I am the unpopular guy who has brought down our high-memory linux machine twice this past week because some processes I have been running ate up way more ram then expected. One way around this would be to set a per process ulimit so if each process goes over a certain amount of ram usage that process will be killed. Sometimes though I need lots of ram for a single process rather than some ram spread out over a bunch of parallel processes. Rather than relying on me remembering to actively manage my ulimits whenever I run a different kind of job at a different level of parallelism, is there some kind of equivalent to ulimit that looks at the summed ram usage for a user, and then kills all or some of that user's processes rather than sinking the whole system?
This is almost exactly why Linux control groups (cgroups) were invented. This allows a group of processes (for example, all the processes decending from your initial interactive login) to be treated as a single entity for purposes of resource control -- such as limiting the total amount of memory consumed (or cpu resources, etc).
Cgroups are relatively new, and convenient support for them is largely missing in most distributions.