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copied!<p>On Windows there is underlying OS support for non-blocking I/O, and Microsoft's CLR takes advantage of that. Other CLR implementations (mono) probably do as well, but I don't know for sure. When performing async I/O on the Microsoft CLR there is not a 1-to-1 correlation between pending async I/O operations and threads (or at least managed threads) waiting for those I/O operations to complete.</p> <p>See <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/aa365683(v=vs.85).aspx" rel="nofollow">http://msdn.microsoft.com/en-us/library/windows/desktop/aa365683(v=vs.85).aspx</a> for some details on the Win32-layer details. Also the information on I/O completion ports here: <a href="http://msdn.microsoft.com/en-us/library/aa365198(VS.85).aspx" rel="nofollow">http://msdn.microsoft.com/en-us/library/aa365198(VS.85).aspx</a></p> <p>My understanding is this:</p> <ol> <li>I begin an async I/O operation on some application thread.</li> <li>If it hasn't already, a queue will be created (well, really a kernel-level construct called an I/O completion port, which is associated with a queue in the kernel space of my application). In the .NET world a specially designated thread called an I/O completion port thread will start waiting for notifications of I/O completion on that queue. The important thing to note here is that I can make any number of async I/O requests without increasing the number of I/O completion ports.</li> <li>The OS will notify the application when I/O completes by enqueueing an I/O completion message on the queue. The I/O completion port thread will then handle that message by invoking the I/O completion callback in my .NET application. In the meantime, if other I/O completes, it's results will be enqueued behind the currently-processing results.</li> </ol> <p>Caveats to the above: </p> <ol> <li><p>I am sure I got part of this wrong, but I believe the overall gist of it is correct. Eric or someone can come in and correct me where I'm off.</p></li> <li><p>In .NET there are multiple I/O completion port threads. I have no idea how async I/O requests are allocated amongst the various I/O completion ports. This may be an operating system feature (wherein I/O may come back on any port that the application has open).</p></li> </ol> <p>For Java I am sure it depends on the JVM implementation and the particular OS. I don't know it nearly well enough to speculate beyond that.</p> <p>EDIT: Historical update, many more details <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/aa365198(v=vs.85).aspx" rel="nofollow">here</a></p>

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