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SatyaKiran. Power Estimation of Digital Systems. 22 September 2003. Introduction. Why power of nano-electronics became so important? Because of Moore’s law still holds true through complex applications Mobile systems – battery “bottleneck” High performance computation – heat extraction

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introduction
Introduction
  • Why power of nano-electronics became so important?
    • Because of Moore’s law still holds true through complex applications
    • Mobile systems – battery “bottleneck”
    • High performance computation – heat extraction
    • Operating cost and reliability
      • Data warehouse of ISP with 8000 servers needs 2 MW
  • Power or Energy? Aren’t they go hand-in-hand?
    • Power varies significantly with time!
    • A given battery has fixed amount of energy
    • Average power consumption = Energy/Execution-time
      • Decides average chip and junction temperature
      • Decides battery life (if peak current < rated current)
    • Peak power and current
      • Voltage drops, hot spots, rate of battery discharge
  • Power-efficient, Energy-efficient, Battery-efficient design paradigms do exist!
components of power consumption
Components of Power Consumption
  • System = hardware platform + software (sys. & app.)
    • Software impacts hardware power consumption
  • Static power
    • Sub-threshold leakage & reverse biased junction leakage
    • Quiescent biasing power (in case of non-CMOS circuits)
  • Dynamic power
    • Charging and discharging of capacitance (switching activity)
    • Short circuit power during transition (rate of change, delay)
    • Alternative grouping (used at component/cell level)
      • Switching power at the boundaries of cells
      • Internal cell power
        • Short circuit power
        • Switching power at internal nodes
system abstractions power
System Abstractions - Power

Functional Specifications and Constraints

System Level Netlist

Register Transfer Level (RTL) Netlist

Component/Cell Level Netlist

Layout or Configuration-bits

Chip

Time complexity

Accuracy of power characterization

Opportunities for optimization

power characterization
Power Characterization
  • Measurement (Chip/Board Level)
    • Most accurate
    • Perhaps the fastest, if setup and tools exist
    • Too late to change hardware details
    • Software/Load control is still possible
    • Typically used for software optimizations
  • Transistor Level (estimation)
    • Spice simulation of transistor level netlist
    • Most accurate in the simulation world
    • Requires complete implementation details
    • Unmanageable time complexity even for simpler designs
    • Typically used for cell/component characterization
    • Synopsys PowerMill (said to provide spice-like accuracy)
power characterization cont
Power Characterization (cont…)
  • Cell Level (estimation)
    • After logic synthesis
    • Requires RTL implementation
    • Simulation to capture switching activity
      • Requires delay simulation if glitches need to be accounted
    • Characterized cells – empirical formulas or table look-up
    • Interconnect power
      • Either unaccounted or
      • Using estimated wire load models (typically based on experience) or
      • Extracted layout (if done after physical synthesis)
    • Still unmanageable time complexity especially to use in design space exploration
    • Synopsys PrimePower
      • Netlist, interconnect capacitance, VCD traces, cell power library
power characterization cont1
Power Characterization (cont…)
  • Register Transfer Level (estimation)
    • Requires conceptual RTL description (detailed micro-architecture)
    • Data-path is modeled as netlist of macro cells, which are characterized offline
    • Control path and glue logic
      • Either unaccounted or estimated based on I/O
    • Simulation to capture switching activity
      • Typically glitches are not considered but methods do exist
    • Interconnect power
      • Typically unaccounted but possible to estimate through floor-planning
    • Typically used in DSE mostly using in-house tools
    • Synopsys PrimeCompiler – but too naive
      • Synthesizable RTL, switching activity, power library
system level power estimation
System Level Power Estimation
  • For Design Space Exploration
  • Least accurate but uncertainty of exploration results can be reduced if models have good fidelity
  • Purpose, target architecture and available system details govern the system-level estimation models
    • Selecting algorithm or designing hardware for given algorithm?
    • ASIC based or processor based?
    • Is ISA fixed or extensible?
  • Typically system-level power estimation models are macro-architecture template specific
  • Major constituents of power consumption
    • Computation, communication, storage units & peripherals
power estimation models
Power Estimation Models
  • Instruction Level Power Estimation
    • First introduced to characterize processor power consumption to drive software optimizations
    • Each instruction is associated with some current
    • Inter instruction effects for better accuracy
  • Later experiments on StrongARM by Amit Sinha & APC
    • Current/instruction ~ 0.2A (averaged over all instructions)
    • Min-max variation of 38% of average current
    • Address mode and data dependent variation is smaller
    • But, max current variation across benchmarks is < 8% !
    • Concluded that first order energy model of a given processor is, E = V I(V, f) T
    • Second order effects can be significant for data-path dominated processors such as DSP, VLIW
power estimation models cont
Power Estimation Models (cont…)
  • Second order effects are best characterized by events which can be obtained from simulation
    • I = ( W1 C1 + W2 C2 + … ) I0(V, f)
      • C denotes event frequency
      • W denotes associated weight (obtained by characterization)
    • Events can be either fine-grained or coarse grained
      • Events can be classes of instructions or
      • Micro-architecture component/sub-component accesses (WATTCH) or
      • Events such as normal accesses, stalls, …
  • Power State Machines by Luca Benini et. al.
    • States of the FSM represent mode of operation
      • Active, Idle, Power off, …
    • Transitions are labeled with triggering events, transition energy cost and perhaps transition times
    • Abstract specification to generate events can be accompanied
    • Useful to analyze power management schemes
power estimation models cont1
Power Estimation Models (cont…)
  • Integration of both the above models results in a more powerful model
    • States of the FSM represent mode of operation
    • Certain events cause state change and certain don’t
    • Power/energy consumption in each state can be event and parameter dependent (empirical formulas, table look-ups)
      • Read and write access of memory in active mode
      • ADD and MUL instructions of a processor
      • # of transitions of a bus
    • Transitions are labeled with triggering events, transition energy cost and perhaps transition times
    • Abstract specification to generate events can be accompanied
    • Useful for both system-level and RTL power estimation of wide variety of components
    • Trade-off of accuracy Vs characterization effort is possible