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Appears in 29th International Symposium on Computer Architecture Slack: Maximizing Performance Under Technological Constraints

By Brian Fields and Mark D. Hill


Abstract Many emerging processor microarchitectures seek tomanage technological constraints (e.g., wire delay, power, and circuit complexity) by resorting to non-uniform designs that provide resources at multiple quality levels (e.g., fast/slow bypass paths, multi-speed func-tional units, and grid architectures). In such designs, the constraint problem becomes a control problem, andthe challenge becomes designing a control policy that mitigates the performance penalty of the non-uniformity.Given the increasing importance of non-uniform control policies, we believe it is appropriate to examine them intheir own right. To this end, we develop slack for use in creating con-trol policies that match program execution behavior to machine design. Intuitively, the slack of a dynamic in-struction i is the number of cycles i can be delayed withno effect on execution time. This property makes slack a natural candidate for hiding non-uniform latencies.We make three contributions in our exploration of slack. First, we formally define slack, distinguishthree variants (local, global and apportioned), and perform a limit study to show that slack is prevalentin our SPEC2000 workload. Second, we show how to predict slack in hardware. Third, we illustrate howto create a control policy based on slack for steering instructions among fast (high power) and slow (lowerpower) pipelines. 1 Introduction Recent years have witnessed a proliferation of technol-ogy constraint-aware design proposals. For example, physical clustering of functional units has attacked wiredelays [7, 8], multi-frequency functional-units have addressed power consumption [13], and grid architectureshave sought to reduce cycle time [11]. More importantly, it appears that wire, power, and circuit-complexity trendswill make constraint-aware designs even more prevalent in the future.A challenging feature of many constraint-aware designs is that they introduce non-uniformity, where one o

Year: 2009
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