| OCR Text |
Show "We estimate that the ratio in processing power should at least be 10-15 if the system were to be used practically." Since we want to cascade directors, the difference in processing power between the executor and the director is unacceptable. The requirement for the difference in processing power can be eliminated by a filter. The filter sits between the executor and the director. It is a special purpose circuit that is able to perform certain filtering functions in real time. Filtering functions cover most common filtering tasks. These functions significantly reduce the work performed by the monitor. Since filtering functions cover only a restricted set of all possible monitoring functions, a monitoring task is divided in a task performed by the filter and a task performed by the director. The amount of work in monitoring depends on the speed of the executor and the complexity of the monitoring task. The speed of the executor is the number of events generated by the executor in a time interval. Since the executor generates one event for each executed instruction, the number of events is equal to the number of executed instructions. The complexity of a monitoring task depends on the part of the monitoring task that is performed by the filter. This part is executed in real time, since the filter throughput matches the speed of the executor. If the filter sufficiently reduces the amount of work for the director, then the entire monitoring task can be performed in real time. The complexity of a monitoring task is thus dependent on the functionality of the filter. The remaining part of the section describes the design of filters. Monitoring involves the evaluation of Boolean functions based on the real time behavior of the system being monitored [22, 54, 68, 69]. The problem is how to develop an efficient approach to formulate and evaluate an arbitrary Boolean expression in real time. Some solutions [22, 54] to this monitoring problem include using a fixed set of Boolean expressions (fixed hardware tools), the manual rewiring of the monitor 120 |
