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What is ASP Caching?

The ASP Template Cache stores templates: pre-compiled ASP pages in text format (#includes have been evaluated, and so on). Its size is governed by the AspScriptFileCacheSize setting in the metabase, which defaults to 250. The ASP Script Engine Cache holds ASP templates that have been compiled into byte code. Its size is governed by the AspScriptEngineCacheMax setting in the metabase, which defaults to 125. The relationship between the two is that an ASP page is cached once in the template cache, but it can appear many times in the script engine cache if it is being executed on many threads simultaneously. A site with a lot of memory and a lot of distinct ASP pages that get hit often will probably want to increase AspScriptFileCacheSize (monitor ASP counters with System Monitor to diagnose). There is much less need to increase AspScriptEngineCacheMax; the main reason would be that the defaults are inadequate for machines with 8 or more processors. AspScriptEngineCacheMax should have a metabase value that is equal to or greater than the number of CPUs plus one, multiplied by AspProcessorThreadMax. AspProcessorThreadMax defaults to 25.

Every process that hosts ASP will have its own ASP Template and Script Engine Caches. By default that is just one process because ASP applications run at medium isolation in the pooled Dllhost process.

When ASP gets a request for a page, it checks the ASP Template Cache first. If there’s an instance of that page cached there, the request is forwarded to the Script Engine Cache. If the requested page is not in the Template Cache, it is compiled into a template and forwarded to the ASP Script Engine Cache. If an instance of the page is cached in the Script Engine Cache and it is ready to run, that engine is run. Failing that, if there is an instance of the page that is already executing, ASP clones that running engine and executes the clone. This avoids the cost of re-parsing the template into byte code. If there is no script engine associated with the page, ASP takes the precompiled template from the ASP Template Cache, creates a new script engine and has it compile the template into byte code, and then executes it. When a page finishes executing, the script engine is put at the head of the free list. If the free list has grown larger than AspScriptEngineCacheMax, the least-recently used script engine is destroyed. A hit in the script engine cache means that ASP can avoid recompiling the template into byte code.

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