Design of real-time systems needs to take into account their performance behaviour before their implementation. This goal can be attained by means of queueing network modelling. To process this kind of models normally simulation is used with its inherent costs of debugging and running times. This paper presents the analysis and comparison of several kinds of performance models of a basic software element used in the construction of real-time systems: the producer-consumer. These models go from the simple FIFO queue to the simulation of the complete element passing through some decomposition-aggregation approximate model. This model can be considered as an element of a library and the model of a real-time system can be built by putting together the models of the real-time components.