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COSC 4349 and 5349 Computer Architecture Fundamentals of Computer Design (Part 4) Dr. Lappoon R. Tang Overview Principles of Computer Design What does it mean to use a quantitative approach to implementing these principles? Amdahl’s Law Readings Section 1.9

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cosc 4349 and 5349 computer architecture

COSC 4349 and 5349Computer Architecture

Fundamentals of Computer Design (Part 4)

Dr. Lappoon R. Tang

overview
Overview
  • Principles of Computer Design
    • What does it mean to use a quantitative approach to implementing these principles?
  • Amdahl’s Law
readings
Readings
  • Section 1.9
    • Skim thru “Processor Performance Equation”
principles of computer design
Principles of Computer Design
  • Principle of Parallelism
    • Idea: Exploit parallelism, you get the tremendous speedup and save your time for party over the weekend
    • Ch. 2 
  • Principle of Locality
    • Idea: You fetch your stuffs close to where you just put them
    • Ch. 5 
  • Principle of Optimizing the Common Case
    • Idea: Live close to where you most frequently visit if you want to save gas money
    • This chapter, right here 
quantitative principles of computer design
Quantitative Principles of Computer Design

Put the ideas in the previous slide into

Mathematical equations and you get a

precise model in each principle for:

  • performance evaluation
  • guidingyour design of a computer system
amdahl s law scenario
Amdahl’s Law: Scenario

Scenario: You have a computer system. One day an apple dropped on your head but did not kill you and instead this idea for improving performance emerged. However, the enhancement can only be used for a fraction of overall execution time

Q: How much actual speedup do we have in the computer system if we employ this enhancement whenever possible?

amdahl s law speedup

Equivalently:

Amdahl’s Law: Speedup

Example: Suppose I can over-clock my CPU to enhance its performance.

When my CPU is over-clocked, execution time on my program1.c is 40 secs.

When it is not over-clocked, execution time on program1.c is 80 secs.

Speedup = 80/40 = 2 (i.e. with the enhancement, my CPU runs 2x as fast)

amdahl s law fraction enhanced
Amdahl’s Law: Fractionenhanced
  • Fractionenhanced is the fraction of time enhancement can be utilized in the entire (original) execution time (without enhancement)
  • Example: If my CPU can be over-clocked for 2 hours straight within a 4 hour period, Fractionenhanced = 2/4 = 0.5 (i.e. 50%)
  • Note: Fractionenhanced =< 1
amdahl s law speedup enhanced
Amdahl’s Law: Speedupenhanced
  • Speedupenhanced measures how much faster the task would run if the enhancement is used for the entire program
  • Exactly the same definition as Speedup 
  • Example: With enhancement turned on, my program completes in 2 seconds; otherwise, 4 seconds.

Speedupenhanced = 4/2 = 2 (i.e. 2x faster with enhancement)

amdahl s law execution time new

Execution TimeOld

20 secs (66.7%)

10 secs (33.3%)

Duration in which

enhancement

can be applied

Duration in which enhancement

cannot be applied

But not

applied

20 secs

5 secs

Duration in which enhancement

cannot be applied

Duration in which

enhancement

can be applied

Applied

Execution TimeNew

Speedupenhanced = ?

Amdahl’s Law: Execution timenew

Q: If we know the execution time without enhancement, Fractionenhanced and

Speedupenhanced, how do we figure out the new execution time with enhancement?

Answer: Speedupenhanced = 2

amdahl s law execution time new11

Total execution

time NOT using

enhancement

Total execution

time when

enhancement

Is used

Amdahl’s Law: Execution timenew
amdahl s law previous example
Amdahl’s Law:Previous Example

10 secs

20 secs

Speedup