Why async await doesn't working on IIS, but it's working on IIS Express

I don't understand, why async/await doesn't resolve problem with IIS threads. I see that we have limit on IIS threads equals 10, when I use IIS and no limits for IIS express. I add 2 methods in HomeController for repeat this problem. One of them (method) use Thread.Sleep and other use async/await. Of course I use logger (NLog) for describe this problem in more details. I use apache-jmeter-3.0 with 100 parallel request for one url as stress test. I was very surprised when the test execution time was approximately the same for IIS and not same for IIS express. Some code from HomeController ``` [SessionState(SessionStateBehavior.Disabled)] public class HomeController : Controller { private const int _waitTimeout = 30000; private const string _beginFmt = "Begin method; Count = {0}; OperId = {1};"; private const string _endFmt = "End method ; Count = {0}; OperId = {1};"; private static volatile int _counter = 0; private static Logger _logger = LogManager.GetCurrentClassLogger(); public JsonResult GetWithThreadSleep() { Guid operId = Guid.NewGuid(); _logger.Info(_beginFmt, Interlocked.Increment(ref _counter), operId); Thread.Sleep(_waitTimeout); _logger.Info(_endFmt, Interlocked.Decrement(ref _counter), operId); return Json(null, JsonRequestBehavior.AllowGet); } public async Task<JsonResult> GetWithTaskAwait() { Guid operId = Guid.NewGuid(); _logger.Info(_beginFmt, Interlocked.Increment(ref _counter), operId); await Task.Delay(_waitTimeout); _logger.Info(_endFmt, Interlocked.Decrement(ref _counter), operId); return Json(null, JsonRequestBehavior.AllowGet); } } ``` Log format ``` <target xsi:type="File" name="f" fileName="${basedir}/logs/${shortdate}.log" layout="${threadid} @ ${date:format=HH\:mm\:ss} @ ${level} @${aspnet-sessionid} @ ${message} @ ${callsite:className=False}" /> ``` Test scenario one: I send 100 parallel GET request to method GetWithTaskAwait and expect that the requests will be processed faster than 5 minutes, because thread should reused. But this is not the case. Test duration equals 5 minutes ``` 1 @ 23:43:54 @ Info @ @ MVC Count = 1; @ .ctor 1 @ 23:43:54 @ Info @ @ APP Count = 1; @ Application_Start 11 @ 23:44:06 @ Info @ @ MVC Count = 2; @ .ctor ... 13 @ 23:45:36 @ Info @ @ End method ; Count = 8; OperId = 28818fba-5535-4c01-9d6d-5efa3943cf37; @ MoveNext 19 @ 23:45:36 @ Info @ @ Begin method; Count = 9; OperId = c3cd3339-ec29-4625-adb9-62d5c72831b0; @ GetWithTaskAwait 12 @ 23:45:36 @ Info @ @ End method ; Count = 9; OperId = d1686c05-ca7a-400c-8c17-43dfb7b9218a; @ MoveNext 16 @ 23:45:36 @ Info @ @ Begin method; Count = 10; OperId = c1ec5517-649c-4733-9db3-e666e648abae; @ GetWithTaskAwait 13 @ 23:45:36 @ Info @ @ End method ; Count = 9; OperId = fbcfc9d9-fb84-456b-82a1-a0d593ef0815; @ MoveNext ... 