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System Idle Process
A component of Microsoft Windows
System idle process.png
Type Kernel
Included with Windows NT

In Windows NT operating systems, the System Idle Process contains one or more kernel threads which run 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. In a multiprocessor system, there is one idle thread associated with each CPU.

The original purpose of the idle process and its threads was to eliminate what would otherwise be a special case in the scheduler. Without the idle threads, there could be cases when no threads were runnable, or "Ready" in terms of Windows scheduling states. Since the idle threads are always in a Ready state (if not already Running), this can never happen. Thus whenever the scheduler is called due to the current thread leaving the CPU, it can always find another thread to run on that CPU, even if it is only the CPU's idle thread.

The scheduler still treats the idle threads as special cases in terms of thread scheduling priority. All other threads have a priority in the range 0 through 31, inclusive, with highest priorities always preempting lower. The idle threads do have a priority member in their thread objects but this data is not used to determine when to run them. Instead the scheduler selects the idle thread for a CPU whenever there are no threads in the priority range 0 through 31 that can run on the CPU. It is as if the idle threads each have a priority of negative one. It is not possible to create additional threads that are scheduling peers of the idle threads, even from kernel mode.

In Windows 2000 and later the threads in the System Idle Process are also used to implement CPU power saving. The exact power saving scheme depends on the operating system version and on the hardware and firmware capabilities of the system in question. For instance, on x86 processors under Windows 2000, the idle thread will run a loop of HLT instructions, which causes the CPU to turn off many internal components and wait until an interrupt request arrives. Later versions of Windows implement more complex CPU power saving methods. On these systems the idle thread will call routines in the Hardware Abstraction Layer to reduce CPU clock speed or to implement other power-saving mechanisms.

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 usage. 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.

See also




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