In stress testing, start out with a light load and load the system along some dimension - such as input rate or input size - until you hit the wall. If the wall is too close to satisfy your needs, figure out which resource is the bottleneck (there is usually a dominant one.) Is it memory, processor, I/O, network bandwidth, or data contention? Then figure out how you can move the wall. Note that moving the wall, that is, increasing the maximum load the system can handle, might not help or might actually hurt the performance of a lightly loaded system. Usually performance under heavy load is more important than performance under a light load.