Demand-based switching refers to the changing of ACPI processor performance states in response to system workloads. For example, demand-based switching can change voltage and frequency in response to system workloads. Windows XP processor power management implements demand-based switching through the Adaptive processor throttling policy. The Adaptive processor throttling policy dynamically and automatically adjusts the processor current performance state in response to the computers CPU use. The Adaptive processor throttling policy works without user intervention.
If you run Windows XP on a computer that has multiple processors, single-threaded workloads may move across available CPUs. This migration behavior is a natural artifact of how Windows schedules work across available CPU resources. However, if a computer is running with the Adaptive processor throttling policy, this thread migration may cause problems. For example, the Windows kernel power manager may not be able to correctly calculate the optimal target performance state for the processor. This problem occurs because the individual logical or physical processor core may appear to be less busy than the processor package actually is. On performance benchmarks that use single-threaded workloads, this artifact may become evident in the following ways:
• Decreased performance
• A high degree of variance between successive runs of the same benchmark tests
The hotfix that is described in this article includes changes to the kernel power manager. These changes make it possible to track CPU use across the processor package. This tracking helps calculate an increased target performance state.
Note This solution favors performance gains over power savings. Although benchmark performance scores may improve, battery life could be negatively affected. Therefore, this kernel policy change must be enabled in the registry to allow for maximum flexibility.