Lect13 notes - Efficient Multicore Global Power Management

Problem being solved

  • Hard limit to temperature distribution
    • Has long time constants (ms range)
  • Hard limit to power distribution
    • Shorter time constant
    • Can cause voltage drops when stressed


  1. Maximize performance for a given power budget
    • Never exceed power budget, also save energy
  2. Minimize power without impacting performance much
  3. How to use (HW only) dynamic global control


Users

  • GPP
  • Multicore (high core count)
  • Multiprogramming only - no improvement/degradation for any specific application
    • Ignore multithreaded apps because of synchronization


What is unique

  • Dynamic, per core control
  • Hardware controller
  • Hierarchical control
    • Local for emergency situations
    • Global for optimizing impact
  • Straightforward engineering/unique evaluation
    • New instruments
  • Experimental infrastructure
    • Multicore simulator with power and fast evaluation mode


Evaluation


What they did

  • Multiprogrammed with different combinations of SPEC2000 applications
    • Chose specific applications to exhibit desired behaviour
  • Power/performance tradeoffs for different schemes
    • Assume hard limit of x% peak power
  • Compare MaxBIPS to 3-mode global DVFS and optimistic static VFS
  • Use an oracle for per-core VFS
  • Changed the number of cores
  • Adapt to budget change


Critiques

  • Didn’t consider cost of per-core DVFS
  • Power overshoots?
    • How often? How high?
  • Should change the oracle explore times and other oracle aspects
    • Oracle may not be the upper bound
    • Oracle needs more information than test subject to really be an oracle
  • Doesn’t match goal of demonstrating maximum power without beating power budget
    • Don’t consider power budget/overshoots
  • Bad to use the same model for heuristic and evaluation (self-fulfilling prophecy)
    • Not a huge flaw, but unconvincing


Other notes


Why power and temperature?

  • Directly related
  • Controlling both is important - can lead to problems
    • Thermal runaway
    • Chip lifetime decreases
  • Lose cooling capacity over time


Linear DVFS model

  • Assumes 10% voltage change translates to 10% frequency change
    • Very optimistic
  • More accurate model likely 10% V → 30–40% freq
  • Not unique about this paper


DVFS alternatives

  • Fetch gating
    • Can’t be better than linear
    • Small gain without voltage change