1 Single-ISA Heterogeneous Multi-Core Architectures: The Potential for Processor Power Reduction Rakesh Kumar, Keith I. Farkas, Norman P. Jouppi, Parthasarathy Ranganathan, Dean M. Tullsen Presenter: Borys Bradel

2 Introduction Different programs have different requirements (e.g. ILP) Extends to phases of a single program Heterogeneous cores Use core that matches the requirements Reuse existing cores Use multiple generations of the same family of processors

3 Outline Methodology Hardware Assumptions Power Experiments Optimal – energy/energy delay product Heuristic based – static/dynamic Related Work Conclusion

4 Single ISA Multi-Core Benefits Small area overhead because of the growth in core sizes between generations Clock frequencies of older cores would scale with technology P3 1 GHz = P4 1.4 GHz Increased pipeline depth precisely because could not scale

5 Hardware – Alpha Family 2 in order cores EV4=21064 EV5= out of order cores EV6=21264 EV8-=21464 (multi thread support removed)

6 Hardware Size 15% more area than just using 21464

7 Assumptions Can switch cores dynamically Private L1 cache and common L2 cache All cores use 0.10 micron technology Single process executing on a single core at any one time 2.1 GHz clock (= micron 600 MHz) Input voltage 1.2V Cores shut down when idle 1000 cycle restart cost (staged, phase lock loop left alone) 150 ms memory access Stall cycles through CACTI

8 Core Configurations

9 Power Model Use Wattch to account for activity based dissipation Use scaling and offset factors to account for other factors This hybrid model is closer to manufacturer’s data points Peak power: data sheets less L2 cache and output pins Typical power: scaled based on Intel chips

10 Power and Area Statistics

11 Performance Modeling Use SMTSIM, a cycle accurate simulator simpoint is used to identify representative instructions of programs and how many instructions need to be fast forwarded

12 Varying Performance Ratio

13 Varying Energy Efficiency Ratio

14 Oracle Switching for Energy Performance always within 10% of EV8-

15 Oracle Switching for Energy

16 Oracle Switching for Energy Delay Product Performance always within 50% of EV8-

17 Oracle Switching for Energy Delay Product

18 Others Voltage/frequency scaling – not as good Static core selection only EV6 and EV8- are used Dynamic heuristic Running average performance within 10% Every 100 time intervals (100 million instructions) cores are sampled for 5 intervals Select best core based on sampling

19 Results for Heuristics

20 Results for Heuristics/Static Core

21 Related Work Gating based power optimization Cannot gate at a fine enough granularity May still have leakage This could be thought of as gating to reduce capabilities of different units Voltage and frequency scaling Chip wide – one size does not fit all Fine grained – granularity problems

22 Conclusions Heterogeneous multi core architectures reduce the energy-delay product More fine grained than other approaches Using several cores from the same family is good Reduces development/testing costs Is it scalable? Just use EV6??