Lec02b - COMP 4300 Computer Architecture Performance...

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1 COMP 4300 Computer Architecture Performance Measurement Dr. Xiao Qin Auburn University http://www.eng.auburn.edu/~xqin xqin@auburn.edu Fall, 2010
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2 Example of Measuring CPU time If a computer has a clock rate of 50 MHz, how long does it take to execute a program with 1,000 instructions, if the CPI for the program is 3.5? Using the equation CPU time = Instruction count x CPI / clock rate gives CPU time = 1000 x 3.5 / (50 x 10 ) CPU time = Seconds = Instructions x Cycles x Seconds Program Program Instruction Cycle CPU time = Seconds = Instructions x Cycles x Seconds Program Program Instruction Cycle 6
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CPI: Cycles Per Instruction Clock Cycles for program = Instructions executed for a program (called “ Instruction Count ”) x Average C lock cycles P er I nstruction (abbreviated “ CPI ”) CPI also gives insight into style of ISA: RISC (e.g., MIPS, DEC Alpha, PowerPC) higher instruction count, lower CPI CISC (e.g., Intel) lower instruction count, higher CPI
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“Iron Triangle” of CPU Performance CPU execution time for program = Clock Cycles for program x Clock Cycle Time Substituting for clock cycles: CPU execution time for program = (Instruction Count x CPI) x Clock Cycle Time = Instruction Count x CPI x Clock Cycle Time CPI Instruction Count Clock Cycle Time
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5 Example
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Lec02b - COMP 4300 Computer Architecture Performance...

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