Ch1

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衡量计算机的性能

• 执行时间

  • “X is n time faster than Y”
  • \(Performance\ e_X/ Performance\ e_Y=Execution\ time_Y/Execution\ time_X=n\)

• 系统时钟(CPU Clocking)

  • Clock period: duration of a clock cycl
  • Clock frequency (rate): cycles per second
• \(CPU\ Time=CPU\ Clock\ Cycles \times Clock\ Cycle\ Time=\frac{CPU\ Clock\ Cycles}{Clock\ Rate}\)
• 提升性能
  • 减少clock cycles数
  • 增加clock rate
  • Hardware designer must often trade off clock rate against cycle count （时钟频率高了，做一个任务cycle count会增加，而cycle count降低到一定程度就不降低）

• 指令数和CPI(cycles per instruction)
  • \(CPU\ Time=Instruction\ Count\times CPI\times Clock\ Cycle\ Time\)
• Average cycles per instruction
  • Determined by CPU hardware
  • If different instructions have different CPI
    • Average CPI affected by instruction mix