1 / 37

PaCo Probability-Based Pa th Co nfidence Prediction

PaCo Probability-Based Pa th Co nfidence Prediction. Kshitiz Malik, Mayank Agarwal, Vikram Dhar, Matthew Frank Implicitly Parallel Architectures Group University of Illinois at Urbana-Champaign. Summary. Path Confidence: likelihood of correct path Pipeline Gating, SMT Fetch

pilis
Télécharger la présentation

PaCo Probability-Based Pa th Co nfidence Prediction

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. PaCoProbability-Based Path Confidence Prediction Kshitiz Malik, Mayank Agarwal, Vikram Dhar, Matthew Frank Implicitly Parallel Architectures Group University of Illinois at Urbana-Champaign

  2. Summary • Path Confidence: likelihood of correct path • Pipeline Gating, SMT Fetch • Conventional: use count of low-conf branches • Inaccurate • PaCo: Directly estimates goodpath probability • Highly accurate, modest hardware • Improves performance on gating, SMT Fetch HPCA-14 February 18, 2008

  3. Outline • Overview • Motivation • Design • Evaluation • Applications HPCA-14 February 18, 2008

  4. Branch Confidence Prediction • Branch Confidence: Single Branches • Low Conf / High Conf • Applications: Checkpointing, Multipath etc HPCA-14 February 18, 2008

  5. Path Confidence Prediction • Path Confidence: likelihood of being on correct path • Contributions from Multiple Branches • Conventionally: Count of (unexecuted) low-confidence branches. HPCA-14 February 18, 2008

  6. Path Confidence: Applications • Path Confidence: Multiple Branches • Count of low-confidence branches. • Applications: Pipeline Gating, SMT Prioritization Gate-Count = 5 (Fetch gated when conf >= 5) Gated HPCA-14 February 18, 2008

  7. Path Confidence: Applications • Path Confidence: Multiple Branches • Count of low-confidence branches. • Applications: Pipeline Gating, SMT Prioritization HPCA-14 February 18, 2008

  8. Issues with Conventional Approach • Path Confidence: Multiple Branches • Count of low-confidence branches. • Applications: Pipeline Gating, SMT Prioritization • Problem: implicit assumption that • High-conf branches never mispredict • All low-conf branches have same misprediction probability HPCA-14 February 18, 2008

  9. Path Confidence using PaCo • Directly estimates goodpath probability • Highly Accurate: RMS error 3.8% • Modest Hardware: 60 bytes of counters • Excellent Performance in applications • Gating: BadpathInstrs. Performance Conv↓ 7%↓ 0.1% PaCo ↓ 32%↓ 0.01% • SMT Fetch Prioritization: ↑ perfupto 23% (5.5% av.) HPCA-14 February 18, 2008

  10. Outline • Overview • Motivation • Design • Evaluation • Applications HPCA-14 February 18, 2008

  11. Branch Confidence Prediction • Classify branches as low conf. or high conf. • JRS predictor: • Count consecutive correct predictions • Below threshold: Low Confidence • Table of Miss-Distance Counters (MDCs) MDC Value < Threshold? + MDC Table MDC Value 5 Mispredict? 1 Branch PC 0 + On Branch Execution Global Hist HPCA-14 February 18, 2008 4 bits

  12. Conventional Path Confidence Prediction • Count of low-confidence branches = measure of path confidence • Threshold-and-Count Approach • Inaccurate • Coarseness • No relation to goodpath probability MDC Table Threshold Function Sum Low Conf / High Conf (1 bit) Miss Distance Counter Value (4 bits) Path Confidence Branch HPCA-14 February 18, 2008

  13. Threshold Function Low Conf / High Conf (1 bit) Coarseness Miss Distance Counter Value (4 bits) a JRS Threshold = 3 • All low-conf branches are equal • Eg: SMT prior. • gcc: • a pending • 0.57 gpathprob • vortex: • 2 bpend. • Prob = 0.88*0.88 = .78 • yet, fetch from gcc! b High Conf Low Conf HPCA-14 February 18, 2008

  14. Threshold Function Low Conf / High Conf (1 bit) Coarseness Miss Distance Counter Value (4 bits) • All low-conf branches are equal • High-conf branches don’t mispredict • twolf, vortex, mdc=3 • Don’t affect conf. HPCA-14 February 18, 2008

