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Generating Supply Voltage Islands In Corebased SystemonChip Designs

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Voltage islands are regions where nearby IP blocks use a supply voltage ... Analyzing the most effective way of detecting possible voltage islands ... – PowerPoint PPT presentation

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Title: Generating Supply Voltage Islands In Corebased SystemonChip Designs


1
Generating Supply Voltage Islands In Core-based
System-on-Chip Designs
  • Final Presentation
  • Steven Beigelmacher
  • Gall Gotfried
  • www.ece.cmu.edu/ggall
  • 04/26/2005

2
Overview
  • Review
  • What are we doing?
  • How are we doing it?
  • How We Did It
  • Methodology
  • Experimental setup
  • Results
  • Future directions

3
Review
  • Voltage islands are regions where nearby IP
    blocks use a supply voltage different from the
    full-chip supply
  • We propose reducing energy consumed in a
    core-based SoC design by generating these voltage
    islands
  • A coarse-grained placement problem

4
Review
  • Different classes of placement algorithms exist
  • Integer Linear Programming, Recursive, Iterative
  • We went with an iterative solver
  • Want to avoid greedy algorithms
  • Simulated annealing locally bad choices can be
    globally good
  • Parquet S. Adya, H. Chan, I. Markov
  • Pronunciation pär-'kA
  • to make of parquetry
  • Parquetry -- work in the form of usually
    geometrically patterned wood laid or inlaid
    especially for floors
  • http//vlsicad.eecs.umich.edu/BK/parquet

5
Review
  • Simulated annealing loops are made up of two
    phases
  • What are the set of perturbations I can make to
    the current solution? (move function)
  • What is the relative goodness of that change?
    (cost function)
  • Move Function
  • Move block to spot (x,y), swap two blocks, block
    rotation, block scaling, etc

6
Review
  • Cost Function
  • Calculate a number quantifying the goodness of
    the solution
  • How good a solution is will depend on the area,
    aspect ration, wire length, etc
  • What are we doing?
  • Analyzing the most effective way of detecting
    possible voltage islands
  • Modifying the cost function to take into account
    these voltage islands
  • Quantifying energy savings

7
Sequence Pairs
  • Used to speed up and simplify move functions
  • Simple graphical approximation
  • Rectangular coordinate system
  • Approximates relative location of blocks in graph
  • Blocks represented by sequential numbering in X
    and Y directions

8
Simulated Annealing (Parquet)
  • The initial locations of blocks in S.P.
  • Arbitrary
  • Only used to simplify the simulated annealing
    move function
  • Upon completion of a random move
  • Sequence pairs
  • Get matched up with (x,y) coordinates
  • Checked and adjusted to not overlap and meet
    aspect ratio requirements

9
Sequence Pair Example
  • X lt 3 2 5 4 1 gt
  • Y lt 1 2 3 4 5 gt

10
Selecting a Move
Randomly picking which move to make
Making the move
11
Checking the Cost
  • Parquet uses different linear cost functions
    depending on emphasis of aspect ratio, wire
    length, or area

AR and minWL
AR
minWL
None
Calculation of these delta components changes as
t ? 0
12
Cost Function Measures
  • How do we assess the goodness of current
    solution with respect voltage islands?
  • Number of islands
  • Number of nodes in islands
  • Size of the largest island
  • Reduction in power
  • The above are all important to our modified cost
    function

13
Tread Lightly
  • Need solutions that increase the quantity of any
    of the previous parameters to have reduced cost
  • but without breaking up the entire placer
  • Voltage islands with blocks laid on top of each
    other dont do us much good
  • Make voltage island friendly solutions good,
    without making them too good

14
Calculating the Modified Cost
  • Calculate the cost (delta) as before
  • If a perturbation improves one of the voltage
    island parameters, scale delta
  • Scaling is cumulative
  • Scaling is weighted towards certain measures
  • Stop doing this as t?0
  • Why do we do this?

15
Accepting a Move
Accept good moves
Else, accept bad moves with some probability that
decreases as t ? 0
Once t equals 0, run a few more greedy iterations
16
What Did Our Changes Do?
  • A delta lt 0 is always accepted
  • A voltage island friendly solution that was
    already good will still be accepted
  • A delta gt 0 is accepted with some probability
    (declines over time)
  • Scaling increases the probability of being
    accepted, without forcing it
  • Only effective on solutions that werent that bad
    to begin with

17
Methodology
  • Benchmarks are tested with set vdds and checked
    for power saved after placement
  • We define power saved as
  • (Num-nodes in an island)(voltage of the island)
  • Quantified by the amount of power saved by
    reduction in voltage converting FIFOs
  • Less islands means more voltage conversion hence
    more power consumed

18
Methodology Purpose
  • Our experiments were designed to show
  • Function of 4 cost functions
  • Voltage islands
  • Nodes in islands
  • Power saved!!! (emphasis here)

19
Experimental Setup
  • Before applying any cost functions we calculate
    the total initial power saved by the pre-placed
    islands
  • These are islands created by luck in the
    initialization phase
  • The total initial power saved is used
  • Compare and determine worth of solution
  • Initial power saved Current power saved -gt more
    negative numbers mean better solution
  • Determine the percent saved in final solution

20
Benchmarks used
21
Results AMI
  • AMI33 has room to increase nodes in islands to
    save power
  • AMI49 has even vdd node distribution
  • Cost function becomes more sensitive
  • increasing distinct voltage islands to save power

22
Results HP
  • HP6 has majority of blocks with high vdd
  • Less room for optimization
  • HP11 shows an increase in nodes in islands used
    to save power
  • Attributed to the fact more nodes have higher vdd

23
Future Work
  • We have created an open source vdd/vt island
    creation tool
  • Move function must be modified to account for
    multiple voltages in selecting moves
  • Cost functions can be easily extended
  • To create various placements based on multiple
    VDD islands
  • To create placements based on multiple Vt islands

24
You Got Questions
  • We Got Answers
  • Thank You
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