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Empirical Study of Execution

Decades of empirical study of program execution have shown that programs spend most of their execution time in a small part of their code.

This is often quoted as the 90/10 rule   (or 80/20 rule or ...):

``90% of the execution time is spent in 10% of the code''

This means that

  • most of the code has little impact on overall performance
  • small parts of the code account for most execution time
We should clearly concentrate efforts at improving efficiency in the 10% of code which accounts for most of the execution time.