MemoryLimit Tuning (ASP.NET v1.1)
IIS 6.0 replaces the MemoryLimit machine.config setting from ASP.NET on Windows 2000 with IIS Manager UI configuration when running on Windows 2003. By default, under Windows 2003, the Memory Recycling feature is turned off. By enabling memory recycling, specifically Maximum used memory, you can keep a healthy memory footprint for your ASP.NET application.
What to set it to?
In the machine.config the default value for MemoryLimit is 60% of physical memory. Although matching that value under Windows 2003 is a good start, tuning based on your specific performance requirements and hardware will allow you to both, keep from swapping W3WP.EXE memory to disk and reaching a common ASP.NET critical mass of 800MB*. Alternatively, setting the Maximum used memory too aggressively will first, force System.Web.Caching.Cache items to be cleared using LRU when Private Bytes reaches 80% of the MaxMemory value and second, recycle the worker process excessively.
What to measure?
Process \ Private Bytes: W3WP.EXE - If exceeds 60% of physical memory and Memory \ Pages /sec are high (> 5/sec) then set the Maximum used memory property to less than 60% of physical memory. If the server fills multiple roles you may need to decrease the value until Pages/sec decrease.
After setting a value for MaxMemory monitor ASP.NET Apps v1.1.4322 \ Cache API Turnover Rate and
ASP.NET v1.1.4322 \ Application Restarts for increases over your acceptable baselines.
There will be a sweet spot between Pages/sec, Cache API Turnover Rate and Application Restarts that will be optimized for performance. I recommend setting Performance Alerts for these counters for increases to monitor change due to load increase or feature changes.
* If the /3GB option is enabled in the boot.ini the common ASP.NET critical mass is increased to 1.2GB
Resources
Improving ASP.NET Application Performance: Chapter 17
Improving ASP.NET Application Performance: Chapter 15
ASP.NET Performance Monitoring, and When to Alert Administrators
http://www.asp.net/learn/whitepapers/aspnet-and-iis6/#2
http://blogs.msdn.com/rayesc/archive/2004/12/10/279399.aspx