user time + system time = CPU time

Before

        sort { -M $b <=> -M $a } readdir D;

After

         @sorted_names = 
           map { $_->[0] }
           sort { $b->[1] <=> $a->[1] } 
           map { [ $_, -M $_ ] } 
           readdir D;

• -M and readdir consume mostly system time

• Everything else is pure user time

• The goal of the Schwartzian Transform is to reduce the number of -M's

  • But optimization is always a tradeoff

  • The cost is a lot more user-mode processing

  • We see this in the timing outputs

                      User   Sys  Total
              Direct  0.80  2.55   3.35
         Schwartzian  1.14  0.39   1.53

• The Schwartzian transform does 43% more processing

  • But it wins by asking the kernel for 84% less service

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Copyright © 2003 M. J. Dominus