Windows® Internals, Sixth Edition, Part 2

Windows keeps track of threads that become inactive because they block on something other than the completion port by relying on the queue pointer in a thread’s control block. The scheduler routines that possibly result in a thread blocking (such as KeWaitForSingleObject, KeDelayExecution-Thread, and so on) check the thread’s queue pointer. If the pointer isn’t NULL, the functions call KiActivateWaiterQueue, a queue-related function that decrements the count of active threads associated with the queue. If the resultant number is less than the maximum and at least one completion packet is in the queue, the thread at the front of the queue’s thread list is awakened and given the oldest packet. Conversely, whenever a thread that is associated with a queue wakes up after blocking, the scheduler executes the function KiUnwaitThread, which increments the queue’s active count.

Finally, the PostQueuedCompletionStatus Windows API function results in the execution of the NtSetIoCompletion system service. This function simply inserts the specified packet onto the completion port’s queue by using KeInsertQueue.

Figure 8-24 shows an example of a completion port object in operation. Even though two threads are ready to process completion packets, the concurrency value of 1 allows only one thread associated with the completion port to be active, and so the two threads are blocked on the completion port.

Figure 8-24. I/O completion port operation

Finally, the exact notification model of the I/O completion port can be fine-tuned through the SetFileCompletionNotificationModes API, which allows application developers to take advantage of additional, specific improvements that usually require code changes but can offer even more throughput. Three notification-mode optimizations are supported, which are listed in Table 8-3. Note that these modes are per file handle and permanent.

Table 8-3. I/O Completion Port Notification Modes

Notification Mode

Meaning

Skip completion port on success

If the following three conditions are true, the I/O manager does not queue a completion entry to the port when it would ordinarily do so. First, a completion port must be associated with the file handle; second, the file must be opened for asynchronous I/O; third, the request must return success immediately without returning ERROR_PENDING.

Skip set event on handle

The I/O manager does not set the event for the file object if a request returns with a success code or the error returned is ERROR_PENDING and the function that is called is not a synchronous function. If an explicit event is provided for the request, it is still signaled.

Skip set user event on fast I/O

The I/O manager does not set the explicit event provided for the request if a request takes the fast I/O path and returns with a success code or the error returned is ERROR_PENDING and the function that is called is not a synchronous function.

I/O Prioritization

Without I/O priority, background activities like search indexing, virus scanning, and disk defragmenting can severely impact the responsiveness of foreground operations. A user launching an application or opening a document while another process is performing disk I/O, for example, experiences delays as the foreground task waits for disk access. The same interference also affects the streaming playback of multimedia content like music from a disk.

Windows includes two types of I/O prioritization to help foreground I/O operations get preference: priority on individual I/O operations and I/O bandwidth reservations.

I/O Priorities

The Windows I/O manager internally includes support for five I/O priorities, as shown in Table 8-4, but only three of the priorities are used. (Future versions of Windows may support High and Low.)

Table 8-4. I/O Priorities

I/O Priority

Usage

Critical

Memory manager

High

Not used

Normal

Normal application I/O

Low

Not used

Very Low

Scheduled tasks, Superfetch, defragmenting, content indexing, background activities

I/O has a default priority of Normal, and the memory manager uses Critical when it wants to write dirty memory data out to disk under low-memory situations to make room in RAM for other data and code. The Windows Task Scheduler sets the I/O priority for tasks that have the default task priority to Very Low. The priority specified by applications that perform background processing is Very Low. All of the Windows background operations, including Windows Defender scanning and desktop search indexing, use Very Low I/O priority.

Prioritization Strategies