Benchmarking Perl Code

(Up-to-date source of this post.)

Sometimes my code takes a really long time to run and I'd like to know which of the alternatives runs faster.

In this example I compare two sorting subroutines; a "naive" approach and "The Schwartzian Transform". The former subroutine just compares all files' sizes to each other while the latter first precomputes the size of each file and then does the comparisons.

use Benchmark qw(timethese);

chdir;    # change to my home directory
my @files = glob '*';

timethese(
    -2,
    {
        naive => sub {
            my @sorted = sort { -s $a <=> -s $b } @files;
        },
        schwartzian => sub {
            my @sorted =
              map  { $_->[0] }
              sort { $a->[1] <=> $b->[1] }
              map  { [ $_, -s $_ ] } @files;
        },
    }
);

The program's output:

Benchmark: running naive, schwartzian for at least 2 CPU seconds...
     naive:  2 wallclock secs ( 0.58 usr +  1.49 sys =  2.07 CPU) @ 11661.84/s (n=24140)
schwartzian:  2 wallclock secs ( 1.57 usr +  0.59 sys =  2.16 CPU) @ 21200.00/s (n=45792)

The output says that the Schwartzian Transform is much faster (the function ran more times in 2 seconds). The reason is that we don't ask for the file size each time we want to compare two files sizes; we ask just once for each file size.

See Also

• http://perldoc.perl.org/Benchmark.html
• Intermediate Perl, 2nd, p. 144
• http://www.perlmonks.com/?node_id=393128