Lec03 - 1 Review Two performance metrics execution time and...

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Unformatted text preview: 1 Review Two performance metrics execution time and throughput. Measuring CPU time: CPI CPU time = Instruction count x CPI x clock cycle time CPU time = Instruction count x CPI / clock rate 2 Suppose that enhancement E accelerates a fraction F of the task by a factor S, and the remainder of the task is unaffected t Enhancemen Without e Performanc t Enhancemen With e Performanc t Enhancemen With Time Execution t Enhancemen Without Time Execution E Speedup _ _ _ _ _ _ _ _ _ _ ) ( = = Speedup due to enhancement E: This fraction enhanced Quantitative Design: Amdahl's Law Amdahls Law gives a quick way to find the speedup from some enhancement. 3 Quantitative Design: Amdahl's Law This fraction enhanced ExTime old ExTime new ExTime new = ExTime old x (1 - Fraction enhanced ) + Fraction enhanced Speedup overall = ExTime old ExTime new Speedup enhanced = 1 (1 - Fraction enhanced ) + Fraction enhanced Speedup enhanced 4 Pictorial Depiction of Amdahl Pictorial Depiction of Amdahl s Law s Law Before : Execution Time without enhancement E After : Execution Time with enhancement E: Enhancement E accelerates fraction F of original execution time by a factor of S Unaffected fraction: (1- F) Affected fraction: F Unaffected fraction: (1- F) F/S Unchanged Execution Time without enhancement E 1 Speedup(E) = --------------------------------------------------------- = ---------------------- Execution Time with enhancement E (1 - F) + F/S shown normalized to 1 = (1-F) + F =1 5 Floating point (FP) instructions improved to run 2X; but only 10% of actual instructions are FP. Suppose the old execution time is ExTime old , What are the current execution time and speedup ? Quantitative Design: Amdahl's Law Speedup overall = 1 0.95 = 1.053 ExTime new = ExTime old x (0.9 + 0.1/2) = ExTime old x 0.95 Speedup = ExTime old ExTime new = 1 (1 - Fraction enhanced ) + Fraction enhanced Speedup enhanced Speedup = 1 (1 - 0.1) + 0.1/2 = 1.053 6 Performance Summary Two performance metrics execution time and throughput. Measuring CPU time: CPI CPU time = Instruction count x CPI x clock cycle time CPU time = Instruction count x CPI / clock rate Amdahl s Law When trying to improve performance, look at what occurs frequently => make the common case fast ....
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Lec03 - 1 Review Two performance metrics execution time and...

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