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Ziavras 3 3 focus on the common case common sense

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Unformatted text preview: adapted from D. Culler & D. Patterson (UCB) Larger Lower Level S. Ziavras 3) 3) Focus on the Common Case • Common sense guides computer design – Since it’s engineering, common sense is valuable • In making a design trade-off, favor the frequent case over the infrequent case – E.g., Instruction fetch and decode unit used more frequently than than multiplier optimize it 1st – E.g., If database server has 50 disks / processor, storage dependability dominates system dependability optimize it 1st • Frequent case is often simpler and can be done faster than the infrequent case – E.g., overflow is rare when adding 2 numbers, so improve performance performance by optimizing more common case of no overflow – May slow down overflow, but overall performance improved by optimizing for the normal case • What is frequent case and how much performance improved by making case faster => Amdahl’s Law Some material is adapted from D. Culler & D. Patterson (UCB) S. Ziavras 4) Amdahl’s Law ExTimenew ⎡ Fraction enhanced ⎤ (1 − Fractionenhanced ) + = ExTimeold × ⎢ Speedupenhanced ⎥ ⎣ ⎦ Speedupoverall = ExTimeold = ExTimenew 1 (1 − Fractionenhanced...
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