DREXEL ECEC 355 - mips_ratings (3 pages)

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mips_ratings



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mips_ratings

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3
School:
Drexel University
Course:
Ecec 355 - Computer Organization & Architecture

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Using MIPS and MFLOPS as Performance Metrics April 26 2008 One alternative way to measure CPU performance is MIPS or million instructions per second For a given program MIPS is given by MIPS Instruction count Execution time 106 Since Execution time Instruction count CPI Clock rate 1 2 Equation 1 becomes Clock rate 3 CPI 106 Since MIPS is a rate of operations per unit time CPU performance can be specified as the inverse of execution time with faster machines having a higher MIPS rating However according to the Patterson and Hennessy there are problems with using MIPS as a performance metric MIPS MIPS is dependent on the instruction set of the CPU making it difficult to compare the MIPS ratings of processors with different instruction sets MIPS can vary inversely to performance Consider the MIPS rating of a processor with an optional floating point unit Since it generally takes more clock cycles per floating point instruction that per integer instruction floating point programs using the optional hardware instead of software floating point routines take less time but have a lower MIPS rating A software floating point routine executes simpler instructions resulting in a higher MIPS rating but it executes so many more instructions that the overall execution time is longer We can see similar anomalies with optimizing compilers as the following example demonstrates Example Let us assume that you have profiled your code and the instruction mix is detailed in Table 1 We now want to build an optimizing compiler for the CPU The compiler discards 50 of the ALU instructions although it cannot reduce loads stores or branches Assuming a 20 ns clock cycle time or a 50 MHz clock what is the MIPS rating for the optimized code versus the unoptimized code Does the MIPS rating agree with the ranking of execution time 1 Table 1 The instruction mix and CPIs of individual instructions Operation ALU Operations Loads Stores branches Frequency 43 21 12 24 CPI 1 2 2 2 Answer We use the CPU



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