Download On-Line Adjustable Buffering for Runtime Power Reduction

On-Line Adjustable
Buffering for Runtime
Power Reduction
Andrew B. KahngΨ
Sherief Reda†
Puneet SharmaΨ
ΨUniversity
of California, San Diego
†Brown University
Outline
Introduction
 Adjustable Buffering Methodology
 Experiments & Results
 Conclusions

Power: First-Class Objective
Power bottleneck to Moore’s law
 Power-frequency tradeoff exists in CMOS circuits


Techniques to exploit
power-frequency tradeoff
are of interest



Allow high freq. operation
Can give significant power
reduction when max.
performance not required
Mainstream approach:
Dynamic voltage and
frequency scaling (DVFS)
Power

Much higher power required to operate at high frequency
Frequency
Power-frequency tradeoff with VDD
scaling
Dynamic VDD & Freq. Scaling

Scale down VDD and freq. when high performance not needed
Limitations of DVFS


VDD cannot be scaled down
indefinitely
Range of VDD scaling is small
and diminishing




Extremely high power at high
VDD  reduce max. VDD
High Vth to reduce leakage,
noise margins, variability, soft
errors  increase min. VDD
Discrete allowed voltages
Power

Frequency
Ideal frequency-power from VDD scaling
Actual frequency-power from DVFS
Our objective: enable additional modes to exploit
frequency-power tradeoff


Useable when VDD cannot be scaled further
Useable without DVFS
Proposal: Adjustable Buffering
Our approach, like DVFS, provides runtime-selectable lowpower modes  supplement or replace DVFS
 Key idea: Lot of logic added for performance, not
functionality  Turn this logic off when high-performance
not needed
 Poor interconnect scaling  large number of repeaters

 20-30%
of cells are repeaters
 Fat repeaters are used to improve delay but consume a lot of power

We modify repeaters to dynamically adjust their driving
capacity
Select
Transform
32X
16X / 32X
Outline
Introduction
 Adjustable Buffering Methodology
 Experiments & Results
 Conclusions

Adjustable Repeater Design
We add PMOS-NMOS pair to turn half the devices off dynamically
Control Gate
Control Gate
Traditional Inverter (INVX8)
Adjustable Inverter
“LPM” = ON  only half devices operational (low-power mode).
“LPM” = OFF  all devices operational (high-performance mode).
What power components are likely to reduce in low-power mode?
• Short-circuit power: during switching, PMOS & NMOS ON momentarily  short
circuit between VDD and VSS
• High when transition time (slew) is large
• Subthreshold leakage: when one of PMOS-NMOS pair between VDD and VSS ON
Adjustable Repeater Requirements

Low area overhead




High performance in high-performance mode


Added PMOS-NMOS pair (LPM devices) takes area
LPM signal to be routed or locally generated
Layout of the new cell must be simple and low area
overhead
On-resistance of LPM devices may reduce
performance
Good power reduction in low-power mode
Area Overhead

Problem: High performance needed when LPM signal OFF
 use large control gates  large area overhead
Control Gate
Control Gate
Delay overhead: increase in delay of
adjustable repeater over traditional repeater
Solution: Share control gates among multiple repeaters
Control Gate Sharing
V’DD
V’SS
LPM devices shared by two inverters
Fewer control gates but virtual VDD (V’DD) and VSS (V’SS) need routing
 How many control gates needed?
 Compute simultaneous switching rate (SSR) by finding the max.
#repeaters that have overlapping timing windows. Time = O(RlogR)
(R = #repeaters)
 Find total width of all repeater devices controlled by CGs (=WR)
 For good performance, width of control gates = 4 x SSR x WR
Typical SSR=~10%  small area overhead
Ensuring High Performance

Problem: Adjustable repeaters ~5% slower
when LPM signal OFF
 Up to ~5% reduction in circuit performance



Solution: do not use adjustable repeaters on
timing-critical paths
Additional constraint: slew constraints not
violated when LPM signal is OFF or ON.
We characterize adjustable repeaters (i.e., find
delay, slew, power, input capacitance) and then
substitute traditional repeaters with adjustable
repeaters subject to delay and slew constraints.
 No loss in circuit performance & no slew violations
Power Reduction in Low-Power Mode
Short-circuit energy and leakage reduce
OFF
OFF
Traditional Inverter
Adjustable Inverter
Reduction in short-circuit energy and leakage for INVX8
Short-Circuit Energy
Leakage
LVT
43%
28%
SVT
35%
26%
HVT
22%
22%
Outline
Introduction
 Adjustable Buffering Methodology
 Experiments & Results
 Conclusions

Experimental Validation



Circuits: s38417 (8,890 cells), AES (15,272),
OpenRisc (46,732)
Tools: Synopsys HSPICE (SPICE), Design
Compiler (synthesis, timing and power analysis);
Cadence SoC Encounter (P&R), SignalStorm
(library characterization); Artisan TSMC 90nm
library models
Other settings: power and timing analysis at
slow corner, VDD of 1.1V and 0.9V, activity factor
of 0.01.
Results: Power Reduction
AES
DVFS
DVFS+LPM
Total Power (mW)
4.5
VDD=0.9
LPM=0
4
3.5
3
VDD=1.1
LPM=0
VDD=0.9
LPM=1
VDD=1.1
LPM=1
2.5
We perform comparative
analysis of:


Circuit with DVFS
Circuit with DVFS + LPM
2
445
438
432 389 354 349
Frequency (MHz)
OpenRisc
343
337
DVFS
DVFS+LPM
12
Total Power (mW)

11


10
9
8
7
192
187
181
173
164
159
Frequency (MHz)
154
149
Both dynamic and leakage
power reduce
6-12% reduction in total
power at low-power mode
Results: Area Overhead

Logic area overhead due to
control gates
 Depends
on SSR
 Smaller if control gates can be
placed in whitespace
6.00%
5.00%
4.00%
3.00%
2.00%
1.00%
0.00%

s38417
(SSR=13.5%)
AES
(SSR=7.95%)
OpenRisc
(SSR=9.77%)
s38417
AES
OpenRisc
Routing overhead
 LPM,
3.46%
LPM routed to control gates 3.44%
routing overhead depends on
locations of control gates
 # control gates small  overhead
small

 V’DD,

V’SS routed to all repeaters
For overhead estimation, nets
assumed to be Steiner trees
3.42%
3.40%
3.38%
3.36%
3.34%
3.32%
Outline
Introduction
 Adjustable Buffering Methodology
 Experiments & Results
 Conclusions

Conclusions

Presented a novel technique that dynamically trades off power and
performance by turning off devices not needed at less than max.
performance




Both leakage and dynamic power reduce; total power reduction is 6-12%
on our testcases
By sharing of control gates, area overhead reduced to <5.57%
No adverse affect on performance of the circuit when LPM signal OFF
Future work:
 Actual layout of adjustable repeaters with routing of V’DD, V’SS,
LPM nets to accurately estimate power, performance, area
impacts
 Customization of more cells especially clock repeaters to further
improve power-performance tradeoff
Thank You

Questions?