Device-Dedicated Threads

The driver of a slow device or a device that is seldom used (like the floppy controller) can solve many "waiting" problems by creating a device-dedicated system thread. Additionally, most file system drivers use system worker threads and supply worker-thread callback routines.

If a device driver has its own thread context or is running in a system-thread context, the device-dedicated thread or highest-level driver's worker-thread callback routine can synchronize operations on a dispatcher object, such as an event object or semaphore object, in a shared communication region of the driver's device extension. For example, a device-dedicated thread can wait for a shared dispatcher object, while the thread's device is not in use, by calling KeWaitForSingleObject for a semaphore. Until the device driver is called to carry out an I/O operation (at which point it sets the semaphore to the Signaled state), its waiting thread uses no CPU time.

A driver can call PsCreateSystemThread to create a driver- or device-dedicated thread, and then call KeSetBasePriorityThread to set the thread's base priority. The driver should specify a priority value that avoids run-time priority inversions in SMP machines. That is, setting the base priority of a driver-created thread too high can create delays in the execution of lower priority threads that submit I/O requests for that driver.

Because thread objects are themselves a type of dispatcher object, a thread can wait for another thread to complete. To obtain the thread object pointer associated with a thread, a driver can call ObReferenceObjectByHandle, passing in the thread handle received from PsCreateSystemThread.

A thread can call KeDelayExecutionThread to wait for an interval that could be a full time slice or longer. The granularity of a KeDelayExecutionThread interval is around 10 milliseconds. Because KeDelayExecutionThread is a timer-driven routine, the granularity of its interval is slightly faster or slower than 10 milliseconds, depending on the platform.



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