Measuring limits of parallelism and characterizing its vulnerability to resource constraints
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This paper addresses a two-fold question: whether there is enough parallelism in numeric and non-numeric workloads, such as the SPEC92 benchmark suite, under ideal conditions, disregarding any resource constraints and more importantly, whether a high ideal parallelism can be further characterized to assess its extractability with finite resources. We have designed and implemented an analysis tool that accepts as input a dynamic execution trace from an IBM RS/6000 environment, and outputs a parallelized instruction trace (schedule) that could be executed on an abstract machine with unlimited functional units and various constraints on the rest of its resources, namely, registers, stack and memory. We also analyze two different instruction scheduling policies: greedy and lazy. This paper further offers a characterization of ideal parallelism (obtainable on a machine with infinite resources) using a measure called slack to assess its sustainability with finite resources. Most of the new floating-point additions to SPEC92 (except ora) offer quite high limits of ideal (oracle) parallelism. But this very high parallelism is mostly associated with not very high average slack, suggesting the vulnerability of this parallelism to resource constraints. While register renaming is much more important for all the programs than stack or memory renaming, for quite a few programs renaming of stack and memory, in addition to the registers, offers a significant performance boost.
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Proceedings of the 26th Annual International Symposium on Microarchitecture
