A method, apparatus, and computer program product are disclosed in a data processing system for sharing data in a cache among multiple threads in a simultaneous multi-threaded (SMT) processor. The SMT processor executes multiple threads concurrently during each clock cycle. The cache is dynamically allocated for use among the multiple threads. Portions of the cache are capable of being designated to store private data that is used exclusively by only a first one of the threads. The portions of the cache are capable of being designated to store shared data that can be used by any one of the multiple threads. The size of the portions can be changed dynamically during execution of the threads.