1. Optimizing Communication and Capacity in 3D Stacked Cache Hierarchies Aniruddha Udipi N. Madan, L. Zhao, N. Muralimanohar, A. Udipi, R. Balasubramonian, R. Iyer, S. Makineni and D. Newell University of Utah and Intel STL

2. University of Utah 2 Motivation Many-core designs requires large cache capacity for performance SRAM has low latency and consumes less power DRAM has 8X density but poor latency/power characteristics Can we design a hybrid SRAM-DRAM cache to take advantage of both technologies? Can we build a customized on-chip network specifically targeted at such a design?

3. University of Utah 3 Proposal - 3D Stacked Hybrid Cache SRAM DRAM Not an option in conventional 2D design 3D Mixed-process stacking enables a single vertical SRAM/DRAM bank

4. University of Utah 4 Executive Summary 3D stacked hybrid cache design Synergistic proposals to improve performance and power efficiency –Optimizing Capacity Reconfigurable cache hierarchy –Optimizing Communication Page coloring for effective data placement - reduced communication Tailor-made on-chip interconnection network - quicker communication Up to 62% performance increase

5. University of Utah 5 Outline Overview of 3D Technology Technique I - Reconfigurable Cache Hierarchy Technique II - Page coloring Technique III - On-chip Interconnection Network Evaluation Conclusions

6. University of Utah 6 3D Technology + Mixed process integration possible + High speed vertical interconnects - Thermal Issues Source: Black et al. MICRO06 Through-Silicon Vias (TSVs) Die-to-die vias Heat sink Die #2 Die #1 Bulk Si #1 Active Si #1 Metal Active Si #2 Bulk Si #2 I/O Bumps

7. University of Utah 7 Baseline Model Lower die - 16 Processing cores Upper die - 16 SRAM Banks, with grid based on-chip network

8. University of Utah 8 Outline Overview of 3D Technology Technique I - Reconfigurable Cache Hierarchy Technique II - On-chip Interconnection Network Technique III - Page coloring Evaluation Conclusions

9. University of Utah 9 Technique I - Reconfigurable hierarchy Increase capacity by stacking a DRAM bank on each SRAM cache bank, reconfigure bank size based on demand More compelling with 3D and NUCA –Space capacity on die 3 does not intrude with layout of second die or steal capacity from neighboring caches –Cache already partitioned into NUCA banks, additional banks do not complicate logic too much –Access time grows less than linearly with capacity –Dramatic increase in capacity, no gradation, only two choices Turn-off DRAM for small working set size

10. University of Utah 10 Proposed Reconfigurable Cache Model Die containing 16 cores Die containing 16 SRAM banks and tree interconnect Die containing 16 DRAM banks and no interconnect Inter-die via pillar to send request from core to L2 SRAM (not shown: one pillar for each core) Inter-die via pillar to access portion of L2 in DRAM (not shown: one pillar per sector)

11. University of Utah 11 Simple heuristic for enabling/disabling DRAM bank: Every Reconfiguration Interval, –If usage is low and cache-bank miss-rate is low disable DRAM bank above –If usage is high and cache-bank miss-rate is high enable DRAM bank above Reconfiguration interval is every 10 million cycles All cores are stalled for 100K cycles during reconfiguration Proposed Reconfiguration Policy

12. Cache Organization University of Utah 12 Tag Array Data Array DRAMDRAM SRAMSRAM Adaptive arrays become tag arrays for ways in DRAM Total Capacity 1 MB9 MB LowHighAccess Pressure Ways 4 2 0 32

13. Cache Organization SRAM banks have three memory arrays – tag array, data array, adaptive array (can act as both tag & data) Whenever DRAM banks are switched on, tags implemented in part of the SRAM –Quick lookup of tag Increased capacity manifests as additional ways –Cache lines in SRAM need not be flushed on reconfiguration –Two ways of data available with low latency, moving MRU data to these ways will further increase efficiency University of Utah 13

14. Why is this better than a L2/L3 hierarchy? Additional access penalty on L2 miss before the L3 is accessed to service the request –In our scheme, we look up all tags in parallel, in the SRAM An additional level implies additional coherence complexity Our experiments show non-trivial performance degradation on implementing SRAM/DRAM as L2/L3 compared to our scheme University of Utah 14

15. University of Utah 15 Outline Overview of 3D Technology Technique I - Reconfigurable Cache Hierarchy Technique II - Page coloring Technique III - On-chip Interconnection Network Evaluation Conclusions

