Starting with Windows 8.1, drivers can use the ExXxxTimer routines to manage high-resolution timers. The accuracy of a high-resolution timer is limited only by the maximum supported resolution of the system clock. In contrast, timers that are limited to the default system clock resolution are significantly less accurate.
However, high-resolution timers require system clock interrupts to—at least, temporarily—occur at a higher rate, which tends to increase power consumption. Thus, drivers should use high-resolution timers only when timer accuracy is essential, and use default-resolution timers in all other cases.
To create a high-resolution timer, a WDM driver calls the ExAllocateTimer routine and sets the EX_TIMER_HIGH_RESOLUTION flag in the Attributes parameter. When the driver calls the ExSetTimer routine to set the high-resolution timer, the operating system increases the resolution of the system clock, as necessary, so that the times at which the timer expires more precisely correspond to the nominal expiration times specified in the DueTime and Period parameters.
A Kernel-Mode Driver Framework (KMDF) driver can call the WdfTimerCreate method to create a high-resolution timer. In this call, the driver passes a pointer to a WDF_TIMER_CONFIG structure as a parameter. To create a high-resolution timer, the driver sets the UseHighResolutionTimer member of this structure to TRUE. This member is a part of the structure starting with Windows 8.1 and KMDF version 1.13.
Controlling timer accuracy
For example, for Windows running on an x86 processor, the default interval between system clock ticks is typically about 15 milliseconds, and the minimum interval between system clock ticks is about 1 millisecond. Thus, the expiration time of a default-resolution timer (which ExAllocateTimer creates if the EX_TIMER_HIGH_RESOLUTION flag is not set) can be controlled only to within about 15 milliseconds, but the expiration time of a high-resolution timer can be controlled to within a millisecond.
If a driver specifies a relative expiration time for a default-resolution timer, the timer can expire up to about 15 milliseconds earlier or later than the specified expiration time. If a driver specifies a relative expiration time for a high-resolution timer, the timer can expire as late as about a millisecond after the specified expiration time but it never expires early. For more information about the relationship between system clock resolution and timer accuracy, see Timer Accuracy.
If no high-resolution timers are set, the operating system typically runs the system clock at its default rate. However, if one or more high-resolution timers are set, the operating system might need to run the system clock at its maximum rate for at least a part of the time before these timers expire.
To avoid unnecessarily increasing power consumption, the operating system runs the system clock at its maximum rate only when necessary to satisfy the timing requirements of high-resolution timers. For example, if a high-resolution timer is periodic, and its period spans several default system clock ticks, the operating system might run the system clock at its maximum rate only in the part of the timer period that immediately precedes each expiration. For the rest of the timer period, the system clock runs at its default rate.
To prevent excessive power consumption, drivers should avoid setting the period of a long-running high-resolution timer to a value that is less than the default interval between system clock ticks. Otherwise, the operating system is forced to continuously run the system clock at its maximum rate.
Starting with Windows 8, a driver can call the ExQueryTimerResolution routine to get the range of timer resolutions that are supported by the system clock.
Comparison to ExSetTimerResolution
Starting with Windows 2000, a driver can call the ExSetTimerResolution routine to change the time interval between successive system clock interrupts. For example, a driver can call this routine to change the system clock from its default rate to its maximum rate to improve timer accuracy. However, using ExSetTimerResolution has several disadvantages compared to using high-resolution timers created by ExAllocateTimer.
First, after calling ExSetTimerResolution to temporarily increase the system clock rate, a driver must call ExSetTimerResolution a second time to restore the system clock to its default rate. Otherwise, the system clock timer continuously generates interrupts at the maximum rate, which might cause excessive power consumption.
Second, a driver that uses the ExSetTimerResolution routine cannot optimize its temporary use of higher system clock rates as effectively as the operating system does for high-resolution timers. Thus, the system clock spends more time running at the maximum rate than is strictly necessary.
Third, if multiple drivers concurrently use ExSetTimerResolution to improve timer accuracy, the system clock might run at its maximum rate for long periods. In contrast, the operating system globally coordinates the operation of multiple high-resolution timers so that the system clock runs at the maximum rate only when necessary to meet the timing requirements of these timers.
Finally, using ExSetTimerResolution is inherently less accurate than using a high-resolution timer. After a driver calls ExSetTimerResolution to increase the system clock to its maximum rate, which is typically about a tick per millisecond, the driver might call a routine such as KeSetTimerEx to set the timer. If, in this call, the driver specifies a relative expiration time, the timer can expire up to about a millisecond earlier than or later than the specified expiration time. However, if a relative expiration time is specified for a high-resolution timer, the timer can expire up to about a millisecond later than the specified expiration time but it never expires early.
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