[CA] Lecture #06

CPU Clocking

CPU Clocking

• Clock period: Duration of a clock cycle.
• Clock frequency (rate): Cycles per second.

 

Order of Magnitude (규모)

Order of Magnitude (규모)

 

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CPU Time (CPU Performance)

• CPU Time = CPU Clock Cycles x Clock Cycle Time
• CPU Time = CPU Clock Cycles / Clock Rate

--> Performance is improved by:

• CPU Clock Cycles 수 ↓
• Clock Rate ↑ (= Clock Frequency ↑)
  예) 3Ghz --> 4Ghz

 

CPU Performance - Example

Problem

Solution

 

Instruction Count & CPI

• Clock Cycles = Instruction Count x Cycles per Instruction (CPI)
• CPU Time = Instruction Count x CPI x Clock Cycle Time
• CPU Time = Instruction Count x CPI / Clock Rate

▶ Instrunction count: Determined by program, ISA, compiler.

▶ CPI: 하나의 instruction을 실행하는데 걸리는 clock cycle 수

 

--> Performance is improved by:

• Instruction Count 개수↓: 수행해야 하는 instruction 개수↓
• CPI ↓

 

Instruction Count & CPI - Example

Instruction Count & CPI - Example

• Clock cycle time: A가 더 빠르다.
• CPI(Cycle per Instruction): B가 1.2배 더 많다.

--> A CPU time = instruction count x 2 x 250 = 500 x IC

--> B CPU time = instruction count x 1.2 x 500 = 600 x IC

Solution

 

CPI (in more details)

CPI (in more details)

 

Comparing Code Segments - Example

Comparing Code Segments - Example

 

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Performance Summary

CPU time == Performance

 

How Each is Measured?

 

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Power Trends

 

Uniprocessor Performance

 

Multiprocessors

 

Amdahl's Law

 

Low Power at Idle

 

MIPS as a Performance Metric

 

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Conclusion

 

Chapter Objectives