System Idle Process

Infobox Windows component
name = System Idle Process


type = Kernel
included_with = Windows NT

In Windows NT operating systems, the System Idle Process is a kernel thread which runs when no other runnable thread can be scheduled on a CPU. For example, there may be no runnable thread in the system, or all runnable threads are already running on a different CPU.

The System Idle Process is used by Windows NT to implement CPU power saving. The exact power saving scheme depends on the hardware and firmware capabilities of the system in question. For instance, on x86 processors, the process will run a loop of HLT instructions, which causes the CPU to turn off many internal components and wait until an IRQ arrives.

The CPU time consumed by the System Idle Process is commonly of interest for end users, as it is a measure of the CPU utilization in their system which is easily accessible through Windows' Task Manager program. There are, however, more detailed sources of such information available through Windows' performance monitoring system (accessible with the perfmon program), which includes more finely grained categorization of CPU time spending. A limited subset of the CPU time categorization is also accessible through the Task Manager, which can display CPU usage by CPU, and categorized by time spent in user vs. kernel code. It should be noted, though, that that information is not calculated from information about the System Idle Process, but from the system's global performance counters.

When nothing else is using CPU processing power, the System Idle Process kicks in, taking up all the rest of the CPU cycles. If you enable the "CPU Time" column in Windows Task Manager, you will see that it is always counting - a good measure of how long your computer's been switched on in the current session. However, on some computers it counts at double speed (probably to do with hyper-threading or multi-threading), and thus the time your computer has been on is half of that time specified.

See also

* List of Microsoft Windows components
* Idle (CPU)
* Microsoft Windows
* HLT

References

* cite book
title = Microsoft Windows Internals
edition = 4th edition
chapter = Chapter 2: System Architecture
pages = pp. 75-76
last = Russinovich
first = Mark
authorlink =Mark Russinovich
coauthors = David Solomon
year = 2005
publisher = Microsoft Press
id = ISBN 0-7356-1917-4