You might have experienced the problem that when you first load a large ASP.NET application (e.g. a CMS), it takes quite some time to load. If you then reload the page or even restart the browser and load the page again, it’s quite fast. There seems to be a few possible cause for this. So this post will not give you an extensive answer as to what all could go wrong but focus on application pool recycling.

What is an Application Pool?

Application Pools are used by IIS to isolate web applications. It allows you to use different settings in different applications. This is especially important when your different applications use different security settings. Applications in different application pools run in different windows processes (w3wp.exe) and thus also provide some kind of segregation by preventing an application from interfering with other applications. So an error in one application won’t cause other applications to crash or behave unexpectedly.

What is Application Pool Recycling?

Application pool recycling basically means that when a given event occurs, the resources related to an application pool will be recovered. It is mainly useful to prevent long running application from crashing or hanging e.g. because of memory leaks. When the recycling happens, a new worker process is started which will receive new web requests and the old one is shut down (once it’s done processing current requests), so there is no downtime while recycling.

Why does recycling cause the first load to last longer ?

Many applications need to first compile scripts or DLLs on the fly. Others need to load a large amount of data and pre-process it. This is basically what’s causing the delay on the first load. Afterwards once everything is compiled and data pre-processed, everything is fast.

Whenever an application pool is recycled and the worker processes are shutdown, next time the application is called, a new worker process needs to be started and these compilation and pre-processing activities need to be performed again.

When does recycling happen?

There are basically two different ways to recycle application pools:

1. Recycling based on configuration
2. On-demand recycling

On-demand recycling can be done:

• in the IIS manager
• from the command line using appcmd
• using Windows Management Instrumentation (WMI) i.e. the ApplicationPool.Recycle method

Automatic application pool recycling can be configured to recycle:

• when reaching a memory threshold (virtual memory or used memory)
• when reaching a certain number of requests
• at a scheduled time
• after a scheduled time

Additionally, recycling will happen, when the configuration of an application is changed or when an unhealthy ISAPI condition is reported.

How to configure when automatic application pool recycling is done?

This can be configured in the Application Pools configuration panel of the IIS manager (Go to Start -> Administrative Tools -> Internet Information Services (IIS) Manager):

Select an application pool on the left-hand side and click on Recycling on the right.

The default configuration is to recycle an application pool after 29 hours (1740 minutes):

If there are long periods of time during which your site/application has few hits, it might be a good idea to increase this default value. Actually, except if you know that you have a memory leak somewhere and do not manage to fix it, there is no big benefit in recycling your application pools at fixed intervals. If your application is running on a server where other applications are running, you may want to used the memory based maximums. Otherwise just removing the time interval based configuration may be a good solution. If nothing else it configured, you application pool will be recycled anyway once all available memory is used.

Idle Time-out

Also note that even though strictly speaking, it’s not an automatic application pool recycling, you can configure the application pool so that worker processes are shutdown after a specified amount on time being idle i.e. not receiving new requests or processing existing requests. The default value is 20 minutes. Actually I guess in most cases it makes no sense. It’s only useful if you have so limited resources that you need to get rid of unused application pools as fast as possible. So in most cases, I’d recommend setting the idle time-out to zero:

How do I know when recycling has occurred?

There are two places where you can configure under which conditions an event log entry is generated when an application pool is recycled.

The first one is when you press “Next” in the dialog shown above. You will then see the following:

The checkbox under “Configurable recycling events” are disable in this screenshot because I haven’t configured any  trigger for an automatic application pool recycling. But if you defined e.g. a number of requests, some checkboxes will be enabled.

Another place where you can set it is the Advanced Settings dialog. Just select your application pool and press “Advanced Settings”. The following dialog will be displayed:

The recycling settings are the bottom and contain a section called “Generate Recycle Event Log Entry”. You can then set the appropriate entries to true to get the required event log entries.

Also remember that starting from Windows 2008, you can have an email sent when an event is triggered.