CPU power dissipation
||It has been suggested that this article be merged with Thermal design power. (Discuss) Proposed since February 2014.|
Central processing unit power dissipation or CPU power dissipation is the process in which central processing units (CPUs) consume electrical energy, and dissipate this energy both by the action of the switching devices contained in the CPU (such as transistors or vacuum tubes) and by the energy lost in the form of heat due to the impedance of the electronic circuits.
CPU power management
Designing CPUs that perform tasks efficiently without overheating is a major consideration of nearly all CPU manufacturers to date. Some implementations of CPUs use very little power; for example, the CPUs in mobile phones often use just a few hundred milliwatts of electricity. Some microcontrollers, used in embedded systems may use a few milliwatts. In comparison, CPUs in general purpose personal computers, such as desktops and laptops, dissipate significantly more power because of their higher complexity and speed. These microelectronic CPUs may consume power in the order of a few watts to hundreds of watts. Historically, early CPUs implemented with vacuum tubes consumed power on the order of many kilowatts.
CPUs for desktop computers typically use a significant portion of the power consumed by the computer. Other major uses include fast video cards, which contain graphics processing units, and the power supply. In laptops, the LCD's backlight also uses a significant portion of overall power. While energy-saving features have been instituted in personal computers for when they are idle, the overall consumption of today's high-performance CPUs is considerable. This is in strong contrast with the much lower energy consumption of CPUs designed for low-power devices. One such CPU, the Intel XScale, can run at 600 MHz with only half a watt of power, whereas x86 PC processors from Intel in the same performance bracket consume roughly eighty times as much energy.
There are some engineering reasons for this pattern.
- For a given device, operating at a higher clock rate always requires more power. Reducing the clock rate of the microprocessor through power management when possible reduces energy consumption.
- New features generally require more transistors, each of which uses power. Turning unused areas off saves energy, such as through clock gating.
- As a processor model's design matures, smaller transistors, lower-voltage structures, and design experience may reduce energy consumption.
Processor manufacturers usually release two power consumption numbers for a CPU:
- typical thermal power, which is measured under normal load. (for instance, AMD's Average CPU power)
- maximum thermal power, which is measured under a worst-case load.
For example, the Pentium 4 2.8 GHz has 68.4 W typical thermal power and 85 W maximum thermal power. When the CPU is idle, it will draw far less than the typical thermal power. Datasheets normally contain the thermal design power (TDP), which is the maximum amount of heat generated by the CPU, which the cooling system in a computer is required to dissipate. Both Intel and Advanced Micro Devices (AMD) have defined TDP as the maximum heat generation for thermally significant periods, while running worst-case non-synthetic workloads; thus, TDP is not reflecting the actual maximum power of the processor. This ensures the computer will be able to handle essentially all applications without exceeding its thermal envelope, or requiring a cooling system for the maximum theoretical power (which would cost more but in favor of extra headroom for processing power).
In many applications, the CPU and other components are idle much of the time, so idle power contributes significantly to overall system power usage. When the CPU uses power management features to reduce energy use, other components, such as the motherboard and chipset, take up a larger proportion of the computer's energy. In applications where the computer is often heavily loaded, such as scientific computing, performance per watt (how much computing the CPU does per unit of energy) becomes more significant.
Sources of power consumption
There are several factors contributing to the CPU power consumption; they include dynamic power consumption, short-circuit power consumption, and power loss due to transistor leakage currents:
The dynamic power consumption originates from logic-gate activities in the CPU. When logic gates toggle, energy is flowing as capacities inside the logic gates are charged and discharged. The dynamic power consumed by a CPU is approximately proportional to the CPU frequency, and to the square of the CPU voltage:
where C is capacitance, f is frequency, and V is voltage.
When logic gates toggle, some transistors inside may change states. As this takes a finite amount of time, it may happen that for a very brief amount of time some transistors are conducting simultaneously. A direct path between the source and ground then results in some short-circuit power loss. The magnitude of this power is dependent on the logic gate, and is rather complex to model on a macro level.
Power consumption due to leakage power emanates at a micro-level in transistors. Small amounts of currents are always flowing between the differently doped parts of the transistor. The magnitude of these currents depend on the state of the transistor, its dimensions, physical properties and sometimes temperature. The total amount of leakage currents tends to inflate for increasing temperature and decreasing transistor sizes.
Both dynamic and short-circuit power consumption are dependent on the clock frequency, while the leakage current is dependent on the CPU supply voltage. It has been shown that the energy consumption of a program shows convex energy behavior, meaning that there exists an optimal CPU frequency at which energy consumption is minimal.
Implications of increased clock frequencies
Processor manufacturers consistently delivered increases in clock rates and instruction-level parallelism, so that single-threaded code executed faster on newer processors with no modification. Now, to manage CPU power dissipation, processor makers favor multi-core chip designs, and software has to be written in a multi-threaded or multi-process manner to take full advantage of the hardware. Many multi-threaded development paradigms introduce overhead, and will not see a linear increase in speed vs number of processors. This is particularly true while accessing shared or dependent resources, due to lock contention. This effect becomes more noticeable as the number of processors increases. Recently, IBM has been exploring ways to distribute computing power more efficiently by mimicking the distributional properties of the human brain.
- Mike Chin (2004-06-15). "Athlon 64 for Quiet Power". silentpcreview.com. p. 3. Retrieved 2013-12-21. "Thermal Design Power (TDP) should be used for processor thermal solution design targets. The TDP is not the maximum power that the processor can dissipate."
- "The technical details behind Intel's 7 Watt Ivy Bridge CPUs". arstechnica.com. 2013-01-14. Retrieved 2013-01-14. "In Intel's case, a specified chip's TDP has less to do with the amount of power a chip needs to use (or can use) and more to do with the amount of power the computer's fan and heatsink need to be able to dissipate while the chip is under sustained load. Actual power usage can be higher or (much) lower than TDP, but the figure is intended to give guidance to engineers designing cooling solutions for their products."
- "Enhanced Intel SpeedStep Technology for the Intel Pentium M Processor (White Paper)" (PDF). Intel Corporation. March 2004. Retrieved 2013-12-21.
- K. De Vogeleer et al. (9 September 2013). The Energy/Frequency Convexity Rule: Modeling and Experimental Validation on Mobile Devices (PDF). 1.0. Springer. Retrieved 2014-1-21.
- See Herb Sutter, The Free Lunch Is Over: A Fundamental Turn Toward Concurrency in Software, Dr. Dobb's Journal, 30(3), March 2005
- Johnson. "IBM demos cognitive computer chips". Retrieved 10/1/11.
- Weik, Martin H. (1955). A Survey of Domestic Electronic Digital Computing Systems. United States Department of Commerce Office of Technical Services.
- http://www.intel.com/Assets/en_US/PDF/datasheet/313079.pdf, page 12
- http://support.amd.com/us/Processor_TechDocs/43374.pdf, pages 10 and 80.
- CPU Reference for all vendors. Process node, die size, speed, power, instruction set, etc.
- Processor Electrical Specifications
- SizingLounge – Online calculation tool for server energy costs.
- For specification on Intel processors.