Windows Server 2012 : Scalable and elastic web platform (part 4) - IIS CPU throttling

CPU throttling configuration

CPU throttling has been included in prior versions of IIS, but for IIS 8.0, it has received a major reworking under the hood.

In earlier versions of IIS, a polling mechanism was used to check the CPU usage periodically and take action if it was above the configured threshold for a long enough time. The problem with this approach is that CPU usage wasn’t truly limited—it could increase far beyond the configured limit and remain high for a period of time before the polling mechanism noticed. When the CPU was determined to be above the threshold, the only “corrective” action available was to kill the IIS worker process (W3pw.exe). When the process was killed, IIS also prevented a new process from being started for the offending application for a period of time so that it would not immediately come back and take over the CPU again. Any requests to the application during that time would fail, resulting in a poor user experience.

For IIS 8.0, we worked with the Windows Kernel team to implement true throttling of CPU usage. In place of the old polling design, the kernel will now ensure that CPU usage stays at the configured level. With this change, we no longer need to kill the W3WP process to halt an offending application, so the application stays active and responsive to user requests even when it is being throttled.

There are two new options for how CPU throttling works in IIS 8.0. The Throttle configuration option will keep the CPU near the configured limit at all times. The ThrottleUnderLoad configuration option will keep the CPU near the configured limit when there is contention for CPU resources, but it will let it consume more CPU if the server would otherwise be idle. In this model, once other processes need additional CPU resources, the IIS worker process are throttled to ensure that the other processes get the resources they need.