You lost me there, when might it be less efficient to "specify a multiple step path" ?

I don't really use File::Find much, so I super searched for some example code to try benchmarking, and from what i can tell, all other factors being equal/irrelevant, no_chdir seems to be faster.

Here's an example from one of your snippets...

#!/usr/local/bin/perl

use Benchmark qw(cmpthese);
use File::Find;

my %results;

my $wanted = sub {
  if (-l) { # it's a symlink
    my ($dev, $ino) = lstat _; # reuse info from -l
    push @{$results{"$dev $ino"}}, $File::Find::name;
    if (-e) { # that points somewhere else
      my ($dev, $ino) = stat _; # reuse info from -e
      push @{$results{"$dev $ino"}}, "symlink:$File::Find::name";
    }
  } else {
    my ($dev, $ino) = stat;
    push @{$results{"$dev $ino"}}, $File::Find::name;
  }
};

my @dirs = qw(/bin /usr/bin /usr/sbin); # change this to "/" to do the


cmpthese(1000, {
    chdir => sub {
    %results=();
    find { wanted=>$wanted }, @dirs;
    }, 
    no_chdir => sub {
    %results=();
    find { wanted=>$wanted, no_chdir=>1}, @dirs;
    }
});

__END__

laptop:~> monk.pl
Benchmark: timing 1000 iterations of chdir, no_chdir...
     chdir: 79 wallclock secs (61.68 usr 13.90 sys +  1.23 cusr  1.03 
+csys = 77.84 CPU) @ 13.23/s (n=1000)
  no_chdir: 82 wallclock secs (61.94 usr 17.99 sys +  1.07 cusr  1.09 
+csys = 82.09 CPU) @ 12.51/s (n=1000)
           Rate no_chdir    chdir
no_chdir 12.5/s       --      -5%
chdir    13.2/s       6%       --
[download]

That's not very deep, to go to /usr/bin/foo instead of using chdir first. What I meant was a deep hierarchy, like /usr/local/lib/X11/app_defaults/foo. Every path that has all those steps has to be looked up step-by-step, repeating the same work for the kernel over and over.

Admittedly, the cost is fairly cheap these days, since modern kernels can cache a lot of the intermediate directories. But it's still a non-zero cost, and while that might not make a difference for a dozen lookups, it will for a thousand lookups.

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

Ah, yeah .. good point.

For those who want to prove this to themselves, here's the script I used to convince myself...

Read more... (2 kB)

And the result...

laptop:~/tmp/monk-test> ~/monk.pl -50
laptop:~/tmp/monk-test> ~/monk.pl 100
Benchmark: timing 100 iterations of chdir, no_chdir...
     chdir:  2 wallclock secs ( 1.39 usr  0.40 sys +  0.12 cusr  0.10 
+csys =  2.01 CPU) @ 55.87/s (n=100)
  no_chdir:  3 wallclock secs ( 1.50 usr  1.08 sys +  0.09 cusr  0.17 
+csys =  2.84 CPU) @ 38.76/s (n=100)
           Rate no_chdir    chdir
no_chdir 38.8/s       --     -31%
chdir    55.9/s      44%       --
laptop:~/tmp/monk-test> ls
0  1  a  aa
laptop:~/tmp/monk-test> rm -rf *
laptop:~/tmp/monk-test> ~/monk.pl -100
laptop:~/tmp/monk-test> ~/monk.pl 100
Benchmark: timing 100 iterations of chdir, no_chdir...
     chdir:  4 wallclock secs ( 2.98 usr  0.71 sys +  0.13 cusr  0.12 
+csys =  3.94 CPU) @ 27.10/s (n=100)
  no_chdir:  8 wallclock secs ( 3.03 usr  3.98 sys +  0.13 cusr  0.21 
+csys =  7.35 CPU) @ 14.27/s (n=100)
           Rate no_chdir    chdir
no_chdir 14.3/s       --     -47%
chdir    27.1/s      90%       --
[download]