26 @ 23:49:06 @ Info @ @ End method ; Count = 8; OperId = 766d4bc5-3739-4e20-a242-1f114dd44442; @ MoveNext 22 @ 23:49:06 @ Info @ @ End method ; Count = 7; OperId = 0ba18b30-11c9-49fe-8c94-40bbfa817e88; @ MoveNext 16 @ 23:49:06 @ Info @ @ End method ; Count = 9; OperId = 6d7afd57-6128-4bee-bf32-92de7cfc6a34; @ MoveNext 26 @ 23:49:06 @ Info @ @ End method ; Count = 6; OperId = 4c022744-9932-4f94-be1d-b982e36460f2; @ MoveNext 22 @ 23:49:06 @ Info @ @ End method ; Count = 5; OperId = 05476e6a-106f-4da5-b99b-098b90dbab89; @ MoveNext 16 @ 23:49:07 @ Info @ @ End method ; Count = 4; OperId = 3aba1ad2-56b0-4cf5-8aee-7e918eca9fba; @ MoveNext 27 @ 23:49:07 @ Info @ @ End method ; Count = 3; OperId = ef972cdd-7992-4816-9452-ecabaf189767; @ MoveNext 15 @ 23:49:07 @ Info @ @ End method ; Count = 2; OperId = 9a0f735e-bf6b-4217-b2ef-8abd9ce01d27; @ MoveNext 30 @ 23:49:07 @ Info @ @ End method ; Count = 0; OperId = cc75b44a-4e8d-4534-9906-40bddc027267; @ MoveNext 33 @ 23:49:07 @ Info @ @ End method ; Count = 1; OperId = bbef36b3-051b-444d-858a-2766bcfcd12b; @ MoveNext ... 26 @ 23:49:49 @ Info @ @ MVC Count = 10; @ Dispose 30 @ 23:50:19 @ Info @ @ MVC Count = 9; @ Dispose 16 @ 23:50:49 @ Info @ @ MVC Count = 8; @ Dispose ``` Test scenario two: I send 100 parallel GET request to method GetWithThreadSleep and expect that the requests will be processed equal 5 minutes, because thread shouldn't reuse. ``` 1 @ 00:15:41 @ Info @ @ MVC Count = 1; @ .ctor 1 @ 00:15:41 @ Info @ @ APP Count = 1; @ Application_Start 11 @ 00:16:14 @ Info @ @ MVC Count = 2; @ .ctor ... 15 @ 00:16:15 @ Info @ @ Begin method; Count = 8; OperId = cedb5075-405c-4c23-93cb-388dc34537ba; @ GetWithThreadSleep 17 @ 00:16:15 @ Info @ @ MVC Count = 10; @ .ctor 17 @ 00:16:15 @ Info @ @ Begin method; Count = 9; OperId = 777d7464-f900-48c1-991f-ef8c8f7e6e86; @ GetWithThreadSleep 18 @ 00:16:16 @ Info @ @ MVC Count = 11; @ .ctor 18 @ 00:16:16 @ Info @ @ Begin method; Count = 10; OperId = f9b4b717-94a5-4196-958f-c1234d19514c; @ GetWithThreadSleep 12 @ 00:16:45 @ Info @ @ End method ; Count = 8; OperId = 9e40acd4-888f-4656-a5a3-c5a1d210cc88; @ GetWithThreadSleep 5 @ 00:16:45 @ Info @ @ End method ; Count = 9; OperId = b068e03e-37e9-446e-ad42-c9ab4c30da93; @ GetWithThreadSleep 11 @ 00:16:45 @ Info @ @ End method ; Count = 7; OperId = 22e510e5-62f7-4bc8-8ab0-fdeb3080e250; @ GetWithThreadSleep ... 18 @ 00:21:15 @ Info @ @ End method ; Count = 2; OperId = 169ef436-6b27-4f4c-a3ca-0b18a5a796cf; @ GetWithThreadSleep 13 @ 00:21:16 @ Info @cubgm2zsojblkhrwwpixaaej @ End method ; Count = 1; OperId = ee346be8-17c5-45ac-88b5-67e724250bbf; @ GetWithThreadSleep 12 @ 00:21:16 @ Info @ @ End method ; Count = 0; OperId = 68d5deaa-95d6-4627-a129-144ffafcc2ef; @ GetWithThreadSleep 13 @ 00:22:11 @ Info @ @ MVC Count = 10; @ Dispose 9 @ 00:22:41 @ Info @ @ MVC Count = 9; @ Dispose ``` Important information: - IIS version 10.0.14393.0 - .NET Framework 4.5.1 - Windows 10 Pro You can download full solution from GitHub