Dismas has asked for the wisdom of the Perl Monks concerning the following question:

Hi, Monks!

An optimizing question for ya. Is it better to leave a variable global in scope (with a "my" declaration), or is there a time penalty associated with declaring it (again, with a "my") inside the while loop which is the only place in the script that uses it?

Thanks!

Dismas

Replies are listed 'Best First'.
Re: Time Penalty vs. Global Symbols
by dave_the_m (Monsignor) on Jan 13, 2005 at 19:05 UTC
    There is a slight time penalty involved each time you execute a statement containing a 'my', so including it in the loop will penalise you slightly. However, this should be tiny compared with everything else your loop is likely to be doing, and except in exceptional circumstances, the loss in code clarity and maintainability isn't worth it.

    Dave.

[reply]
Re: Time Penalty vs. Global Symbols
by redhotpenguin (Deacon) on Jan 13, 2005 at 19:59 UTC
    Here's a simple benchmark showing the differences on a sample program I wrote. I don't know how accurately it answers this question, but it gives an idea of the differences.

    #!perl

use strict;
use warnings;

use Benchmark qw(:all);

my $foo;

timethese('50000000', {
    'test_1' => sub { $foo = 1; },
    'test_2' => sub { my $bar = 1; },
});

1;

__END__

fred@fishman ~ $ perl scope.pl 
Benchmark: timing 10000000 iterations of test_1, test_2...
    test_1:  0 wallclock secs ( 1.03 usr +  0.01 sys =  1.04 CPU) @ 96
+15384.62/s (n=10000000)
    test_2:  2 wallclock secs ( 1.38 usr +  0.01 sys =  1.39 CPU) @ 71
+94244.60/s (n=10000000)


fred@fishman ~ $ perl scope.pl 
Benchmark: timing 50000000 iterations of test_1, test_2...
    test_1:  6 wallclock secs ( 5.19 usr +  0.01 sys =  5.20 CPU) @ 96
+15384.62/s (n=50000000)
    test_2:  6 wallclock secs ( 6.51 usr +  0.00 sys =  6.51 CPU) @ 76
+80491.55/s (n=50000000)

    [download]
[reply]
[d/l]

      The benchmark above shows the cost of the "my" in the loop. This compares both lexical approaches with the package variable (aka "global") approach: 

      my $foo;
our $foo2;

cmpthese(shift, {
        'test_1' => sub { $foo = 1; },
        'test_2' => sub { my $bar = 1; },
        'test_3' => sub { $foo2 = 1; },
    });

__END__
            Rate test_2 test_1 test_3
test_2 3731343/s     --   -30%   -37%
test_1 5347594/s    43%     --   -10%
test_3 5952381/s    60%    11%     --

      [download]

      But the apparent gaps diminish rapidly as you start doing even a little work, particularly if system interaction is involved:

      my $foo;
our $foo2;

cmpthese(shift, {
        'test_1' => sub { $foo = time; $foo += 1 if __PACKAGE__ eq 'ma
+in' },
        'test_2' => sub { my $bar = time; $bar += 1 if __PACKAGE__ eq 
+'main' },
        'test_3' => sub { $foo2 = time; $foo2 += 1 if __PACKAGE__ eq '
+main' },
    });

__END__
           Rate test_2 test_3 test_1
test_2 442478/s     --    -4%   -10%
test_3 462963/s     5%     --    -6%
test_1 492611/s    11%     6%     --

      [download]

      Interesting that the order reverses, thought it might just margin of error

      -xdg

      Code written by xdg and posted on PerlMonks is public domain. It is provided as is with no warranties, express or implied, of any kind. Posted code may not have been tested. Use of posted code is at your own risk.

[reply]
[d/l]
[select]

Back to Seekers of Perl Wisdom