Core parking is a feature that was introduced in Windows Server 2008 R2. The processor power management (PPM) engine and the scheduler work together to dynamically adjust the number of cores that are available to run threads. The PPM engine chooses a minimum number of cores for the threads that will be scheduled. Cores that are chosen to “park” generally do not have any threads scheduled, and they will drop into very low power states when they are not processing interrupts, DPCs, or other strictly affinitized work. The remaining set of “unparked” cores are responsible for the remainder of the workload. Core parking can potentially increase energy efficiency during lower usage periods on the server because parked cores can drop into deep low-power states.
For most servers, the default core-parking behavior provides a reasonable balance of throughput and energy efficiency. On processors where core parking may not show as much benefit on generic workloads, it can be disabled by default. If your server has specific core parking requirements, you can control the number of cores that are available to park by using the Processor Performance Core Parking Maximum Cores parameter or the Processor Performance Core Parking Minimum Cores parameter in Windows Server 2012.
One scenario that core parking has difficulty with is when there are one or more active threads affinitized to a non-trivial subset of CPUs in a NUMA node (that is, more than 1 CPU, but less than the entire set of CPUs on the node). When the core parking algorithm is picking cores to unpark (assuming an increase in workload intensity occurs), it does not know to pick the cores within the active affinitized subset (or subsets) to unpark, and thus may end up unparking cores that won’t actually be utilized.
The values for these parameters are percentages in the range 0 – 100. The Processor Performance Core Parking Maximum Cores parameter controls the maximum percentage of cores that can be unparked (available to run threads) at any time, while the Processor Performance Core Parking Minimum Cores parameter controls the minimum percentage of cores that can be unparked. To turn off core parking, set the Processor Performance Core Parking Minimum Cores parameter to 100 percent by using the following commands:
Powercfg -setacvalueindex scheme_current sub_processor CPMINCORES 100
Powercfg -setactive scheme_current
To reduce the number of schedulable cores to 50 percent of the maximum count, set the Processor Performance Core Parking Minimum Cores parameter to 50 as follows:
Powercfg -setacvalueindex scheme_current sub_processor CPMAXCORES 50
Powercfg -setactive scheme_current
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