Editing Underclocking (section)

==In practice==
===Linux===
The [[Linux (kernel)|Linux kernel]] supports CPU frequency modulation. In supported processors, using ''cpufreq'' to gain access to this feature gives the system administrator a variable level of control over the CPU's [[clock rate]]. The kernel includes six governors by default: Conservative, Ondemand, Performance, Powersave, Userspace and Schedutil.<ref>{{Cite web |last=Brodowski |first=Dominik |date=2017-04-12 |title=CPUFreq Governors |url=https://www.kernel.org/doc/Documentation/cpu-freq/governors.txt |website=kernel.org}}</ref> The Conservative and Ondemand governors adjust the [[clock rate]] depending on the CPU load, but each with different algorithms. The Ondemand governor jumps to maximum frequency on CPU load and decreases the frequency step by step on CPU idle, whereas the Conservative governor increases the frequency step by step on CPU load and jumps to lowest frequency on CPU idle. The Performance, Powersave and Userspace governors set the [[clock rate]] statically: Performance to the highest available, Powersave to the lowest available, and Userspace to a frequency determined and controlled by the user. The Schedutil governor estimates the load through the [[Scheduling (computing)|scheduler]]'s Per-Entity Load Tracking (PELT) mechanism.

===Windows===
Underclocking can be done manually in the [[BIOS]] or with [[Microsoft Windows|Windows]] applications, or dynamically using features such as Intel's [[SpeedStep]] or AMD's [[Cool'n'Quiet]]. In Windows 7 and 10, underclocking can be set within the "advanced" settings of a power management plan.<ref>{{cite web |title=How does windows "limit" the CPU (Power Options / Battery saving) |url=https://superuser.com/questions/565347/how-does-windows-limit-the-cpu-power-options-battery-saving |website=superuser StackExchange|access-date=30 April 2020}}</ref><ref>{{cite web |last1=Hunt |first1=Cale |title=7 ways to get better battery life from your Windows 10 PC |url=https://www.windowscentral.com/how-get-best-battery-life-your-windows-10-pc#clock |website=Windows Central |date=14 September 2017 |access-date=30 April 2020}}</ref>

=== Asus Eee PC ===
Earlier models of the [[Asus Eee PC]] used a 900&nbsp;MHz [[Intel]] [[Celeron]] M processor underclocked to 630&nbsp;MHz.

=== Mac OS X ===
Underclocking can be performed in the EFI.

=== Smartphones and PDAs ===
Most [[smartphone]]s and [[Personal digital assistant|PDA]]s, such as the [[Motorola Droid]], [[Palm Pre]], and Apple [[iPhone]], use underclocking of a more powerful processor, rather than the full clocking of a less powerful processor, to maximize battery life. The designers for such mobile devices often discover that a slower processor gives worse battery life than a more powerful processor at a lower [[clock rate]].
They select a processor on the basis of the [[performance per watt]] of the processor.<ref>[[Wikibooks:Microprocessor Design/Performance Metrics#MIPS/mW|"Microprocessor Design: Performance Metrics"]]
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===Performance===
The performance of an underclocked machine will often be better than might be expected. Under normal desktop use, the full power of the CPU is rarely needed. Even when the system is busy, a large amount of time is usually spent waiting for data from memory, disk, or other devices. Such devices communicate with the CPU through a [[Bus (computing)|bus]] which operates at a much lower bandwidth. Generally, the lower the [[CPU multiplier]] (and thus clockrate of a CPU), the closer its performance will be to that of the bus, and the less time it will spend waiting.