Chapter 4 Performance

The Role of Performance

Example of Performance Measure

Performance Metrics Response Time –Delay between start end end time of a task –Performance = 1/Execution Time Throughput –Numbers of tasks per given time

Examples (Throughput/Performance) Replacing the processor with a faster version ? Adding additional processor to a system ?

Amdahl’s Law Distance from College Station to Austin is 100 miles, no flight. –Driving speed is 50 miles/hour => 2 hours from CLL to AUS Distance from Austin to Los Angeles is 1200 miles –The speed of Boeing 777 is 600 miles/hour => 2 hours from AUS to LAX, for a total of 4 hours If we want to improve the performance –F-22: The speed is 1200 miles/hour => 1 hour from AUS to LAX, speedup = 4/3 = 1.3 –The speed of Space Shuttle is 3,000 miles/hour (lower space) => 0.4 hour, speedup = 4/2.4=1.7

Amdahl’s Law [contd…] Opportunity for improvement is affected by how much time the event consumes Make the common case fast

Amhdahl’s Law [contd…] A program runs in 100 seconds on a machine, with multiply operations responsible for 80 seconds of this time. How much do I have to improve the speed of multiplication if I want my program to run five times faster ? Execution Time After improvement = (exec time affected by improvement/amount of improvement) + exec time unaffected exec time after improvement = (80 seconds / n) + (100 – 80 seconds) We want performance to be 5 times faster => 20 seconds = 80/n seconds / n + 20 seconds 0 = 80 / n !!!!

Summary Performance “eye of the beholder” Running Time = (Instruction Count)xCPIx(cycle Time) Amdahl’s Law “Make the Common Case Faster”