Cuzz is a very effective tool for finding concurrency bugs. Cuzz works on unmodified executables and is designed for maximizing concurrency coverage for your existing (unmodified) tests. It randomizes the thread schedules in a systematic and disciplined way, using an algorithm that provides probabilistic coverage guarantees.
Cuzz is very scalable and can run on large programs that create lots of threads. It is available as part of AppVerifier. You can find out more about Cuzz from this video or the paper below.