7 - Performance (1).pptx - PERFORMANCE Introduction The...

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PERFORMANCE
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Introduction The performance of a system depends on: algorithms programming language architecture hardware elements Assessing performance can be challenging
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Introduction What is performance? Airplane Passengers Range Speed Throughput Boeing 747 470 4150 610 286,700 Concorde 132 4000 1350 178,200 DC-8-50 146 8720 544 79.424
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Definitions of Performance Response time How long it takes to do a task Execution time Throughput Total work done per unit time e.g., tasks/transactions/… per hour
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Performance as Response Time To increase performance, reduce response time. “X is n time faster than Y” Performance(X) Execution Time(Y) –––––––––––––– = –––––––––––––––– = n Performance(Y) Execution Time(X) 1 Performance(X) = –––––––––––––––– Execution Time(X)
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Example If computer A runs a program in 10 seconds and computer B runs the same program in 15 seconds, how much faster is A than B? A is 1.5 times faster than B. Execution Time(B) 15 –––––––––––––– = –––––––––––––––– = n Execution Time(A) 10 Execution Time(B) 15 –––––––––––––– = –––––––––––––––– = 1.5 Execution Time(A) 10 Performance(A) Execution Time(B) –––––––––––––– = –––––––––––––––– = n Performance(B) Execution Time(A)
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Measuring Performance Time is our metric for determining performance Time can be defined in different ways
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Measuring Execution Time Elapsed time Wall-clock” time, Response time Total time to complete task, including all aspects Processing, I/O, OS overhead, idle time Determines system performance CPU time Time spent processing a given job Discounts I/O time, other jobs’ shares Comprises user CPU time and system CPU time Different programs are affected differently by CPU and system performance CPU Performance System Performance
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CPU Clocking Clock period: duration of a clock cycle e.g. 250 ps Clock frequency: cycles per second e.g. 4 GHz Clock (cycles) Data transfer and computation Update state Clock period
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CPU Time Performance improved by Reducing number of clock cycles Increasing clock rate Trade-offs: Designers often trade off clock rate against cycle count CPU Time = CPU Clock Cycles x Clock Cycle Time
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Example Our favorite program runs in 10 seconds on computer A, which has a 4 GHz clock . We are trying to help a computer designer build a computer, B, that will run this program in 6 seconds. The designer has determined that a substantial increase in the clock rate is possible , but this increase will affect the rest of the CPU design, causing computer B to require 1.2 times as many clock cycles as computer A for this program.
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