Energy and Power Lecture notes S. Yalamanchili and S. Mukhopadhyay
Some Useful Reading • http://en.wikipedia.org/wiki/CPU_power_dissipation • http://en.wikipedia.org/wiki/CMOS#Power:_switching_and_leakage • http://www.xbitlabs.com/articles/cpu/display/core-i5-2500t-2390t-i3-2100t-pentium-g620t.html • http://www.cpu-world.com/info/charts.html
GATE DRAIN SOURCE BODY Technology Scaling GATE • 30% scaling down in dimensions doubles transistor density • Power per transistor • Vddscaling lower power • Transistor delay = CgateVdd/ISAT • Cgate, Vddscaling lower delay DRAIN SOURCE tox L
Fundamental Trends Source: Shekhar Borkar, Intel Corp.
ITRS Roadmap for Logic Devices From: “ExaScaleComputing Study: Technology Challenges in Achieving Exascale Systems,” P. Kogge, et.al, 2008
Where Does the Power Go in CMOS? • Dynamic Power Consumption • Charging and discharging capacitance • Short Circuit Power • Short circuit path between supply rails during switching • Nominally 10%-20% of dynamic power and can be ignored for a first order analysis • Leakage • Leaky transistors
Dynamic Power Voltage VDD VDD iDD VDD CL iDD CL 0 T Time Output Capacitor Discharging Output Capacitor Charging Input to CMOS inverter PDYNAMIC = CL x VDD x VDD x Frequency Dynamic power is used in charging and discharging the capacitances in the CMOS circuit.
Static Power • Technology scaling has caused transistors to become smaller and smaller. As a result, static power has become a substantial portion of the total power. Gate Leakage Output = VDD Input = 0 Junction Leakage Sub-threshold Leakage PSTATIC = VDD x ISTATIC
Energy-Delay Interaction • Delay decreases with supply voltage but energy/power increases Delay Energy EDP Energy or delay VDD VDD
Static Energy-Delay Interaction GATE • Static energy increases exponentially with decrease in threshold voltage • Delay increases with threshold voltage DRAIN SOURCE leakage delay tox leakage or delay L Vth
Power Vs. Energy • Energy is a rate of expenditure of energy • One joule/sec = one watt • Both profiles use the same amount of energy at different rates or power P2 Power(watts) P1 P0 Same Energy = area under the curve Time Power(watts) P0 Time
Optimizing Power vs. Energy Maximize battery life minimize energy Thermal envelopes minimize peak power
The Problem • Historically performance scaling was accompanied by power scaling • This is no longer true power densities are increasing
The End of Dennard Scaling GATE • Voltage is no longer scaling at the same rate • Slower scaling in power per transistor increasing power densities DRAIN SOURCE tox L From R. Dennard, et al., “Design of ion-implanted MOSFETs with very small physical dimensions,” IEEE Journal of Solid State Circuits, vol. SC-9, no. 5, pp. 256-268, Oct. 1974.
Chip Power Densities From: “ExaScaleComputing Study: Technology Challenges in Achieving Exascale Systems,” P. Kogge, et.al, 2008
What is the Problem? Mukhopadhyay and Yalamanchili (2009) • Based on scaling using Pentium-class cores • While Moore’s Law continues, scaling phenomena have changed • Power densities are increasing with each generation
Power per transistor scales with frequency but also scales with Vdd Lower Vddcan be compensated for with increased pipelining to keep throughput constant Power per transistor is not same as power per area power density is the problem! Multiple units can be run at lower frequencies to keep throughput constant, while saving power The Power Wall
The Advent of Dark Silicon? In-order core Out of-order core • Cannot afford to turn on all devices at once • How do we manage the power and thermals? 64-core asymmetric chip multiprocessor layoutand failure probability distribution
What are my Options? • Better technology • Manufacturing • New Devices non-CMOS? • Be more efficient – activity management • Clock gating • Power gating • Power management • Improved architecture • Simpler pipelines • Parallelism
Activity Management Clock Gating Power Gating • Turn off clock to a block of logic • Eliminate unnecessary transitions/activity • Clock distribution power Vdd Power gate transistor Core 0 Core 1 • Turn off power to a block of logic, e.g., core • No leakage Combinational Logic input clk cond clk clk
Power Management • Software controlled power management • Optimize power and/or energy • Orchestrated by the operating system or application libraries • Industry standard interfaces for power management • Advanced Configuration and Power Interface (ACPI) • https://www.acpica.org/ • http://www.acpi.info/ • Hardware power management • Optimized power/energy • Failsafe operation, e.g., protect against thermal emergencies
Processor Power States • Performance States – P-states • Operate at different voltage/frequencies • Recall delay-voltage relationship • Lower voltage lower leakage • Lower frequency lower power (not the same as energy!) • Lower frequency longer execution time • Idle States - C-states • Sleep states • Differ is how much state is saved • SW or HW managed transitions between states!
Multiple Voltage Frequency Domains Intel Sandy Bridge Processor • Coresand ring in one DVFS domain • Graphics unit in another DVFS domain • Cores and portion of cache can be gated off From E. Rotem et. Al. HotChips 2011
Power States From: http://www.intel.com/content/www/us/en/processors/core/2nd-gen-core-family-mobile-vol-1-datasheet.html
Power Gating • Turn off components that are not being used • Lose all state information • Costs of powering down • Costs of powering up • Smart shutdown • Models to guide decisions Intel Sandy Bridge Processor
Simplify Core Design AMD Bulldozer Core • Support for out of order execution, schedulers, branch prediction, etc. consumes more energy per instruction • Can fit many more simpler cores on a dies ARM A7 Core (arm.com)
Parallelism and Power IBM Power5 AMD Trinity Source: forwardthinking.pcmag.com Source: IBM • How much of the chip area is devoted to compute? • Run many cores slower. Why does this reduce power?
