Issue 416072 and  issue 605842  mention background processes running on the machine and affecting the results of performance benchmarks. Ned suggested in  issue 605842  comment 21 to track whether something is running at the beginning and end of each perf test run.

I don't think it will be sufficient to merely diff the process list. Most background processes are always running, but are sleeping, waiting for a signal to wake up. We may have to run top or something to watch for CPU activity.