Consider the following scenario. You configure the Power Options profile in Microsoft Windows XP so that the computer enters standby after a particular period has elapsed since the last significant system activity. For example, assume that the computer is running on AC power and that you configure the standby setting to 20 minutes. When you watch the computer for 20 minutes, it does not enter standby even though the system appears to be idle.
Even though the system appears to be idle, the Windows XP System Idle Task Scheduler service may be running system maintenance tasks. These tasks may trigger a reset of the System Idle counter. In this scenario, the computer does not enter standby.
Note Third-party programs and services may also be running during system idle time. These programs and services may use more than 10 percent of the computer's CPU and disk resources. This level of CPU and disk utilization may reset the System Idle counter and therefore delay the system from entering standby.
Windows schedules certain maintenance tasks when the following conditions are true:
The system is idle.
The system running on AC power.
Some idle tasks may cause high CPU utilization and high disk utilization. Under these conditions, the system will experience delays in entering standby. By default in the following versions of Windows XP, the System Idle Task Scheduler service starts when the computer is idle for approximately 15 minutes:
Windows XP Service Pack 2 (SP2)
Windows XP Service Pack 1 ( SP1)
Windows XP with the hotfix from the following Microsoft Knowledge Base article installed:
310601 Computer does not enter standby mode if Power Options profile is set to standby after 45 or more minutes
The System Idle Task Scheduler service tries to schedule system maintenance tasks so that there is sufficient time to complete a particular task without triggering a reset of the System Idle counter. With this goal,the System Idle Task Scheduler service monitors the system for idle periods. This service considers the system to be idle if the following conditions are true for at least the last 10 minutes:
There is no user input.
CPU and disk utilization levels are less than 10 percent.
The system is not running on battery power.
No presentation program, such as a slide show or a movie player, is running.
If these conditions are true, the service then takes a system snapshot one time per minute for three consecutive minutes to monitor for any new tasks that may have started. If the service determines that no new tasks are running, the service is ready to schedule any queued tasks.
Note A task that is started by the System Idle Task Scheduler service, such as a virus scan or a disk optimization, may trigger a reset of the System Idle counter.
If a scheduled task runs and, in doing this, uses more than 10 percent of CPU and disk resources for an extended period, the Kernel-mode System Idle Detection thread determines that the system is not idle. Therefore, the System Idle counter is reset to zero. This reset process delays the system from entering standby.
If the following tasks are queued, they can be scheduled to run during system idle time without triggering a reset of the System Idle counter:
The disk layout task
When you start the computer, the disk layout task is queued to run during system idle time. When the System Idle Task Scheduler service schedules this task, Windows examines the registry to determine when this task was last run. If the disk layout task has not been run during the past three days, the process continues. Otherwise, the process quits. If the process continues, it determines whether there have been many changes to the disk layout since the last time this task was run. The System Idle Task Scheduler service requeues the task after 32 processes are created on the system.
To determine whether there have been significant changes in the disk layout, the System Idle Task Scheduler service examines the scenario files in the Windows\Prefetch folder. These scenario files show which files are used during program startup and during computer startup. If this file-examination process continues, the disk layout is updated. This update process may generate CPU or disk utilization of more than 10 percent. And at this level of CPU or disk utilization, the Kernel-mode System Idle Detection thread resets the System Idle counter.
The system restore task
This task prepares automatic system restore points to improve system reliability.
The Help services and data collection task
To optimize system performance and reliability, Windows XP is designed to automatically run system maintenance tasks. Therefore, when the computer is idle and is running on AC power, Windows XP does not immediately put the computer into standby. Instead, Windows XP waits several minutes for system maintenance tasks to run. These tasks do not delay for more than 15 minutes the process by which the computer enters standby or by which it enters the other Power Option states, such as hibernation or suspend. Additionally, these tasks are typically performed during the first idle period after system startup.