  15. Sum ≠ Probability Sum Low Conf / High Conf (1 bit) Path Confidence • Gating at count=5 • Too aggressive for gzip, not useful for route • SMT Fetch: • gzip and route, conf = 5. Equal bandwith? Goodpath Likelihood when 5 low-confidence branches are pending HPCA-14 February 18, 2008

  16. Sum ≠ Probability Goodpath Likelihood when 5 low-confidence branches are pending HPCA-14 February 18, 2008

  17. Sum ≠ Probability Sum Low Conf / High Conf (1 bit) Path Confidence • Hard to choose optimal gate-count • Different gate-counts for different benchmarks • Different gate-counts for different phases • Hard to compare path confidence of diff. apps • SMT Fetch Prioritization sub-optimal HPCA-14 February 18, 2008

  18. Outline • Overview • Motivation • Design • Evaluation • Applications HPCA-14 February 18, 2008

  19. Design • Calculate Goodpath Probability directly • Finding ‘correct prediction probability’ for a branch • MDC table good differentiator of misprediction rates • Find misprediction rate for each MDC value • No thresholding! • Other, more h/w intensive approaches possible HPCA-14 February 18, 2008

  20. Threshold-and-count vs. PaCo MDC Table Threshold Function Sum Low Conf / High Conf (1 bit) Miss Distance Counter Value (4 bits) Path Confidence Branch Mispredict Rate Calculator (MRT) Product Mispredict Probability Path Confidence Mispred Rate Table 0 PaCo 1 2 . . . Misprediction Rate 13 14 15 HPCA-Practice Talk February 13, 2008 MDC Value

  21. Hardware Complexity • Remove floating point: scale to integer values • Hardware Complexity • Floating point MUL and DIV required • Use logarithms, remove mul/div HPCA-14 February 18, 2008

  22. MDC Table PaCo Hardware 5 Branch PC Feedback from Backend 60 bytes of counters, 10-bit shift register Global Hist Mispredict Rate Calculator Path Confidence Predictor Encoded Probability Correct Preds MDC 0 Mispreds MDC 0 MDC 1 Correct Preds MDC 1 Log Circuit Mispreds . . . . . . . . . . . Path Confidence 6 bits + + 10 bits Correct Preds MDC 15 MDC 15 Mispreds 12 bits

  23. Outline • Overview • Motivation • Design • Evaluation • Applications HPCA-14 February 18, 2008

  24. Evaluation: Prediction Accuracy • RMS Error = 0.038. • Example: • Predicted 60% goodpath likelihood • Should be within (60 ± 3.8) = 56.2% - 63.8% HPCA-14 February 18, 2008

  25. Accuracy: Reliability Diagram RMS Error

  26. Outline • Overview • Motivation • Design • Evaluation • Applications HPCA-14 February 18, 2008

  27. Applications: Pipeline Gating GateCount=1 GateCount=2 GateCount=10 HPCA-14 February 18, 2008

  28. Applications: Pipeline Gating 0.1% perf loss 32% redn. in badpath instructions HPCA-14 February 18, 2008

  29. Applications: SMT Fetch Prioritization

  30. Conclusions • Threshold-and-Count predictors are inaccurate • PaCo: Directly produces goodpath probability • Uses modest h/w by using logarithms • Highly accurate: low RMS error (3.8%) • PaCo does very well in Pipeline Gating and SMT Fetch Prioritization HPCA-14 February 18, 2008

  31. Questions? HPCA-14 February 18, 2008

  32. Backup1: Comparison with WPUP • WPUP: Perfect Fetch gating improves average performance by 2.3% (excl. mcf and parser) • Badpath Instructions • Good: prefetching (useful: small ROBs, wide machines) • Bad: BTB/Cache pollution (prob: smaller BTBs/caches) • Prefetching much less useful with 512 ROB

  33. Pipeline Parameters: Fetch Gating HPCA-14 February 18, 2008

  34. Pipeline Parameters: SMT HPCA-14 February 18, 2008

  35. MDC Table Threshold Function Sum Low Conf / High Conf (1 bit) Miss Distance Counter Value (4 bits) Path Confidence Branch HPCA-Practice Talk February 13, 2008

  36. + MDC Table 5 Mispredict? 1 Branch PC 0 + Global Hist HPCA-14 February 18, 2008 HPCA-14 February 18, 2008 4 bits

  37. Applications: SMT Fetch Prioritization

More Related