use strict;
use warnings;
use Time::HiRes qw(time);

# Create a large hash
my %hash;
my $count = 3_000_000;
$hash{$count} = rand(10) while $count--;

# Undefine it
print "Undefining hash.\n";
my $before = time;
undef %hash;
printf "Done in %.03f seconds\n", time - $before;
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On my linux box it's about 1.5s for both perl 5.8.8 and perl 5.10.0.

I suspect that might be due to a different malloc in windows and linux, so my next step will be do compile a perl with a different malloc, and try again.

Update: A perl configured with -Uusemymalloc took 1.4s, so no real difference.

Your times reflect my own for Windows, and your Linux test suggests that Linux does not show this problem either. This is perhaps a problem specifically with Perl on FreeBSD?

my macbook pro (2.16Ghz Core2duo) took 3.17 seconds.. Darwin being a BSD derived OS.

$ perl hash.pl ; perl hash.pl ; perl hash.pl
Create:  5.26445007324219
Destroy: 1.82375288009644
Create:  5.18459510803223
Destroy: 1.82551097869873
Create:  5.14320707321167
Destroy: 1.8236780166626
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$ cat hash.pl
#!/usr/bin/perl

use strict;
use warnings FATAL => 'all';
use Time::HiRes 'time';

my $t0 = time;
my %hash;
$hash{$_} = 1 for 1..3_000_000;
my $t1 = time;
undef %hash;
my $t2 = time;

print "Create:  ", $t1-$t0, "\nDestroy: ", $t2-$t1, "\n";
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$ uname -a
FreeBSD XXX.XXX.XXX 7.0-RELEASE-p2 FreeBSD 7.0-RELEASE-p2 #0: Wed Jun 
+18 06:48:16 UTC 2008     root@amd64-builder.daemonology.net:/usr/obj/
+usr/src/sys/GENERIC  amd64
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$ perl -v

This is perl, v5.8.8 built for amd64-freebsd
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And I get very similar results on linux with perl 5.10.0 and 5.8.8

My 14 secs is just for the undef.

I'm testing on FreeBSD 6.2-RELEASE-p1 with Perl "v5.8.8 built for i386-freebsd".

A less messy variant of that idea is to use just one global array called @garbage, and push references to your large hashes onto it.
That way you would at least only have one global to worry about, and your structures would still stick around until the end.

The response to the post you linked to sounds like it should solve your problem, provided that you have the ability to recompile Perl on the target systems: Re^3: Sort taking a long time to clean up

Thanks: I like the @garbage idea which slightly reduces the hackiness of the workaround.

(I'm afraid recompiling Perl isn't an option.)

Maybe you could try a two-step approach:

%hash = ();
undef %hash;
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May you can also try delete. It is supposed to be slower but it would be interesting to compare. I have tried your example on Windows XP and Redhat (both Perl 5.8) and it runs fine. Strangely enough it takes a bit longer under Redhat?! Unfortunately I don't have FreeBSD available.

Thanks for suggestions! Results:

"%hash = ();" appears to do much the same as undef(), at least, it creates the same long delay.

Iterating all the keys and deleting them is slower than undef, although not by much: around 16 secs as opposed to 14 secs for undef.

So, no solutions there, but thanks for the ideas.

Frankly speaking that didn't surprise me. I just thought it was worth a try. I tried it myself on Redhat also with increased values up to 30_000_000 same results as you.

FWIW here are my results on MacOSX 10.5 PPC using MacPort's perl 5.8.8:

              s/iter    With undef Without undef
With undef      22.5            --           -9%
Without undef   20.6           10%            --
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And with the system perl 5.8.8:

              s/iter    With undef Without undef
With undef      22.5            --           -8%
Without undef   20.7            9%            --
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So on this platform the undef time is certainly measurable, but exactly an order less than the creation time.

Here is the code I used for the benchmark comparisons:

use strict;
use warnings;

use Benchmark qw(:all);

sub without_undef {
    # Create a large hash
    my %hash;
    my $count = 3_000_000;
    $hash{$count} = rand(10) while $count--;
}

sub with_undef {
    # Create a large hash
    my %hash;
    my $count = 3_000_000;
    $hash{$count} = rand(10) while $count--;

    # Undefine it
    undef %hash;
}

cmpthese(20, {
    'Without undef' => \&without_undef,
    'With undef' => \&with_undef,
});
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Thanks for the code, I used it to benchmark Perl 5.10.0 on Linux:

              s/iter    With undef Without undef
With undef      17.2            --          -26%
Without undef   12.8           34%            --
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which, although a bigger difference than yours, isn't as bad as what oxone reported. This was on:

Linux 2.6.25 #6 SMP Tue Aug 5 17:42:15 SGT 2008 i686 GNU/Linux
This is perl, v5.10.0 built for i486-linux-gnu-thread-multi
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