A parallel algorithm cannot be evaluated apart from the architecture it is implemented on. So, we define a parallel system as the combination of a parallel algorithm and a parallel architecture. The paper is devoted to the extension of well-known isoefficiency scalability metrics to heterogeneous parallel systems. Based on this extension the scalability of SUMMA (Scalable Universal Matrix Multiplication Algorithm) on parallel architecture with homogeneous communication system supporting simultaneous point-to-point communications is evaluated. Two strategies of data distribution are considered: (i) homogeneous – data are distributed between processors evenly; (ii) data are distributed between processors according to their performance. It is shown that under some assumption both strategies ensure the same scalability of heterogeneous parallel system. This theoretical result is corroborated with experiment.