The system provides one DPC queue for each processor. Drivers can control which queue the system assigns a DPC to, the location of the DPC within the queue, and when the queue is processed.
DPCs that are assigned to a particular processor's queue are run on that processor. By default, when the driver calls KeInsertQueueDpc or IoRequestDpc, the DPC is queued on the currently active processor. Drivers can specify the processor queue by calling KeSetTargetProcessorDpc before calling KeInsertQueueDpc or IoRequestDpc.
On Windows Vista and later versions of Windows, the system also has one threaded DPC queue for each processor. Drivers can use KeSetTargetProcessorDpc to specify the processor queue for threaded DPCs.
The KeSetImportanceDpc routine controls where a DPC is placed within the queue. Typically, the DPC is placed at the end of the queue; but if the driver first calls KeSetImportanceDpc with the Importance parameter equal to HighImportance, the DPC is placed at the beginning of the queue.
For ordinary (non-threaded) DPCs, KeSetImportanceDpc also determines whether KeInsertQueueDpc or IoRequestDpc will immediately begin processing the DPC queue. The following list describes the rules for processing the queue:
Processing of the DPC queue begins immediately if the DPC is assigned to the current processor and Importance is not equal to LowImportance, or if Importance is equal to LowImportance and the DPC queue depth of the current processor exceeds a system-defined limit or the DPC request rate has fallen below a system-defined minimum. Otherwise, processing of the DPC is deferred until the appropriate queue depth and rate requirements are met.
Processing of the DPC queue begins immediately on the target processor if the DPC is assigned to a processor that is different than the current processor and Importance equals MediumHighImportance or HighImportance, or if the DPC queue depth of the target processor exceeds a system-defined limit. Otherwise, processing of the DPC is deferred until the appropriate queue depth and rate requirements are met.