GREEN COMPUTINGPower Consumption Basics

in ICT Products

Maziar Goudarzi

Outline

• Metrics• Energy consumption in ICT products• Some common energy optimization

techniques

Acknowledgements: Some slides/parts from http://www.ida.liu.se/~TDDD50/

Electrical Units

Power Metrics

Performance related energy metrics

• Energy-per-instruction (EPI)– Energy spent to execute an instruction

• Used to compare micro-architectural traits• Sometimes to model software consumption

– Not all the instructions consume the same

• Application energy consumption– Power vs. Time

Comparing CPU energies

• Example: Same program, – AMD CPU, 2GHz, 150W, 10s– Intel CPU, 2.5GHz, 200W, 8sWhich one is better?

• Another (perhaps better) example– Same program– Atom processor, 1.5GHz, 10W, 20s– Core i7 processor, 2GHz, 55W, 5sWhich one is better?

Performance related energy metrics

• Energy delay product (EDP)– Encourages low consumption and fast

runtime– Energy or delay increase → EDP increases

EDP = Watts * runtime2 Energy = Watts * runtimeDelay = runtime

Outline

• Metrics• Energy consumption in ICT products• Some common energy optimization

techniques

Power Consumption Fundamentals

• Most widely used technology today– CMOS (complementary Metal Oxide

Semiconductor) technology– Technology name

• Minimum feature size: 65nm, 45nm, …• Latest technology?

Power Consumption Fundamentals

• Elements of power consumption– Dynamic power

• Dissipated when charging /discharging capacitors

• Inevitable!

– Static power • Leakage• Total waste!• Was negligible until recently• Increased with technology

scaling (<180nm)• 20 to 40% in today processors

• AMD Opteron X2: 300mm wafer, 117 chips, 90nm technology

• Opteron X4: 45nm technology

CMOS Leakage

• Transistor is not a perfect digital switch!– Subthreshold leakage– Gate leakage -> high-k dielectric– Junction leakage

Subthreshold Leakage

• Subthreshold leakage depends on

Outline

• Metrics• Energy consumption in ICT products• Some common energy optimization

techniques– Static power reduction– Dynamic power reduction

Leakage reduction techniques

• Subthreshold leakage depends on

• Architectural techniques to reduce leakage– Stacking effect and gated Vdd– Drowsy effect– Threshold voltage manipulation

Stacking effect and gated Vdd

• Connection of transistors in series source to drain– Reduces the Vds of each

transistor

• Popular stacking technique: Gated Vdd

– Sleep transistor gates the ground (disconnects power supply)

Gated Vdd for SRAM

• Dynamically Resized Instruction Cache• Cache decay

– Disable individual lines– Managed with counters to estimate dead lines

– Disabled lines lose the state– Expensive management

Stefanos Kaxiras, Zhigang Hu, Margaret Martonosi, Cache Decay: Exploiting Generational Behavior to Reduce Cache Leakage Power, ISCA, 2001.

Drowsy effect

• Voltage-scale of idle memory cells– Two levels of supply voltage (Vdd and VddLow)

– Transistors leak much less than with full Vdd

– No loss of memory state

• High level policies for drowsy caches– No need for complex management mechanisms

• Reading delay (cell voltage scaled back to Vdd)– Worst case are few cycles of delay

– Examples• Simple: whole cache periodically put in drowsy mode• Petit et al.: Simple with heuristics, such as avoid setting the Most

Recently Used (MRU) line to drowsy mode

Threshold voltage manipulation

• The lower the VT, the higher the leakage– Technology scaling enforces

• Reduce Vdd to reduce power consumption and temperature

• Reduce VT to reduce delay

• Architectural level techniques– Combination of high-VT and low-VT devices

• High-VT : low leakage, long latency

• Low-VT : high leakage, short latency

– Gated-Vdd using a high-VT device

Variable Threshold CMOS• Body Biasing• Body effect to change

device Vth

• Standby leakage reduction with maximum reverse bias

• Triple well structurehttp://mtlweb.mit.edu/researchgroups/icsystems/pubs/tutorials/jkao_2002_iccad_I.pdf

Outline

• Metrics• Energy consumption in ICT products• Some common energy optimization

techniques– Static power reduction– Dynamic power reduction

Capacitance and switching activity

• Capacitance and Switching factor intertwined

P=C V⋅ 2 A f⋅ ⋅

• Capacitance (C)– Fixed at design time– Dependant on

• number of transistors• Interconnections

• Switching activity or factor (A)• Fraction between 0 and 1• Factor of capacitance charged/discharged each CPU cycle

Capacitance

• Description of capacitance (Burd and Brodersen)

CL=CW + Cfixed

– CW: Product of technology constant and device width• Optimized at circuit level

– Cfixed: Capacitance of the interconnections• Optimized at architectural level• Reduction of wire length• Effective placement and routing (locality)• Break up large memory banks in smaller chunks

Excess switching activity

• Avoidable charge/discharge activity

• Types– Idle-unit– Idle-width– Idle-capacity– Parallel-speculative– Cacheable– Speculative

Idle-unit switching activity

• Triggered by clock activity in unused units

Idle-width switching activity

• Processor structures wider than needed– Example

• Units with support for 64 bit operands• Most common operations use 16 bit operands

• Solutions– Adapt width of machine according to

operands– Pack multiple narrow-width operations

Width adaptation

Width adaptation

Idle-capacity switching activity

• Over-provisioned processor resources– Resource partitioning or re-sizing

• Grounds– Wire delay increases as technology scale decreases– Long wires imply

• Non affordable delay• High capacitance and consumption

– Buffered wires reduce circuit delay

Complexity-adaptive structures

• Complexity-adaptive structures (Albonesi)– Trade latency & consumption with capacity– Structures become faster as they become smaller

• Solution– Partitions with tri-state buffers

• When structures are reduced– Faster processing– Less energy consumed

– Suitable for SRAM

Parallel speculative switching activity

• Parallel activity is spent for performance– Associative caches

• All but one associative ways fail to produce a hit• All ways are accessed in parallel for speed

– Solution: Smart way access approaches

Phased Cache

Sequential cache

Cache Way Memorization

Upon failure

Voltage-Frequency Scaling

• Basic dynamic power equation:

P = C V⋅ 2 A f⋅ ⋅

– Voltage reduction decreases power by the square of it• Maximum frequency is limited by voltage

– Potential cubic reduction in power dissipation• Considering f and V

– Performance decreases linearly

Dynamic voltage/frequency scaling (DVFS)

• Dynamic adjustment of voltage/frequency– Tradeoff power dissipation / performance

• DVFS decision level– Hardware level

• Exploits different timings of hardware components

– Program level• Program behavior drives decision• E.g. scale down when program knows that has to wait

– System level (OS)• Idleness of the system drives decision• Voltage/frequency scaled to eliminate idle periods

Dynamic voltage/frequency scaling (DVFS)

• Examples of commercial systems– Intel SpeedStep– AMD PowerNow! (for laptops)

• Cool'n'Quiet (for desktop and servers)

• Decision taken at system level

• Changes through specific CPU register

Enhanced Intel ® SpeedStep ® Technology for the Intel ® Pentium ® M Processor (White Paper)http://download.intel.com/design/network/papers/30117401.pdf

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Coming Next

• Power Aware Computing• Higher-level power reduction techniques