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Proceedings.International Conference on Parallel Architectures and Compilation Techniques (2002)
Charlottesville, Virginia
Sept. 22, 2002 to Sept. 25, 2002
ISSN: 1089-795X
ISBN: 0-7695-1620-3
pp: 30
Gautham K. Dorai , University of Maryland at College Park
Donald Yeung , University of Maryland at College Park
<p>Simultaneous Multithreading (SMT) processors achieve high processor throughput at the expense of single-thread performance. This paper investigates resource allocation policies for SMT processors that preserve, as much as possible, the single-thread performance of designated "foreground" threads, while still permitting other "background" threads to share resources. Since background threads on such an SMT machine have a near-zero performance impact on foreground threads, we refer to the background threads as transparent threads. Transparent threads are ideal for performing low-priority or non-critical computations, with applications in process scheduling, subordinate multithreading, and on-line performance monitoring.</p> <p>To realize transparent threads, we propose three mechanisms for maintaining the transparency of background threads: slot prioritization, background thread instruction-window partitioning, and background thread flushing. In addition, we propose three mechanisms to boost background thread performance without sacrificing transparency: aggressive fetch partitioning, foreground thread instruction-window partitioning, and foreground thread flushing. We implement our mechanisms on a detailed simulator of an SMT processor, and evaluate them using 8 benchmarks, including 7 from the SPEC CPU2000 suite. Our results show when cache and branch predictor interference are factored out, background threads introduce less than 1% performance degradation on the foreground thread. Furthermore, maintaining the transparency of background threads reduces their throughput by only 23% relative to an equal priority scheme.</p> <p>To demonstrate the usefulness of transparent threads, we study Transparent Software Prefetching (TSP), an implementation of software data prefetching using transparent threads. Due to its near-zero overhead, TSP enables prefetch instrumentation for all loads in a program, eliminating the need for profiling. TSP, without any profile information, achieves a 9.52% gain across 6 SPEC benchmarks, whereas conventional software prefetching guided by cache-miss profiles increases performance by only 2.47%.</p>

D. Yeung and G. K. Dorai, "Transparent Threads: Resource Sharing in SMT Processors for High Single-Thread Performance," Proceedings.International Conference on Parallel Architectures and Compilation Techniques(PACT), Charlottesville, Virginia, 2002, pp. 30.
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