Quantitative Performance Comparison

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Quantitative Performance Comparison

• Computer Architecture

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Need for quantitative performance comparisons

• Modern CPUs utilize a number of architectural
  advances
• Instruction sets of CPUs are different
• RISC versus CISC
• General purposes versus ASIC (Application
  Specific Integrated Circuit) CPUs
• Different CPUs have different pipeline depths
• The number level of caches are different
• The clock speed of CPUs are different
• The number of ALUs vary impacting the type of
  superscalar operations that CPUs can perform
• The aforementioned factors affect overall CPU
  performance
• All of the above factors (including others) must
  accounted to identify a good CPU

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Measuring Performance

• Performance is a relative quantitative metric
  that is used to compare two CPUs
• Comparisons assume all other factors are
  identical
• Performance is represented as a ratio
• Given two computers ? and ? and a benchmark
  program, then performance ratio is defined as

Performance Ratio Benchmark Execution time on ?
Performance Ratio Benchmark Execution time on ?

• Note that execution time is always measured as
  the CPU time and not wall clock time.
• This is important on multi-tasking machines where
  multiple programs may be using the CPU

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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?
• The performance ratio is

Execution time on A 15 1.5
Execution time on B 10 1.5

• Therefore computer A is 1.5 times faster than B

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Program Execution Time

• CPU Time for executing a program is determined
  by
• The number of instructions to be executed
• The number of Clock cycles Per Instruction (CPI)
• Clock cycle time (seconds) or clock rate (Hertz)
• Recollect that cycle time and rate are inversely
  related

CPU Time Instruction Count CPI
CPU Time Clock rate
The above formula can be rewritten as
CPU Time Instruction Count CPI Cycle time
Note that changing any one of the above
parameters impacts the CPU Time for executing a
program
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Example

• Suppose we have two CPUs with the same
  instruction set running the same program.
  Computer A has a clock cycle time of 250 ps and a
  CPI of 2.0. Computer B has a clock cycle time of
  500 ps and CPI of 1.2. Which computer is faster
  and by how much?
• Assume program as K instructions.
• CPU Time for A 250 2 K 500K ps
• CPU Time for B 500 1.2 K 600K ps
• Therefore computer A is faster than computer B.

Performance Ratio 600K 1.2
Performance Ratio 500K 1.2
Computer A is 1.2 times faster than Computer B
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More on CPI

• CPUs have a wide variety of instructions
• The CPI for instructions vary
• Some instructions require more cycles to complete
• Some instructions require fewer cycles to
  complete
• The overall average CPI of a program depends on
  the mix of instructions used by the program
• If CPI changes the execution times of programs
  can also change
• Consequently, two program with exactly the same
  number of instructions (but instructions have
  different CPI) can have different execution times!

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Example

• The typical CPI for x86 architecture is shown
  below
• Integer Instructions 1 CPI
• Floating point instructions 2 CPI
• Which program will run faster
• Program P1 that uses 100 floating point
  instructions
• Program P2 that uses 50 floating point and 75
  integer instructions?
• Solution
• Total cycles for P1 100 2 200 cycles
• Total cycles for P2 (50 2) (75 1) 175
  cycles
• Therefore program P2 will run faster than program
  P1 (even though P2 has more instructions)