Windows 8 Registered I/O - Single threaded RIO IOCP UDP Example Server

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This article presents the third in my series of example servers using the Windows 8 Registered I/O Networking extensions, RIO. This example server uses the I/O Completion Port notification method to handle RIO completions, but only uses a single thread to service the IOCP. I’ve been looking at the Windows 8 Registered I/O Networking Extensions since October when they first made an appearance as part of the Windows 8 Developer Preview. Whilst exploring and understanding the new API I spent some time putting together some simple UDP servers using the various notification styles that RIO provides. I then put together some equally simple UDP servers using the “traditional” APIs so that I could compare performance. This series of blog posts describes each of the example servers in turn. You can find an index to all of the articles about the Windows 8 Registered I/O example servers here.

Using an I/O Completion Port for RIO completions

As I mentioned back in October, there are three ways to receive completion notifications from RIO; polling, event driven and via an I/O Completion Port. Using an IOCP for RIO completions allows you to easily scale your completion handling across multiple threads, though in this first IOCP example server we use a single thread so as to allow us to compare the performance against the polled and event driven servers. The next example server will adapt this server for multiple threads and allow us to scale our completion processing across more CPUs.

Creating an IOCP driven RIO completion queue

We start by initialising things in the same way that we did with the earlier example RIO servers.

int _tmain(int argc, _TCHAR* argv[])
   SetupTiming("RIO IOCP UDP");



   g_hIOCP = ::CreateIoCompletionPort(

   OVERLAPPED overlapped;


   completionType.Type = RIO_IOCP_COMPLETION;
   completionType.Iocp.IocpHandle = g_hIOCP;
   completionType.Iocp.CompletionKey = (void*)0;
   completionType.Iocp.Overlapped = &overlapped;

   g_queue = g_rio.RIOCreateCompletionQueue(

   if (g_queue == RIO_INVALID_CQ)

Once that is done we create an IO Completion Port and then create a RIO completion queue which uses the IOCP for notification. In this simple design we have no need for a completion key as we only have a single completion queue so there’s no need to differentiate between completion types. We also use a plain old OVERLAPPED rather than extending it to carry more information. More complex designs could use either the completion key, or an extended overlapped structure to pass queue specific information to our completion handler in much the same way that we do with normal IOCP server designs.

Creating a RIO request queue

Creating the request queue and posting our receives is identical to the polled example. The only difference is how we handle the completions.

Calling RIODequeueCompletion() and processing results

Processing completions is almost identical to processing event driven completions. We simply change the call to WaitForSingleObject() that we were using in the event driven example to the following to retrieve a completion notification from the IOCP.

   DWORD numberOfBytes = 0;

   ULONG_PTR completionKey = 0;

   OVERLAPPED *pOverlapped = 0;

   if (!::GetQueuedCompletionStatus(

Everything else is identical. Things change somewhat when we switch to using multiple threads for our completion handling.

The code for this example can be downloaded from here. This code requires Visual Studio 11, but would work with earlier compilers if you have a Windows SDK that supports RIO. Note that Shared.h and Constants.h contain helper functions and tuning constants for ALL of the examples and so there will be code in there that is not used by this example. You should be able to unzip each example into the same directory structure so that they all share the same shared headers. This allows you to tune all of the examples the same so that any performance comparisons make sense.

Join in

Comments and suggestions are more than welcome. I’m learning as I go here and I’m quite likely to have made some mistakes or come up with some erroneous conclusions, feel free to put me straight and help make these examples better.

Code is here

Code - updated 15th April 2023

Full source can be found here on GitHub.

This isn’t production code, error handling is simply “panic and run away”.

This code is licensed with the MIT license.