Open-loop versus closed-loop control of processor loading

Abstract

In many processing systems, the goodput (the number of succesfully completed tasks per time unit) may decrease as the offered load increases. Such systems need some form of overload control. We distinguish between open-loop control that limits the number of tasks in the system with a fixed maximum and closed-loop control that rejects new tasks if the observed processor loading is too high. In most control systems closed-loop control performs better than open-loop control. Here we present a remarkable illustration of the converse: we show that higher goodput can be obtained with an open-loop control-law. The results are obtained with a simplified analytical model of a multi-tasking processing system, such as the central processor of a telephone exchange. Compared to open-loop control, closed-loop control is more robust with respect to a change in task idle-time. Under conditions of overload, the mentioned differences between the two types of control law become more pronounced. The computed results are confirmed by the outcome of simulation runs. In the simulations we used a more realistic definition of goodput than was used in the analytical model.