What is the impact of async action method on IIS? <p>I need help in understanding how async action method can improve performance of IIS (and eventually my application).</p> <p><strong>First I tried to create Server Unavailable (503) with following code and setup</strong></p> <ul> <li>Application Queue Length: 10</li> <li>IIS - Threads per processor limit: 2</li> <li>Processors: 4</li> </ul> <p><a href="https://i.stack.imgur.com/KZ5FR.png" rel="nofollow"><img src="https://i.stack.imgur.com/KZ5FR.png" alt="enter image description here"></a></p> <p>Sent 20 requests with SOAP UI and when tried from the browser, the browser showed 503, which is what I expected.</p> <p>Then I made following changes in code</p> <p><a href="https://i.stack.imgur.com/JCwkm.png" rel="nofollow"><img src="https://i.stack.imgur.com/JCwkm.png" alt="enter image description here"></a></p> <p>Sent 20 requests again with SOAP UI and when tried from the browser, the browser again showed 503, which I wasnt expecting.</p> <p><strong>Wouldn't the async and await release the thread to accept new connections?</strong> </p> <p>Is my understanding correct on following</p> <ol> <li>Maximum number of concurrent requests the server can accept at any given time is : (IIS Threads per processor limit * Number of processors + Application Queue Length) 18 in my case?</li> <li>19th request would get a 503 in case of synchronous action methods?</li> </ol>