Exploiting full computational power of hierarchical multiprocessor machines with irregular multithreaded applications requires a very careful distribution of threads and data. To achieve most of the available performance, programmers often have to forget about portability and wire down ad hoc placement strategies that highly depend on the architecture. To guarantee the portability of performance, we have defined abstractions called ``bubbles'' for capturing both the hierarchical structure of the application's parallelism, and the hierarchical architecture of the targeted machine. We have defined a set of high level primitives to ease the implementation of dedicated, efficient and portable schedulers. We show the relevance of our approach and describe the mechanisms we developped for easily implementing such schedulers.