16. University of Utah 16 Technique II - Page Coloring OS can control what Physical Page Number is assigned to each virtual page, thus controlling the index It can be manipulated to redirect cache line placements Cache TagOffsetIndex Physical Page NumberOffset Page Color CACHE VIEW PHYSICAL ADDRESS

17. University of Utah 17 Page Coloring Page coloring employed to map data to banks based on proximity to cores. We assume an offline oracle page-coloring implementation Policies depend upon 2 criteria: –Knowledge of a page being private or shared –Knowledge of a page being data or code More capacity pressure on banks carrying shared data

18. University of Utah 18 Proposed Page Coloring Schemes Share4:D+IRp:I+Share4:D Share16:D+I Shared Data + Code Private Page Shared Data Private Code Shared data & code mapped to central 4 banks Shared data to central 4 banks; code replicated Shared data + code distributed to all 16 banks

19. University of Utah 19 Outline Overview of 3D Technology Technique I - Reconfigurable Cache Hierarchy Technique II - Page coloring Technique III - On-chip Interconnection Network Evaluation Conclusions

20. University of Utah 20 Technique III - Interconnection network TREE Links Router Routers saved!

21. On chip tree network Predictable traffic pattern Data moves between shared central banks/private overhead banks and the core Decreased router overhead Saves energy and time University of Utah 21

22. University of Utah 22 Synergy between proposals Page coloringTree networkHybrid 3D cache - No search (S-NUCA) - Radiating traffic pattern - No spills into neighboring banks - Increased bank capacity with low latency

23. University of Utah 23 Outline Overview of 3D Technology Technique I - Reconfigurable Cache Hierarchy Technique II - Page coloring Technique III - On-chip Interconnection Network Evaluation Conclusions

24. University of Utah 24 Methodology Intel ManySim trace-based simulator CACTI cache model for area, power and access latencies HotSpot 4.0 for thermal evaluation 16 cores, 32nm process, 4GHz clock 4KB page granularity 1MB SRAM bank and 8MB DRAM bank SAP, SPECjbb, TPC-C and TPC-E commercial multi-threaded workload traces

25. University of Utah 25 Workload Characterization Working set size of code pages is 0.6% of data pages Average code page access count is 57%

26. University of Utah 26 Page Coloring Evaluation Capacity constraint favors distributing shared pages Code Replication favorable when capacity is available

27. University of Utah 27 Interconnect Evaluation Network power savings up to 48% Most accesses are local due to code replication Most accesses are random

28. University of Utah 28 Hybrid Cache Evaluation Cores SRAM Cores SRAM Cores SRAM DRAM Base-No-PCBase-2x-No-PCBase-3- level L2 L3 L2 Cores SRAM DRAM Proposed Chip L2 Re-configurable Cache (with code replication) performs 55% better than Base-1 ~ 5% IPC drop, to get power savings

29. University of Utah 29 SRAM-DRAM Hits without Reconfiguration Most accesses are to SRAM ways except in shared banks (5,6,9,10)

30. SRAM-DRAM Hits with Reconfiguration University of Utah 30

31. University of Utah 31 Reconfiguration Policy Shared Banks have DRAM always enabled SPECJbb – DRAM always enabled – majority pages are private

32. University of Utah 32 Related Work Reconfigurable Caches in 2D –Ranganathan et al. (ISCA 00), Balasubramonian et al. (MICRO 00), Zhang et al. (ISCA 03) 3D Cache hierarchy –Lie et al. (IEEE D&T 05), Loi et al. (DAC 06), Kgil et al. (ASPLOS 06), Loh (ISCA 08) Page coloring for NUCA – Cho et al. (MICRO 06), Awasthi et al. (HPCA09), Chaudhuri (HPCA 09) 3D NUCA interconnect –Li et al. (ISCA 06) Our is the first paper to propose SRAM/DRAM, targeted tree network, and combining all these into a 3D hierarcy

33. University of Utah 33 Key Contributions A synergistic cache design Communication- and capacity-optimized 3D cache –Reconfigurable cache to improve performance while reducing power –OS-based page coloring for reduced communication –Tailor-made on-chip network for quicker communication Significant increase in efficiency –Performance improvement of up to 62% –Network power savings of up to 48% Typical thermal effect +7 Celsius

34. University of Utah 34 Thank you.. Questions?