SERVER-16829 exposed tcmalloc_aggressive_decommit via the setParameter framework. This has turned out to be not very useful, as aggressive decommit comes with a huge performance penalty. (-75% in a small YCSB workload.)
We recently found that tcmalloc_release_rate can be used to reduce memory overhead, with little or no performance overhead. We should add a setParameter knob to do this at runtime too.