Multicore technology provides a way to improve the performance of embedded systems in response to the demand in many domains for more and more complex functionality. However, increasing the number of processing units also introduces the problem of deciding which task to execute on which core in order to best utilize the platform. In this paper we present a model-based approach for automatic allocation of software tasks to the cores of a soft real-time embedded system, based on design-time performance predictions. We describe a general iterative method for finding an allocation that maximizes key performance aspects while satisfying given allocation constraints, and present an instance of this method, focusing on the particular performance aspects of timeliness and balanced computational load over time and over the cores.