Asymmetric Multiprocessing for Simultaneous Multithreading Processors

Simultaneous Multithreading (SMT) has become common in commercially available processors with hardware support for dual contexts of execution. However, performance of SMT systems has been disappointing for many applications. Consequently, many SMT systems are operated in a single-context configuration to achieve better average throughput, depending on the application domain. This paper first examines the performance of several benchmarks programs in both uniprocessor mode (one hardware thread context) and SMT mode (two contexts) on a Hyperthreaded Pentium 4. Based these results, three classes of applications can be distinguished: 1) those that prefer to execute in SMT, 2) those that prefer uniprocessor-mode, and 3) those that do not care. Our classification takes into account two aspects: (a) the performance of the application itself when sharing the system and (b) the performance of other applications whose concurrent execution is being affected. The paper introduces a novel bi-modal scheduler for Linux that continuously alternates between two distinct modes: SMT and UNI. The SMT mode uses the standard Linux SMP scheduler with its support for SMT while the UNI mode uses the standard uniprocessor scheduler, activating only one hardware context. Thus, the system alternates between operating as an SMT and operating as a uniprocessor even though its hardware is multithreaded. An application labeled as UNI will only execute in the UNI mode phase while SMT applications run concurrently. In addition, we provide a third class, called ANY, for processes that are agnostic with regard to SMT. Need a results sentence here!