Efficiently programming shared-memory machines is a difficult challenge because mapping application threads onto the memory hierarchy has a strong impact on the performance. However, optimizing such thread placement is difficult: architectures become increasingly complex and application behavior changes with implementations and input parameters, e.g problem size and number of threads. In this work, we propose a fully automatic, abstracted and portable affinity module. It produces and implements an optimized affinity strategy that combines knowledge about application characteristics and the platform topology. Implemented in the back-end of our runtime system (ORWL), our approach was used to enhance the performance and the scalability of several unmodified ORWL-coded applications: matrix multiplication, a 2D stencil (Livermore Kernel 23), and a video tracking real world application. On two SMP machines with quite different hardware characteristics, our tests show spectacular performance improvements for these unmodified application codes due to a dramatic decrease of cache misses and pipeline stalls. A comparison to reference implementations using OpenMP confirms this performance gain of almost one order of magnitude.