If you have a few sites running on your server, and want to know which worker process/application pool is taking up the most memory. There is a very simple script you can run.

1.) Make sure you have the PID column showing in your processes tab.

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2.) Open the cmd window and type the following to get you into the 'inetsrv' folder

cd C:\Windows\System32\inetsrv

and then type the following

appcmd list wp

This will give you a list of all the worker processes with their PID's and show you which AppPool it relates to.