We upgraded our web server from Windows Server 2003 to Windows Server 2012 R2. The web application in question runs against ASP.NET 4.0. The web application in turn communicates with a third-party Web API.
Shortly after the upgrade, the Web API latency increased, which in turn lead to notable timeouts. I suspected that on IIS 8.5, the number of allowed concurrent requests increased, causing an increase in throughput that the Web API was unable to handle. However:
When we rolled back the upgrade, the latency shortly after dropped. Unlikely a coincidence, so with everything else remaining the same, there must be something intrinsic about Windows 2012 R2 or IIS 8.5 that's affecting the Web API. The 3rd party Web API developers confirmed that nothing had changed in their space, and unfortunately there isn't any additional information that I can gather.
I checked the IIS logs for both version 6 and 8.5: the average (and median) requests per second, minute, and hour indicate there were more requests being processed on IIS 8.5 (I used Log Parser Studio to analyse the logs). However, there is nothing to indicate that this should be the case. Does anyone have any insight or suggestions? If there are settings controlling this, then for now I'd like the web application to behave in the same on Windows Server 2003 and 2012 R2.
Since each new request is processed on a new worker thread from the thread pool, I queried the thread pool limits to verify that the auto-configured settings match (aspx file):
<%@ Page Language="C#" %>
<%@ Import Namespace="System.Threading" %>
<html>
<body>
<div>
<%
int workers;
int io;
ThreadPool.GetMaxThreads(out workers, out io);
%>
The maximum number of worker threads is <%=workers.ToString()%> and
the maximum number of IO threads is <%=io.ToString()%>
</div>
</body>
</html>
The results are as follows (the virtual machine had 2 cores):
This implies that the newer setup is able to handle more requests concurrently.