Re: [理工] [計組] performance
※ 引述《mqazz1 (無法顯示)》之銘言：
: assume that multiply instructions take 12 cycles and account for 15% of the
: instructions in a typical program, and the other 85% of the instructions
: require an average of 4 cycles for each instruction
: (b) your hardware engineering team has indicated that it would be possible to
: reduce the number of cycles required for multiplication to 8, but this will
: require a 20% increase in the cycle time. Nothing else will be affected by the
: change. Should they proceed with the modification?
: 請問這題要怎麼解?
: 張凡上冊243頁 謝謝
execution time = cycle time * cpi * # of instructions
old machine:
cycle time = 1 (normalized)
cpi = 15% * 12 + 85% * 4 = 1.8 + 3.4 = 5.2
# of instructions = 1 (normalized)
execution time = 1 * 5.2 * 1 = 5.2
new machine:
cycle time = 1.2 (normalized)
cpi = 15% * 8 + 85% * 4 = 1.2 + 3.4 = 4.6
# of instructions = 1(normalized)
execution time = 1.2 * 4.6 * 1 = 5.52
因為新的機器 execution time 較長所以 performance 較差，
不應該為了效能的原因作這個修改。
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