Parallelism • Concurrency + lower frequency greater energy efficiency Example • 4X #cores • 0.75x voltage • 0.5x Frequency • 1X power • 2X in performance Core Core Core Core Core Cache Cache Cache Cache Cache
Microarchitectural Level Models • How can we study power consumption without building circuits? • Models • Models can are available at multiple levels of abstraction. We are interested in microarchitectural models
Processor Microarchitecture Fetch Decode Execute/Writeback ALU Register Files MUL Instruction Decoder Instruction Cache Instruction Queue Fetch Queue FPU LD Branch Prediction Instruction TLB ST L1 Data Cache Data TLB Network Memory On-Chip Network L2 Data Cache NoC Router
Energy/Power Calculation • How do we calculate energy or power dissipation for a given microarchitecture? • Energy/Power varies between: • Different ISA; ARMvsIntel x86 • Different microarchitecture; in-ordervsout-of-order • Different applications; memoryvscompute-bound • Different technologies; 90nmvs22nm technology • Different operation conditions; frequency, temperature
Architecture Activity (1) fbuffer.write++; icache.read++; ALU Register Files Activity 1: Instruction Fetch MUL Instruction Decoder Instruction Cache Instruction Queue Fetch Queue FPU LD Branch Prediction Instruction TLB ST • Collect activity counts of each architecture component (through simulation or measurement). • List of components differs between microarchitectures. • Activity counts at each component differs between applications. L1 Data Cache Data TLB On-Chip Network L2 Data Cache NoC Router
Architecture Activity (2) idecoder.logic++; fbuffer.read++; ALU Register Files Activity 2: Instruction Decode MUL Instruction Decoder Instruction Cache Instruction Queue Fetch Queue FPU LD Branch Prediction Instruction TLB ST • Read/write accesses to caches, buffers, etc. • Logical accesses to logic blocks such as decoder, ALUs, etc. • Tradeoff of differentiating more access types (accuracy) vs simulation speed (complexity). L1 Data Cache Data TLB On-Chip Network L2 Data Cache NoC Router
Power and Architecture Activity • For example, At nth clock cycle, collected counters are: • Data cache: • read = 20, write = 12; • per-read energy = 0.5nJ; per-write energy = 0.6nJ; • Read energy = read*per-read energy = 10nJ • Write energy = write*per-write energy = 7.2nJ • Total activity energy = read+write energies = 17.2nJ • If n = 50th clock cycle and clock frequency = 2GHz,Total activity power = energy*clock_freq/n = 688mW • *Note: n/clock_freq = n clock periods in sec power = time average of energy
Things to consider (1) How do we calculate per-read/write energies? • Per-access energies can be estimated from circuit-level designs and analyses. • There are various open-source tools for this. Architecture Specification Circuit-level Estimation Tool Estimation Results: Area, Energy, Timing, etc. Technology Parameters
Things to consider (2) Is per-access energy always the same? • Per-access energy in fact depends on: • how many bits are switching • how they are switching (0→1 or 1→0) • It is reasonable to assume constant per-access energy in long-term observation (e.g., n = 1M clock cycles); the number of switching bits are averaged (e.g., 50% of bits are switching). • Most architecture simulators do not capture bit-level details due to simulation complexity.
Things to consider (3) If a register file didn’t have read/write accesses but held data, what is the energy dissipation? • Energy (or power) is largely comprised of dynamic and static dissipations. • Dynamic (or switching) energy refers to energy dissipation due to switching activities. • Static (or leakage) energy is dissipation to keep the electronic system turned on. • In this case, the register file has no dynamic energy dissipation but consumes static energy.
Thermal Issues • Heat can cause damage to the chip • Need failsafe operation • Thermal fields change the physical characteristics • Leakage current and therefore power increases • Delay increases • Device degradation becomes worse • Cooling solution determines the permitted power dissipation
Thermal Design Power (TDP) • This is the maximum power at which the part is designed to operate • Dictates the design of the cooling system • Max temperature Tjmax • Typically fixed by worst case workload • Parts are typically operating below the TDP • Opportunities for turbo mode? AMD Trinity APU http://ecs.vancouver.wsu.edu/thermofluids-research
Trinity TDP Source: http://www.anandtech.com/show/6347/amd-a10-5800k-a8-5600k-review-trinity-on-the-desktop-part-2
Exploiting the Physics • Most of time the part is operating well below its thermal limit • Leaving performance on the table • Can temporarily boost frequency (and therefore power dissipation) for short periods of time, e.g., seconds • Temperature changes slowly
Boosting Intel Sandy Bridge • Exploit package physics • Temperature changes on the order of milliseconds • Use the thermal headroom Turbo boost region Max Power TDP Power 10s of seconds Low power – build up thermal credits
Conclusions • Power/energy is the leading driver of modern architecture design • Power and energy management is key to scalability • Need integrated power/energy, performance, thermal management in fielded systems • What about energy/power efficient algorithms?
Study Guide • Explain the difference between energy dissipation and power dissipation • Distinguish between static power dissipation and dynamic power dissipation • Be able to apply the simplified McPAT power model to a simple datapath and instruction sequence • Explain dynamic voltage frequency scaling • What are power states? • Why is this an advantage? • What is the impact of DVFS on i) energy, ii) execution time, and iii) power
Study Guide (cont.) • How is thermal design power (TDP) calculated? • When using boost algorithms, what determines the duration of the high frequency operation? • How does a power virus work? • Describe how throttling works • Know the power dissipation in some modern processor-memory systems drawn from the embedded, server, and high performance computing segments