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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.
Idle is a state that a computer processor is in when it is not being used by any program. Every program or task that runs on a computer system occupies a certain amount of processing time on the CPU. If the CPU has completed all tasks it is idle. Modern processors use idle time to save power.
ACPI 1.0 (1996) defines a way for a CPU to go to idle "C states", but defines no frequency-scaling system. ACPI 2.0 (2000) introduces a system of P states (power-performance states) that a processor can use to communicate its possible frequency–power settings to the OS. The operating system then sets the speed as needed by switching between ...
However, the idle process does not use up computer resources (even when stated to be running at a high percent). Its CPU time "usage" is a measure of how much CPU time is not being used by other threads. In Windows 2000 and later the threads in the System Idle Process are also used to implement CPU power saving.
For example, one can interpret a load average of "1.73 0.60 7.98" on a single-CPU system as: During the last minute, the system was overloaded by 73% on average (1.73 runnable processes, so that 0.73 processes had to wait for a turn for a single CPU system on average). During the last 5 minutes, the CPU was idling 40% of the time, on average.
The power measurement is often the average power used while running the benchmark, but other measures of power usage may be employed (e.g. peak power, idle power). For example, the early UNIVAC I computer performed approximately 0.015 operations per watt-second (performing 1,905 operations per second (OPS), while consuming 125 kW).
The number of available hardware counters in a processor is limited while each CPU model might have a lot of different events that a developer might like to measure. Each counter can be programmed with the index of an event type to be monitored, like a L1 cache miss or a branch misprediction.
Measure the performance of the system before modification. Identify the part of the system that is critical for improving the performance. This is called the bottleneck. Modify that part of the system to remove the bottleneck. Measure the performance of the system after modification. If the modification makes the performance better, adopt it.