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:

  • IIS 6 isn't limiting the number of concurrent requests. The help file reads: You can configure Internet Information Services (IIS) to allow an unlimited number of concurrent connections, or to limit the number of connections it accepts for this Web site. We currently have this set to unlimited.
  • Both IIS 6 and IIS 8.5 are using ASP.NET 4 which also has the ability to limit the number of concurrent requests. Both IIS versions are set to auto-config in the machine.config file; since both servers have the same processor and RAM configuration, they should be using the same settings.

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.


Did you try running the application pool under classic-mode (IIS 6 architecture) vs integrated mode while under IIS 8.5? Maybe something with the new integrated pipeline model affected the transition.

Were you seeing requests queued during the event?

Have you tuned your IIS settings for requests and connections? Machine.config's processModel section.

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