The Innermost Loop

     %Time ExclSec CumulS #Calls sec/call Csec/c  Name
      63.7   0.830  0.829    109   0.0076 0.0076  main::letter_histogram

This is a typical situation
- Often, a program is structured as a series of nested loops
For example, this program:
- For each input file,
  - For each message in the file,
    - For each character in the file
      - Append it to the histogram.
The code inside the innermost loop gets run many, many times
- Here, once for each character in the entire input
Other parts of the program are run much less frequently
- A small speedup in this innermost loop can have a disproportionate effect on run time