One of our clients uses WordPress extensively to develop their sites.
On a regular basis they have issues where one or two sites on a server will down the entire server.
In /var/log/messages, just before the outage, we see a number of "System running out of memory. Availability of the system is in risk"
Their servers typically have two (2) to four (4) GB of RAM; and typically they have less than a dozen sites on the server (they far from oversell).
How can I best help them trouble shoot this serious issue as to whether it is WordPress (and then what) or a module they are using with WordPress, etc.?