# Computer Organization and Architecture 18 th March, 2008.

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Computer Organization and Architecture 18 th March, 2008

Measuring Performance

Defining Performance Which plane has the best performance? Range? Highest speed? Capacity? Fastest?

Defining Performance (Cont.. ) Response time? Execution time? Throughput?

Example: Do the following changes to a computer system increase throughput, decrease response time, or both? 1.Replacing the processor in a computer with a faster version 2.Adding additional processors to a system that uses multiple processors for separate tasks– for example, handling an airline reservation system

Mainly we ‘ll measure performance in terms Execution time. What would be better? More execution time or less execution time?

Performance = 1/execution time Performance of X > Performance of Y Execution time of Y ?? Execution time of X

Example If machine A runs program in 10 seconds and machine B runs the same programs in 15 sec, how much faster is A than B?

Performance Equation CPU Time = IC * CPI * cc time CPU Time = IC * CPI / Clock rate

Example Suppose we have two implementations of the same instruction set architecture. Machine A has a clock cycle time of 1ns and a CPI of 2.0 for some program, and machine B has a clock cycle time of 2ns and a CPI of 1.2 for the same program. Which machine is faster and by how much?

Example A compiler designer is trying to decide between two code sequences for a particular machine. The hardware designer has supplied the following facts: Two code sequences that require the following instruction counts. What code sequence executes the most instructions? Which will be faster? What is the CPI for each sequence Instruction classCPI for this Instruction class A1 B2 C3 Code Sequence Instruction counts for instruction class ABC 1212 2411

Amdahl’s Law Execution time new = Execution time old * { (1- Fraction enhanced)+ Fraction Enhanced/ Speedup Enhanced } Speedup = Execution time old / Execution time new

Example Suppose that we want to enhance the processor used for Web serving. The new processor is 10 times faster on computation in the Web serving application than the original processor. Assuming that the original processor is busy with computation 40% of the time and is waiting for I/O 60% of the time, what is the overall speedup gained by incorporating the enhancement?

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