CSCE 212 Chapter 4 Assessing and Understanding Performance

Measuring Performance • Basic problem: how can we tell if one processor is faster

Example • A program runs in 10 seconds on computer A, which has a

Performance Factors • Execution time = instructions per program * cycles per instruction *

Performance Measures • CPI = cycles per instruction – CPI provides one way of

CPI instruction class CPI A B C 1 2 3 Code sequence Instruction counts

Example • Suppose we have two implementations of the same ISA. Computer A has

Performance Measures • Program execution time vs CPU execution time – Must account for

Relative Performance Computer A Computer B Program 1 (seconds) 1 10 Program 2 (seconds)

MIPS • Instructions per second • MIPS is problematic – Does not take into

Benchmarks • Set of programs that are used to evaluate a machine’s performance •

Amdahl’s Law • Basic idea: improve the common case • Example: – Program runs

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CSCE 212 Chapter 4: Assessing and Understanding Performance Instructor: Jason D. Bakos

Measuring Performance • Basic problem: how can we tell if one processor is faster than another? • Execution time is analogous to performance • Execution time (per processor core) = 2

Example • A program runs in 10 seconds on computer A, which has a 4 GHz clock. We are designing computer B that must run the program in 6 seconds. We can increase clock rate but the computer will require 1. 2 times as many clock cycles. What clock rate should be target for computer B? 3

Performance Factors • Execution time = instructions per program * cycles per instruction * seconds per cycle • Problems: – Cycles per instruction depends on many factors • • i. e. instruction mix, I/O, memory Performance factors: – – Algorithm Programming language Compiler ISA instruction count, CPI CPI clock rate, CPI 4

Performance Measures • CPI = cycles per instruction – CPI provides one way of comparing two different implementations of the same instruction set architecture – CPI is tricky! • Instructions from the same ISA require different number of cycles • Depends on program • Memory behavior affects CPI – CPI is usually defined for a particular program on a particular CPU 5

CPI instruction class CPI A B C 1 2 3 Code sequence Instruction counts A B C 1 2 2 4 1 1 6

Example • Suppose we have two implementations of the same ISA. Computer A has a cycle time of 250 ps and a CPI of 2. 0 for some program, and computer B has a clock cycle time of 500 ps and a CPI of 1. 2 for the same program. Which computer is faster and by how much? 7

Performance Measures • Program execution time vs CPU execution time – Must account for other programs and I/O – User CPU time – CPU time spent executing a program • System CPU time – Amount of CPU time the operating system spends on behalf of a program – Ex. system calls, such as I/O, synchronization, etc. 8

Relative Performance Computer A Computer B Program 1 (seconds) 1 10 Program 2 (seconds) 1000 100 • Solutions: – Total execution time – Arithmetic mean – Weight arithmetic mean (based on workload for each program) 9

MIPS • Instructions per second • MIPS is problematic – Does not take into account the capabilities of the instructions – MIPS varies between programs on the same computer! • Depends on average CPI of the instruction mix – MIPS can vary inversely with performance 10

Benchmarks • Set of programs that are used to evaluate a machine’s performance • SPEC CPU 2000 suite – 12 integer and 14 floating-point – Measured as speedup relative to Sun Ultra 5_10@300 MHz – CINT 2000 and CFP 2000 is geometric mean of ratios 11

Amdahl’s Law • Basic idea: improve the common case • Example: – Program runs in 100 s, multiplies accord for 80 s – I want a 5 X overall speedup 12