When a thread is in the "warnable state", all pending APCs are processed in a first-in first-out order ( FIFO ). Secondly, due to the unique processing mechanism of APC in the system, application threads may wait indefinitely without being notified of completion. many extended functions do not allow the use of APC ( Asyncroneus Procedure Call ) to complete notifications. The overlapped I/O model that uses completion routine notification is not the best choice for developing high-performance servers for the following reasons.
Because the operating system can handle limited event objects at the same time, the I/O model based on event objects does not have scalability. If you want this model to manage more than 64 sockets at the same time, you must create additional worker threads to wait for more event objects. Using the event object notification of overlapping I/O model is not scalable, because for issuing WSAWaitForMultipleEvents each thread call, the I/O model can only support a maximum of 64 sockets.
When using the overlapped I/O model, you can choose to use different completion notification methods. Several other advantages of the overlapped I/O model are that Microsoft provides some unique extension functions for the overlapped I/O model.
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