Power Optimization
Chapter 9
Contributed by Alex Turek
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Outline
System Design Considerations
Power Optimization (hardware)
Power Optimization (software)
Constant vs. Dynamic Power in ES
Advanced Configuration and Power Interface (ACPI)
Constructing software for maximum power performance
Summary
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Computer System Design
Considerations
Cost
Performance
Power
Size
Every decision is a trade off
Generally, lowering power consumption has become more important
than increasing performance
Initially developed for mobile platforms
Power optimization is important for all platform types, from mobile
devices and embedded systems to multi-million dollar servers
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Power Basics
Power can be optimized on both the hardware and software
levels
Current computers consist of many MOSFETs (Metal Oxide
Semiconductor field Effect Transistor)
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Active Power Consumption
P = CV2f
C = capacitive load
V = voltage
f = Frequency
Voltage affects active power more so than the frequency or
capacitive load of a circuit due to its exponential relation
Voltage and frequency are first order determinants of
performance
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Power Consumption in CMOS
Charging and discharging capacitors
Voltage and frequency are first order determinants of performance
Short circuit currents:
Short circuit between supply rails during switching
Leakage (static power consumption)
Leaking diodes and transistors
Minimizing leakage is extremely important for
low-power systems, which are not active most
of the time
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Intel 3D Transistor
Intel's 3-D Tri-Gate transistors enable chips to operate at lower voltage with lower
leakage
The 22nm 3-D Tri-Gate transistors provide up to 37 percent performance increase at low
voltage versus Intel's 32nm planar transistors
the new transistors consume less than half the power when at the same performance as
2-D planar transistors on 32nm chips.
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Intel 3D Transistor
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Optimizing Power Using Software
Power Profiles of Embedded Systems:
Always on/continuous power
for devices that are required to provide near-peak performance,
nearly all the time (displays, controllers, networking devices, etc)
Advanced Power Management such as Intel
SpeedStep™
Policy and usage based frequency and voltage control
Initially designed for laptops: plugged in vs battery modes
Newer Intel CPUs such as the Core i7 series handle usage power
management, where as the OS is used for policy based power
management
Intel Core i7-3960X 3.3 GHz (3.9 Ghz Turbo) 6 core 12 thread
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Reducing Power Consumption
Mobile computers/devices created the need for CPUs with better
power consumption
Now, CPUs are using less of the overall system power budget,
manufacturers are looking for ways to improve the power
consumption of other components
The First Step to Optimizing System Power
Maximize the range of power consumption between lowpower states and high power states
If the max/min states have negligible differences in power
consumption, then power management will have no impact
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Constant vs. Dynamic Power
Constant Power: an embedded computer consumes a constant,
minimum amount of power when powered on, regardless of the level
of system activity
Other than by powering a system down completely or transitioning to
a sleep state, software cannot influence a system’s constant power
Dynamic Power: power consumption based on load
If most of the components in the system support low-power modes,
then dynamic power consumption can be significantly controlled
through the careful use of resources
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Idle – Peak Ratio
Ratio of Idle to Peak power consumption:
estimate: 6/9 = 0.66;observed: 9/11 = 0.81
~75% of total system power consumption
is completely independent of softwarelevel activity on the platform
In comparison:
FIGURE 9.1 Advertised and Measured Dynamic Power
Range for CompuLab’s fit-PC2. The Advertised Power
Range Covers 66% of Peak, whereas the Measured
Value, When Accounting for Active I/O Devices, Covers
81% of the Observed Peak.
Intel Core i7-3960X 3.3 GHz (3.9 Ghz
Turbo) 6 core 12 thread
Idle Power: 62 W;Peak Power: 210 W
62/210 = 0.295
At 0% utilization, 29.5% of peak power
consumed
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FIGURE Top: Power Consumption and
Efficiency for a System With a
Dynamic Power Range Down to 50%
of Peak Performance. Power is
Normalized to Peak Consumption.
Efficiency is Calculated by Dividing
Utilization by Normalized Power.
FIGURE Bottom: Power
Consumption and Efficiency for a
System With a Dynamic Power
Range Down to 10% of Peak
Performance.
Goal of Power Efficiency: high efficiency over entire operating range
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ACPI
ACPI: (Advanced Configuration and Power Interface): platform
specific industry standard that defines the roles, responsibilities, and
specific mechanisms provided to achieve reliable power management
and system configuration
Developed through the cooperation of HP, Intel, Microsoft, Phoenix
Technologies, and Toshiba
Previous interfaces replaced by ACPI such as plug-and-play BIOS and
Advanced Power Management (APM) were platform and operating
system specific
If most of the components in the system support low-power modes,
then dynamic power consumption can be significantly controlled
through the careful use of resources
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ACPI continued…
The operating system is granted exclusive control over all systemlevel management tasks, including the boot sequence, device
configuration, power management, and external event handling (such
as thermal monitoring or power button presses)
ACPI Registers: well defined locations that can be rea
d and
written to monitor and change the status of hardware
resources
ACPI BIOS: This firmware manages system boot and
transitions between sleep and active states.
ACPI Tables: describe the interfaces to the underlying
hardware and represent the system description.
