Designing CPUs that perform tasks
efficiently without
overheating is a major consideration of nearly all CPU manufacturers to date. Historically, early CPUs implemented with
vacuum tubes consumed power on the order of many
kilowatts. Current CPUs in general-purpose
personal computers, such as
desktops and
laptops, consume power in the order of tens to hundreds of watts. Some other CPU implementations use very little power; for example, the CPUs in
mobile phones often use just a few
watts of electricity, while some
microcontrollers used in
embedded systems may consume only a few milliwatts or even as little as a few microwatts. There are a number of engineering reasons for this pattern: • For a given CPU core, energy usage will scale up as its clock rate increases. Reducing the clock rate or
undervolting usually reduces energy consumption; it is also possible to undervolt the microprocessor while keeping the clock rate the same. • 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 a 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. CPUs 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
power supplies. 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. == Sources ==