The more common mode is "fixed tick," in which the timer interrupts at a regular rate whether or not there is anything for the scheduler to do. The other approach is "variable tick," whereby the kernel can adjust the tick interval based on current conditions. While fixed tick timing is generally easier to implement, variable tick offers the potential to save power in portable devices, by eliminating unnecessary interrupts, Viswanathan writes.
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Understanding the Windows CE variable tick timer
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