Written in a domain specific language, ACPI Machine
Language (AML). The operating system's ACPI driver in
cludes an interpreter for AML.
These tables are provided by the ACPI BIOS
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Idle vs. Sleep
Two dominant forms of user-facing power management
Sleep States: system appears to be powered off completely, while in
fact some software may periodically execute to service external events,
such as network packet arrivals.
First introduced with laptop use in mind
Idle Modes: take advantage of the variation in system utilization when
the system is powered up
Transitions between idle modes are made in milliseconds to reduce
active power consumption, without the involvement or awareness of the
user
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ACPI System States
Global System States (Gx States)
Describe the entire system
Transitions between these states are typically
observed by the user
G0: Working
G1: Sleeping (suspend or “instant boot” mode)
G2 (S5): Soft Off
G3: Mechanical Off
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ACPI System States Continued
Sleep States (Sx States)
Sleep states within the global sleep state G1
S1: All system context is maintained by hardware upon entry and thus provides
the lowest waking latency
S2: Operating system is responsible for storing and restoring CPU and cache
hierarchy context. Upon entry to S2, CPU and cache state are lost
S3: Powers down more internal units than S2, but otherwise is similar
S4: Write all system states, including main memory, to nonvolatile storage
S5: Does not store system context.
Enables system to be powered on electronically (Wake on LAN)
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ACPI System States Continued
Device Power States (Dx States)
Device class-specific approach to power management that allows devices of
the same type to be treated in the same way
Device Classes: Audio, COM Port, Display, Input, Modem, Network, PC Card Controller, Storage
D0 (Fully On): fully powered up, fully operational state. Highest Power Consumption
D1: Initial sleep state for a device. Capable of waking itself or the entire system in response to
an external event
D2: Incremental state beyond D1. Operates at a lower power and requires a greater waking
latency than D1
D3hot: Saves a device-specific state so it can be awakened without a complete reboot
D3 (Off, or d3cold): Complete power down of the device. Device context not restorable
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ACPI System States Continued
Processor Power States (Cx States)
ACPI allows processors to sleep while in the G0 working state. The Cx States
only apply when the global state is G0.
C0: The processor executes instructions and operates normally
C1: Lowest transition latency CPU sleep state. Latency is so low that it is negligible and thus
not a deciding factor in the operating system’s decision to transition to this state.
C2: Lower-power sleep state than C1. Operating system does consider the transition latency
when deciding whether to transition to this state or another.
Transitions to both C1 and C2 are not apparent to software and do not alter system operation.
C3: Offers great power reduction at the cost of an increased transition latency. Processor
caches maintain their state but do not emit cache coherence traffic
C4: Optional additional power states included in ACPI revision 2.0. Even lower power
consumption and greater transition latency. Vendor specific.
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ACPI System States Continued
Processor Performance States (Px States)
Within processor power state C0, ACPI defines a range of performance states
that are intended to enable a fully working system to vary its power
consumption and performance by operating at different voltage and frequency
levels. The operating system explicitly controls transitions between these
states. For multiprocessor systems, each processor must support the same
number and type of processor performance state.
P0: Maximum performance, maximum power consumption state.
P1: Next highest performing processor performance state and is expected to have
second-greatest power consumption
Pn: ACPI allows for a maximum of 16 distinct performance states.
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More on Intel SpeedStep
Technology
Features performance and power
management through voltage and
frequency control, thermal monitoring, and
thermal management features
Provides a central software control
mechanism through which the processor
can manage different operating points
In a FSB-based system, processors drive
seven output voltage identification (VID)
pins to facilitate automatic selection of
processor voltages VCC from the
motherboard voltage regulator
Each core in a multi-core has its own
machine state register (MSR) to control
the VID value. However, each core must
work at the same voltage and frequency.
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Software Construction for
Maximum Power Performance
Linux PowerTOP Tool
Race to Sleep
Modern systems transition between power
states in response to load
Open-source power profiling tool for Linux
that reports the occupation frequency of
processor sleep and performance states
Software should be organized to complete
all available useful work in a continuous
batch and then transition to an idle state
and avoid unnecessary interruptions
Traditional software development tools
emphasize code execution frequency.
Modern software dev tools must also take
into account how often a program
interferes with transitions to low-power
CPU states
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Evaluating Software and Systems with
PowerTOP
PowerTOP can be used to measure and improve performance of code as it is
written
The measurement harness is implemented in Python. To begin, the developer
identifies the target code for measurement, referred to as the code-under-test
(CUT)
Basic Operation of the Measurement Harness
Phase 1. Discover the maximum rate of execution of the CUT
Phase 2. Measure the power efficiency at the maximum execution rate
Phase 3. Measure the power efficiency as execution rate is scaled down from
maximum
Two key parameters manipulated in Phase 3:
- total fraction of “sleep” time during the experimental observational period
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Concluding Remarks
System Design Considerations – “Everything’s a tradeoff”
Active Power Consumption – P = CV2f
Idle vs. Peak Ratio – decreasing constant power consumption is most favorable
Advanced Configuration and Power Interface
(ACPI) – Standard developed by the industry to provide universal power
management
Software techniques for measuring and managing power consumption – race to
sleep
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