The FastCGI module in IIS has a configuration setting maxInstances , which controls the number of concurrently running php-cgi.exe processes that IIS creates to process PHP requests. To achieve optimal performance, it is recommended to fine tune this
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Read the complete post at http://blogs.iis.net/ruslany/archive/2009/09/02/windows-cache-extension-for-php-and-cpu-usage.aspx