IIS 8 What’s new – Application pool settings

Here is a list of enhancements to Application Pools in IIS 8.

Start Mode

A few web applications take a significant amount of time to start up. IIS by default only launches a worker process when the first request for the web application is received. So for the web applications that require a longer time to initialize, users might see slow responses.

For such applications it is a good idea to launch the worker process as soon as IIS is started. The application pools have a startMode setting which when set to AlwaysRunning launches the worker process for the application pool as soon as IIS is started.

IIS 8 provides you this setting in the Application Pool Settings UI.


This feature is also available in IIS 7.5, but you will have to edit the applicationHost.config directly to enable this feature.

    <add name="DefaultAppPool" startMode="AlwaysRunning" />

CPU Throttle

IIS 7.5 and earlier have a CPU monitoring feature that you could use to terminate a worker process, when its CPU usage goes beyond a certain limit. Its very useful in shared hosting scenarios.


In IIS 8 this feature has been enhanced. You can now instead of terminating the worker process, throttle it.


The KillW3wp will terminate the worker process like IIS 7.5. But if set to Throttle or ThrottleUnderLoad instead of terminating the worker process it will limits its CPU usage to the value set.

This is an application pool setting, so the limit you set applies to all the worker processes (web garden) of an application pool. So for an application pool with 5 worker processes, if the value for limit is set to 50000 the combined CPU usage of all the 5 worker processes is throttled at 50%.

Process Affinity Mask (64 Bit)

The UI now also exposes the smpProcessorAffinityMask2 metabase key. This is used when you enable the Processor Affinity setting.


Generate Process Model Event Log Entries

The logEventOnProcessModel metabase setting controls if an event has to be written when an application pool is shutdown because it was idle.


            <add name="DefaultAppPool" startMode="OnDemand">
                < processModel logEventOnProcessModel="IdleTimeout" />

When set to True an event similar to the one below is logged in the System Event Log when WAS shutdown the application pool.


Comments (3)

  1. Chris Marisic says:

    Why is  processModel logEventOnProcessModel="IdleTimeout"  not enabled by default? Being an ASP.NET dev i've seen hundreds of issues/questions all stem from app pool recycling and this seems like something that would really really really always want to be on. I can't even think of a single use-case you would want it off!

  2. Rosemari says:

    My profile picture got deleted and won't let me upload my profile picture.

  3. Tom A - says:

    I know this blog is a bit old, but I have been looking for someone who can describe the throttle behavior if you have: say 3 (or more) app pools all set to 50% throttle-under-load – and then all pools need as much cpu as possible?  I have iis 8/.5 not 8, but should behave same.

    I would also have been nice to have the killwp still included it, say the wp stayed at the max for a given amount of time.  

Skip to main content