How to constrain the resources an application can use on a linux web server

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http://unix.stackexchange.com – This is the situation: I have a (php/mysql) web application that does some pdf processing and thumbnail creation. This is done by using some 3rd party command line software on the server. Both kinds of processing consume a lot of resources, to the point of choking the server. I would like to limit the amount of resources these applications can use in order to enable the server to keep serving users without too much delay, because now when some heavy pdf is processed my users don't get any response. Is it possible to constrain the amount of RAM and CPU an application can use (all processes co (HowTos)