Rakesh Kumar Keith Farkas Norman P Jouppi,Partha Ranganathan,Dean M.Tullsen University of California, San Diego MICRO 2003 Speaker ： Chun-Chung Chen Single-ISA Heterogeneous Multi-Core Architectures: The Potential for Processor Power Reduction

Outline Introduction Motivation Architecture Experiment Results Conclusion 2

Motivation By 2015 processors will consume 300W Existing CMP designs use only homogeneous cores Applications with high ILP can be exploited on wider cores but applications with low ILP use less power on narrower cores with little loss in performance No need to design cores from scratch because existing Alpha cores run on practically the same ISA

General Idea Single-ISA heterogeneous multi-core architecture –A mechanism to reduce power dissipation System software dynamically choose the most power efficient processor under some performance constraints  Power efficiency

Modeling of CPU Cores EV4: Alpha EV5: Alpha EV6: Alpha EV8-: single-threaded version of Alpha Assumption Only one application runs at a time on only one core Unused cores are completely powered down (therefore no leakage)

Cores, cont. Assuming all cores are implemented in 0.10 micron technology We assume the four cores have private L1 data and instruction caches and share a common L2 cache, phase- lock loop circuitry, and pins. All cores run at 2.1GHz. ISA differences solved by. Either programs are compiled to the least common denominator (the EV4), or we use software traps for the older cores.

Modeling of Power 7

Core Switching Switching done at the operating system level OS switch involves cache flush and saving and loading user states for the cores Estimate that a core can be powered up in ~1000 cycles at 2.1 GHz Switching overhead turns out to be negligible (~1%)

Variation in Power & Performance Benchmark, applu Relative performance of the cores varies between phases. 9

Switching Algorithms: Oracle based dynamic switching using energy heuristic With oracle knowledge of power requirements and performance potential, chose the core that would have the lowest energy consumption, as long as it performs within 10% of EV8- applu

Switching Algorithms: Oracle based dynamic switching using energy-delay heuristic Oracle chooses the core that has the lowest energy–delay product. Choose the core that would maximize IPS 2 /Watt, as long as it performs within 50% of EV8- applu

Switching Algorithms: Realistic Dynamic Switching Every 100 time intervals, one or more cores are sampled for five intervals each. Neighbor –One of the neighboring cores is chosen at random to be sampled Neighbor-global –Similar to neighbor, except selecting the accumulated energy- delay product. Random –A core is chosen at random to be sampled All –All cores are sampled

Realistic Dynamic Switching Results Results shown normalized to EV8- performance Performance degradation of realistic schemes is less than in oracle-based schemes Realistic schemes resulted in more core switching

Conclusion Realistic dynamic switching algorithms show a decrease in energy and energy-delay with only a small decrease in performance. Single ISA heterogeneous multi-core processors using existing technology may be a way to curb power usage.