Http.sys Response Cache

Applies To: Windows Server 2003, Windows Server 2003 with SP1

Kernel caching with the Http.sys response cache can be one of the most effective means of scaling and improving Web server performance. Cached responses are served from the kernel, which greatly improves response times and increases the number of requests per second that IIS can serve because requests for cached content never enter IIS user mode.


The HTTP response cache is known as the kernel URI cache in System Monitor.

For each request, the IIS worker process tells Http.sys whether or not to cache a response based on the activity-period cache algorithm. If a file is requested twice within 10 seconds (the default value for the ActivityPeriod registry entry) the IIS worker process tells Http.sys to cache the full response by URI. All subsequent requests for the cached response will be served from the cache. For more information about the activity-period cache algorithm, see File Cache.

Every 120 seconds, the Http.sys response cache runs a flush algorithm, which flushes cached files that have not been requested within the 120-second interval. The flush algorithm is also called when IIS receives change notification for a file — that is, when the file has been edited or changed in some way.