I/O completion ports are used with asynchronous I/O.

The CreateIoCompletionPort function associates
an I/O completion port with one or more file handles.

When an asynchronous I/O operation started
on a file handle associated with a completion port
is completed, an I/O completion packet is queued to the port.

This can be used to combine the synchronization point
for multiple file handles into a single object.

A thread uses the GetQueuedCompletionStatus function
to wait for a completion packet to be queued to
the completion port, rather than waiting directly
for the asynchronous I/O to complete.

Threads that block their execution on a completion port
are released in last-in-first-out (LIFO) order.

This means that when a completion packet is queued to
the completion port, the system releases the last thread
to block its execution on the port.

When a thread calls GetQueuedCompletionStatus,
it is associated with the specified completion port until it exits,
specifies a different completion port, or frees the completion port.

A thread can be associated with at most one completion port.

The most important property of a completion port is the concurrency value.

The concurrency value of a completion port is specified when the completion port is created.

This value limits the number of runnable threads associated with the completion port.

When the total number of runnable threads associated with the completion port
reaches the concurrency value, the system blocks the execution of any subsequent threads
that specify the completion port until the number of runnable threads associated
with the completion port drops below the concurrency value.

The most efficient scenario occurs when there are completion packets waiting in the queue,
but no waits can be satisfied because the port has reached its concurrency limit.

In this case, when a running thread calls GetQueuedCompletionStatus,
it will immediately pick up the queued completion packet.

No context switches will occur, because the running thread is
continually picking up completion packets and the other threads are unable to run.

The best value to pick for the concurrency value is the number of CPUs on the machine.

If your transaction required a lengthy computation,
a larger concurrency value will allow more threads to run.

Each transaction will take longer to complete, but more transactions will
be processed at the same time.

It is easy to experiment with the concurrency value
to achieve the best effect for your application.

The PostQueuedCompletionStatus function allows an application to queue
its own special-purpose I/O completion packets to the completion port
without starting an asynchronous I/O operation.

This is useful for notifying worker threads of external events.

The completion port is freed when there are no more references to it.

The completion port handle and every file handle associated
with the completion port reference the completion port.

All the handles must be closed to free the completion port.
To close the port handle, call the CloseHandle function.