A page template, such as default.aspx, is compiled into an assembly dynamic-link library (DLL) and loaded into memory just once per Web application. However, this page template and its efficient usage of memory can still be used to serve up pages for thousands of sites. This is an obvious advantage toward scalability.
When a user customizes a site page by using the SharePoint Designer and then saves those changes, a customized version of the page definition is stored in the content database. While this provides flexibility from a customization standpoint, it also can have a negative impact on performance and scalability. When the customized page is requested, its page definition must be retrieved from the Backend database server by the SPVirtualPathProvider component and then fed to the ASP.NET compiler, where it is parsed and loaded into memory. You can imagine that a Web application with thousands of customized pages requires more memory because each customized page definition must be separately parsed and loaded into memory within the application pool that is hosting the current Web application.
You should note that customized pages are not processed by using the standard ASP.NET model in which a page is compiled into an assembly DLL. Instead, customized pages are parsed by the ASP.NET page parser and then processed using the no-compile mode feature that was introduced with ASP.NET 2.0.
As a developer, your initial reaction to this might be to question why customized pages are processed in no-compile mode. Your instincts likely tell you that compiled pages run faster than no-compile pages. However, no-compile pages can be more efficient and more scalable in certain scenarios. This is especially true in a large WSS environment where the number of customized pages can reach into the thousands or tens of thousands
No-compile pages can be loaded into memory and then unloaded in a manner that is not possible for compiled pages because the .NET Framework doesn’t really support the concept of unloading an assembly DLL from memory. The closest equivalent would be to recycle the current Windows process or the current .NET AppDomain. However, this type of recycling involves unloading all assembly DLLs from memory, not just those assembly DLLs that haven’t been used recently. Furthermore, the .NET Framework places an upper limit on the number of assembly DLLs that can be loaded into a .NET AppDomain.
No-compile pages provide higher levels of scalability because they do not require loading new assembly DLLs or managed classes into memory. Instead, the processing of no-compile pages involves loading control trees into memory. WSS can manage the memory usage for the control trees associated with customized pages more efficiently because they are not compiled into assembly DLLs. For example, once WSS has finished processing a customized page, it can unload the page’s control tree to free up memory for other purposes. Furthermore, nocompile pages eliminate the need to go through the compilation process, which actually provides faster response times for pages upon first access.