Slide 1 U.Va. Department of Computer Science LAVA Architecture-Level Power Modeling N. Kim, T. Austin, T. Mudge, and D. Grunwald. “Challenges for Architectural Level Power Modeling.” In Power Aware Computing, (R. Melhem and R. Graybill eds.), Kluwer, Kevin Skadron Mircea Stan

Slide 2 U.Va. Department of Computer Science LAVA Who Cares?  Power is now a first-level design constraint for both embedded/mobile and high-performance/general-purpose processors  Battery life (eg, laptops)  Heat removal and package cost  Degradation and lifetime Architects don’t have good tools to model this

Slide 3 U.Va. Department of Computer Science LAVA Modeling  What architects normally do: model behavior/performance at the cycle level (eg, SimpleScalar)  Many abstractions and simplifications –Examples: I-cache, memory buses  Faster than a more detailed model; still good enough  Power and heat, however, require more implementation detail  Current power-performance simulators try to omit the extra detail by using abstractions or analytic models

Slide 4 U.Va. Department of Computer Science LAVA Current Arch.-Level Power Simulators  Wattch (Brooks et al.)  Doesn’t model anything outside the core (eg, external bus)  CACTI-based models for large structures (cache, branch predictor, register file, instruction window, etc.) –Tuned using Intel data  SimplePower (Vijaykrishnan et al.)  Adds bus and memory modeling, also I/O pads  Look-up-tables (LUTs)  Tempest (Cai & Lim)  Power density  Chip-level thermal modeling  No one: data sensitivity, clock tree, global interconnect

Slide 5 U.Va. Department of Computer Science LAVA Typical Power-Performance Modeling Technology parameters Micro-arch. config Static power model Cycle- accurate performance model Dynamic power estimation activity factors cycle-by-cycle statistics

Slide 6 U.Va. Department of Computer Science LAVA Power Basics P = ½ACV 2 f +  AVI short + VI leak  A = activity factor  C = capacitance  V = dynamic voltage  f = frequency  I short = short-circuit current during switching  I leak = leakage current

Slide 7 U.Va. Department of Computer Science LAVA Power Basics P = ½ACV 2 f +  AVI short + VI leak P = ACV DD V swing f +  AVI short + VI leak  A = activity factor  C = capacitance  V = dynamic voltage  f = frequency  I short = short-circuit current during switching   = duration of short-circuit current  I leak = leakage current  Why averages don’t work: Bursty behavior, dI/dt, peak current, temperature

Slide 8 U.Va. Department of Computer Science LAVA Better Simulation Method P = ACV DD V swing f +  AVI short + Vi leak sum over all blocks

Slide 9 U.Va. Department of Computer Science LAVA Typical Power-Performance Modeling Technology parameters Micro-arch. config Static power model Cycle- accurate performance model Dynamic power estimation activity factors cycle-by-cycle statistics

Slide 10 U.Va. Department of Computer Science LAVA Metrics  What metrics do we care about?  Execution time [sec or IPC]  Energy (battery life)[W]  Energy-delay product[Ws]  Energy-delay 2 product?[Ws 2 ]  Power density (temperature)[W / mm 2 ]  Temperature[K or °C]  DI/dt  Peak power dissipation  Might also care about these at finer granularities: micro-arch. blocks, decoders, circuits, etc.

Slide 11 U.Va. Department of Computer Science LAVA Basic Techniques for Power Efficiency  Leakage: turn things off (but you lose the data)  Resize structures  Turn off idle structures  Turn off entries, eg cache decay  4T RAM cells?  Dynamic: reduce activity factors  Clock gating  Resize structures  “Utility” predictor  Throttle processor width (eg, fetch width)  Filter caches  Datapath resizing  etc. P = ½ACV 2 f +  AVI short + VI leak

Slide 12 U.Va. Department of Computer Science LAVA Simulating Power for Greater Accuracy P = ½ACV 2 f +  AVI short + VI leak In all these cases, we want to find relevant power-related parameters (esp. effective switching capacitance C) -- performance model provides A  More detailed block-specific power information (circuit design style, etc.)  Switching activity (Hamming distance)  Interconnect (floorplanning, approx. area)  Clock tree (H-tree vs. balanced H-tree)  Random logic (empirical models)  Busses, transactions, durations (pull-up/pull-down/hi-Z, read vs. write, etc.)

Slide 13 U.Va. Department of Computer Science LAVA Simulation Challenges  Need leakage models - f(T)  Need temperature models  Eventually want to integrate all these into a fast simulator  This is a research challenge in its own right