The common third level trigger and forth level reconstruction farm for the future HERA-B experiment will have to perform full on-line event reconstruction and calibration for an expected input rate of 2000 events/s. More than a hundred powerful RISC processors connected in a network capable of distributing several hundreds of MB/s with low latency are likely to be necessary for this task. Proper simulation of the real time multi-processor systems is central for an optimal design (hardware and software protocol) of a scalable and flexible parallel data processing architecture. A discrete event, process oriented simulation developed in concurrent μC++ is used as a framework for modelling and evaluating different farm architectures. An object oriented graphic interface to the simulation allows the monitoring of various features and provides an easier way to optimize the system.