Scheduling in Linux: run a task when computer is idle (= no user input)

When the machine in question is a desktop, you could hook a start/stop script into the screensaver so that the process is stopped when the screensaver is inactive and vice versa.

It's fiddly to arrange for the process to only be present when the system is otherwise idle.

Actually starting the program in those conditions isn't the hard bit. You have to arrange for the program to be cleanly shut down, and figure out how and when to do that.

You have to be able to distinguish between that process's own CPU usage, and that of the other programs that might be running, so that you can tell whether the system is properly "idle".

It's a lot easier for the process to only be scheduled when the system is otherwise idle. Just use the 'nice' command to launch the Folding@Home client.

However that won't solve the problem of insufficient RAM. If you've got swap space enabled, the system should be able to swap out any low priority processes such that they're not consuming and real resources, but beware of a big hit on disk I/O each time your Folding@Home client swaps in and out of RAM.

p.s. RAM is very cheap at the moment...

p.p.s. see this article

may be You need to set on idle task lowest priority via nice.