Low Power Cache Design

1. Low Power Cache Design M.Bilal Paracha Hisham Chowdhury Ali Raza

2. Acknowlegements • Ching-Long Su and Alvin M Despain from University of Southern California,”Cache Design Trade-offs for Power and Performance Optimization:A Case Study” • C.L and Alvin M.Despain “ Cache Designs for Energy and Efficiency” • Zhichun Zhu Xiadong Zhang, College of William and Mary, “Access Mode predictions for low-power cache design” • M. D. Powell and A. Agrawal and T. N. Vijaykumar and B. Falsafi and K. Roy, Reducing Set-Associative Cache Energy via selective Direct –Mapping and Way Prediction.”. MICRO 2001.

3. Today’s talk • Abstract • Introduction • Use of cache in microprocessors • Different designs to optimize cache energy and power consumption • Design Trade-offs for Power & Performance Optimization • Vertical Cache Partitioning • Horizontal Cache Partitioning • Gray Code Addressing • Set-Associative Cache Energy Reduction • Way Prediction • Selective direct-mapping • Access Mode Prediction (AMP) • Advantages over Way Prediction and Phased cache • Different prediction techniques • Evaluation Results • Cache Access Times • Miss Rates • Cache Energy consumption

4. Today’s talk…. • Conclusion • Acknowledgements

5. Abstract • Usage of caches in modern microprocessors. • Caches designed for high performance, ignore power consumption • Research activities towards low power cache design

6. Introduction • Cache uses 30-60% processor energy in embedded systems • Use of caches in high performance machines • Various designs to optimize energy consumption

7. Use of cache in microprocessors • High performance products go mobile (Notebooks, PDA’s etc) • Cache’s as temporary storage devices • Design of components with low power consumption

8. Designs to optimize cache energy consumption

9. Vertical Cache Partitioning • Block Buffer • Block Hit/Miss • Block Size

10. Horizontal Cache Partitioning • Cache segments • Cache sub-banks • Reduction cache accesses • Hit time, an advantage

11. Gray Code Addressing • Gray code vs 2’s compliment • Minimizes bit switches • 2s Compliment:31 bits change • Gray Code:16 bits change

12. Evaluation Results • <dm,2> A direct mapped cache with block size 2 words • <dm,4> A direct mapped cache with block size 4 words • <dm,8> A direct mapped cache with block size 8 words • <2lru,2> A 2-way set associative cache with block size 2 words • <2lru,4> A 2-way set associative cache with block size 4 words • <2lru,8> A 2-way set associative cache with block size 8 words • <4lru,2> A 4-way set associative cache with block size 2 words • <4lru,4> A 4-way set associative cache with block size 4 words • <4lru,8> A 4-way set associative cache with block size 8 words

13. Cache Access Time • Takes less time to access direct –mapped than set associative • Cache access of 1K byte for dm=4.79 ns, for set assoc=7.15 ns • 2 way set associative is approx 50% slower than dm cache

14. Energy consumption vs Cache Size

15. Energy Consumption

16. Reducing Set Associative Cache Energy Via Way Prediction and Selective Direct mapping

17. Cache Access Energy Reduction Techniques • Energy Dissipation in Data Array is much larger than in Tag Array so Energy Optimizations in Data Array only are done. • Selective Direct Mapping for D-Caches • Way Prediction for I-Caches

18. Different Design Techniques a) Conventional Parallel Access

19. b) Sequential Access

20. c) Way Prediction

21. d) Selective Direct Mapping (DM)

22. Prediction Framework for Selective Direct mapping (DM)

23. Access Mode Prediction for Low Power Cache Design

24. Different Cache accessing mode • Phased Cache: • Compares tag with all the tag in a particular set, If the tag matches only then, it accesses the data • Consumes energy, not efficient Access the set ↓ Access all n tags ↓ Access the data corresponding to the tag

25. Way Prediction: • Access only the predicted tag and data • Efficient when hit rate is high • Not very efficient when there is a miss (has to access rest of the tag and data elements) Access the set ↓ Way Prediction ↓ Access the predicted data and tag sub array in the set ↓ Prediction Correct Yes ↓→No Compare the rest of the data and tag array Proceed

26. Access Mode Prediction (AMP) • Prediction based approach • Better to use Way Prediction when hit rate is very high • When hit rate is low, it is preferable to use Phased Cache approach • Predicts whether cache access will result in a hit or a miss. If it predicts a hit then Way prediction is used, other wise use Phased Cache approach • Accuracy of the access mode determines the efficiency of the approach

27. Power Consumption: • Perfect AMP and perfect Way Prediction has a power consumption which is the lower bound of conventional set associative cache. • predicted hit in the way-prediction cache, the energy consumed is Etag +Edata, compared with n × Etag+ Edata in the phased cache • miss in the way-prediction cache will consume (n + 1) ×Etag + (n + 1) × Edata, in comparison with (n +1) × Etag + Edata in the phased cache.

28. Different Predictors • Saturating Counter: • Similar to the saturating counter of branch prediction used in project2 • Maintains a two bit counter which increments on a cache hit and decrements on a cache miss • Two-level adaptive predictor: • Adaptive two level branch prediction using global pattern-history table (GAg) • K bit history register records the result of most recent K accesses • For a hit register records a 1, otherwise 0 • This K bit is used to index global pattern history table which has 2^K entries, each entry is a 2 bit saturation counter • Per address two level global pattern history table (PAg) • Each set has its own access history register • All history register index a single history pattern table • Correlation predictor • Gshare predictor: • XOR of global access history with current reference set provides the index for global pattern history table

29. Misprediction rate of different predictors

30. Conclusion • Cache Designs can be modified to obtain maximum performance and optimal energy consumption • Experiments suggest that • direct-mapped caches (inst and data) consume less energy for dynamic logic • Set Associative consume less energy for static logic • Circuit level techniques can no longer keep power dissipation under a reasonable level. • Reduction of power is done on architectural level. By producing different schemes for reducing on-chip cache power consumption

31. Questions…???