Download Lecture 10: Reducing Power through Multicore

ELEC 7770
Advanced VLSI Design
Spring 2007
Reducing Power through Multicore Parallelism
Vishwani D. Agrawal
James J. Danaher Professor
ECE Department, Auburn University
Auburn, AL 36849
[email protected]
http://www.eng.auburn.edu/~vagrawal/COURSE/E7770_Spr07
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ELEC 7770: Advanced VLSI Design (Agrawal)
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Power Dissipation in CMOS
Logic (0.25µ)
Ptotal (0→1) = CL VDD2 + tscVDD Ipeak + VDDIleakage
VDD
VDD
CL
%75
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%20
%5
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Low-Power Datapath Architecture
 Lower supply voltage



 This slows down circuit speed
 Use parallel computing to gain the speed back
Works well when threshold voltage is also lowered.
About 60% reduction in power obtainable.
Reference: A. P. Chandrakasan and R. W. Brodersen,
Low Power Digital CMOS Design, Boston: Kluwer
Academic Publishers (Now Springer), 1995.
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Combinational
logic
Register
Input
Register
A Reference Datapath
Output
Cref
CK
Supply voltage
Total capacitance switched per cycle
Clock frequency
Power consumption:
Pref
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ELEC 7770: Advanced VLSI Design (Agrawal)
= Vref
= Cref
=f
= CrefVref2f
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Comb.
Logic
Copy 2
Multiphase
Clock gen.
and mux
control
f/N
Register
f/N
N = Deg. of
parallelism
Register
Comb.
Logic
Copy 1
Supply voltage:
VN ≤ V1 = Vref
N to 1 multiplexer
Input
Register
Each copy processes
every Nth input,
operates at
f/N
reduced voltage
Register
A Parallel Architecture
Output
f
Comb.
Logic
Copy N
CK
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Level Converter: L to H
VDDH
Transistors
with thicker
oxide and
longer channels
Vout_H
Vin_L
VDDL
N. H. E. Weste and D. Harris, CMOS VLSI Design, Third
Edition, Section 12.4.3, Addison-Wesley, 2005.
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Level Converter: H to L
VDDL
Vin_H
Transistors
with thicker
oxide and
longer channels
Vout_L
N. H. E. Weste and D. Harris, CMOS VLSI Design, Third
Edition, Section 12.4.3, Addison-Wesley, 2005.
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Control Signals, N = 4
CK
Phase 1
Phase 2
Phase 3
Phase 4
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Power
PN
=
Pproc + Poverhead
Pproc
=
N(Cinreg+ Ccomb)VN2f/N + CoutregVN2f
=
(Cinreg+ Ccomb+Coutreg)VN2f
=
CrefVN2f
CoverheadVN2f
PN
[1 + δ(N – 1)]CrefVN2f
=
PN
──
P1
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≈ δCref(N – 1)VN2f
Poverhead =
=
VN2
[1 + δ(N – 1)] ───
Vref2
ELEC 7770: Advanced VLSI Design (Agrawal)
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Voltage vs. Speed
Delay of a gate, T
≈
CLVref
────
I
=
CLVref
──────────
k(W/L)(Vref – Vt)2
Normalized
gate delay, T
where I is saturation current
k is a technology parameter
W/L is width to length ratio of transistor
Vt is threshold voltage
4.0
1.2μ CMOS Voltage reduction
slows down as we
N=3
3.0
get closer to Vt
N=2
2.0
N=1
1.0
0.0
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Vt
V V2=2.9V Vref =5V
ELEC 7770: Advanced VLSI Design (Agrawal)
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Supply voltage
10
Increasing Multiprocessing
1.0
1.2μ CMOS, Vref = 5V
0.8
Vt=0.8V
0.6
PN/P1
Vt=0.4V
0.4
0.2
Vt=0V (extreme case)
0.0
1
2
3
4
5
6
7
8
9
10
11
12
N
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Extreme Cases: Vt = 0
Delay, T α 1/ Vref
For N processing elements, delay = NT → VN = Vref/N
PN
──
P1
=
[1+ δ (N – 1)]
1
──
N2
→
1/N
For negligible overhead, δ→0
PN
──
P1
≈
1
──
N2
For Vt > 0, power reduction is less and there will be an
optimum value of N.
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Example: Multiplier Core
 Specification:
 200MHz Clock
 15W dissipation @ 5V
 Low voltage operation, VDD ≥ 1.5 volts
Relative clock rate
 Problem:
=
(VDD – 0.5)2
───────
20.25
 Integrate multiplier core on a SOC
 Power budget for multiplier ~ 5W
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Multiphase
Clock gen.
and mux
control
40MHz
Reg
40MHz
Output
Reg
Multiplier
Core 2
5 to 1 mux
Input
Reg
40MHz
Multiplier
Core 1
Reg
A Multicore Design
200MHz
Multiplier
Core 5
200MHz
CK
Core clock frequency = 200/N, N should divide 200.
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How Many Cores?
 For N cores:
 clock frequency = 200/N MHz
 Supply voltage, VDDN= 0.5 + (20.25/N)1/2 Volts
 Assuming 10% overhead per core,
VDDN 2
Power dissipation =15 [1 + 0.1(N – 1)] (───) watts
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Design Tradeoffs
Number of cores
N
Clock (MHz)
Core supply VDDN
(Volts)
Total Power
(Watts)
1
200
5.00
15.0
2
100
3.68
8.94
4
50
2.75
5.90
5
40
2.51
5.29
8
25
2.10
4.50
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Power Reduction in Processors
 Just about everything is used.
 Hardware methods:


