Perl Cwd Performance

The Camel book says -- in the Efficiency section of Chapter 24 -- to use the Cwd module instead of calling pwd repeatedly.

Boy, it's not kidding. I wrote a test script that does almost nothing but change directories and print the current directory, repeating that about 7500 times. Using `pwd` to get the directory took an average wall-clock time of 11.7 seconds. If instead you import Cwd's version of chdir(), the script can use $ENV{PWD}, which took an average wall-clock time of 0.3 seconds.

Oddly, using Cwd::cwd() took an average of 15.4 seconds. This wasn't a very scientific test, but those numbers bore up under repeated tries (doing several runs in a row, throwing out the time for the first run). I used wall-clock time, because the sys/user time reported by Unix time was quite a bit lower, even though it was an unloaded system. Maybe because the directories were NFS mounts.

So here's a good strategy for using Cwd:

  use Cwd qw(chdir);
  sub cwd {
    return $ENV{PWD};
  }

Note: the Cwd documentation points out that $ENV{PWD} is only kept up to date if every module used in the script uses Cwd::chdir to change directories.