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The Liberty Research Group

Pipelined Multithreading Transformations and Support Mechanisms [abstract] (PDF)
Ram Rangan
Ph.D. Thesis, Department of Computer Science, Princeton University, June 2007.

Even though chip multiprocessors have emerged as the predominant organization for future microprocessors, the multiple on-chip cores do not directly result in improved application performance (especially for legacy applications, which are predominantly sequential C/C++ codes). Consequently, parallelizing applications to execute on multiple cores is essential to their success. Independent multithreading techniques, like DOALL extraction, create partially or fully independent threads, which communicate rarely, if at all. While such strategies keep high inter-thread communication costs from impacting program performance, they cannot be applied to parallelize general-purpose applications which are characterized by difficult-to-break recurrences. Even though cyclic multithreading techniques, such as DOACROSS, are more applicable, the cyclic inter-thread dependences created by these techniques cause them to have very low tolerance to rising inter-core latencies.

To address these problems, this work introduces a pipelined multithreading (PMT) transformation called Decoupled Software Pipelining (DSWP). DSWP, in particular, and PMT techniques, in general, are able to tolerate inter-core latencies, while still handling codes with complex recurrences. They achieve this by enforcing an acyclic communication discipline amongst threads, which allow threads to use inter-thread queues to communicate in a pipelined fashion. This dissertation demonstrates that DSWPed codes not only tolerate inter-core communication costs, but also effectively tolerate variable latency stalls in applications better than single-threaded execution on both in-order and out-of-order issue processors with comparable resources. It then performs a thorough analysis of the performance scalability of automatically generated DSWPed codes and identifies the conditions necessary to achieve peak PMT performance.

Next, the dissertation shows that even though PMT applications tolerate inter-core latencies well, the high frequency of inter-thread communication (once every 5 to 20 dynamic instructions) in these codes, makes them very sensitive to the intra-thread overhead imposed by communication operations. In order to understand the issues surrounding inter-thread communication for PMT applications, this dissertation undertakes a methodical exploration of the design space of communication support options for PMT. Three new communication mechanisms with varying cost-performance tradeoffs are introduced and are shown to perform 38% to 200% better than the state of the art.