 Voltage reduction for dynamic power
 Dual-threshold devices for leakage reduction
 Clock gating, frequency reduction
 Sleep mode
Architecture:
 Instruction set
 hardware organization
Software methods
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Parallel Architecture
Processor
Input
Output
Processor
Input
Output
f/2
f
Processor
Capacitance = C
Voltage = V
Frequency = f
Power = CV2f
f/2
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ELEC 7770: Advanced VLSI Design (Agrawal)
f
Capacitance = 2.2C
Voltage = 0.6V
Frequency = 0.5f
Power = 0.396CV2f
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Output Input
½
Proc.
Register
Processor
Register
Input
Register
Pipeline Architecture
½
Proc.
Output
f
f
Capacitance = C
Voltage = V
Frequency = f
Power = CV2f
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Capacitance = 1.2C
Voltage = 0.6V
Frequency = f
Power = 0.432CV2f
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Approximate Trend
n-parallel proc.
n-stage pipeline proc.
Capacitance
nC
C
Voltage
V/n
V/n
Frequency
f/n
f
Power
CV2f/n2
CV2f/n2
Chip area
n times
10-20% increase
G. K. Yeap, Practical Low Power Digital VLSI Design, Boston: Kluwer
Academic Publishers, 1998.
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Performance based on
SPECint2000 and SPECfp2000 benchmarks
Multicore Processors
Spring 07, Feb 20
Computer, May 2005, p. 12
Multicore
Single core
2000
2004
ELEC 7770: Advanced VLSI Design (Agrawal)
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Multicore Processors
 D. Geer, “Chip Makers Turn to Multicore Processors,”


Computer, vol. 38, no. 5, pp. 11-13, May 2005.
A. Jerraya, H. Tenhunen and W. Wolf, “Multiprocessor
Systems-on-Chips,” Computer, vol. 5, no. 7, pp. 36-40,
July 2005; this special issue contains three more
articles on multicore processors.
S. K. Moore, “Winner Multimedia Monster – Cell’s Nine
Processors Make It a Supercomputer on a Chip,” IEEE
Spectrum, vol. 43. no. 1, pp. 20-23, January 2006.
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Cell - Cell Broadband Engine
Architecture
© IEEE Spectrum, January 2006
Nine-processor chip:
192 Gflops
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L to R
Atsushi Kameyama, Toshiba
James Kahle, IBM
Masakazu Suzoki, Sony
ELEC 7770: Advanced VLSI Design (Agrawal)
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Cell’s Nine-Processor Chip
© IEEE Spectrum, January 2006
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Eight Identical
Processors
f = 5.6GHz (max)
44.8 Gflops
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