

  19. “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. better TROB Replacing the ROB/512-Entry Window • 64-entry TROB == ROB on the Average • Pathological cases exist  256-entry needed • 512-Entry TROB better than ROB

  21. better TROB Replacing the ROB/128-Entry Window • 64-Entry 50% better than ROB • Fewer pathological cases • 128-Entry TROB better than ROB

  22. better sTROB and Global Checkpoints/128-Entry Window • TROB + 1 GC better than 4GCs

  23. Summary • TROB vs. ROB • Replacement • Same resources  better performance • Fewer resources  often better performance • Except when accuracy is high • Acceleration: • ¼ resources  35% improvement • TROB vs. GCs • Reduce pressure from the critical path • With just 1 GC match the performance of four GCs • One more alternative for designers • Allows different area/performance/power tradeoffs

  24. TurboROBA Low Cost Checkpoint/Restore Accelerator Patrick Akl and Andreas Moshovos AENAO Research Group Department of Electrical and Computer Engineering University of Toronto {pakl, moshovos}@eecg.toronto.edu

  25. better TROB Replacing the ROB/512-Entry Window • 64-entry TROB == ROB on the Average • Pathological cases exist  256-entry needed • 512-Entry TROB better than ROB

  26. better TROB Replacing the ROB/128-Entry Window • 64-Entry 50% better than ROB • Fewer pathological cases • 128-Entry TROB better than ROB

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