  The universal scalability law (USL) is an analytic model used to quantify
application scaling. It is universal because it subsumes Amdahl's law and
Gustafson linearized scaling as special cases. Using simulation, we show: (i)
that the USL is equivalent to synchronous queueing in a load-dependent machine
repairman model and (ii) how USL, Amdahl's law, and Gustafson scaling can be
regarded as boundaries defining three scalability zones. Typical throughput
measurements lie across all three zones. Simulation scenarios provide deeper
insight into queueing effects and thus provide a clearer indication of which
application features should be tuned to get into the optimal performance zone.
