#!/usr/bin/env perl
use strict;
use warnings;

use Linux::Smaps;

$SIG{CHLD} = sub { waitpid(-1, 1) };

# comment this block
{
    my %h;
    # pre-growth PL_strtab hash, kind of: keys %$PL_strtab = 2_000_000;
    foreach my $x (1 .. 2_000_000) {
        $h{$x} = undef;
    }
}

my $pid = fork // die "Cannot fork: $!";
unless ($pid) {
    # child

    my $s = Linux::Smaps->new($$)->all;
    my $before = $s->shared_clean + $s->shared_dirty;

    {
        my %h;
        foreach my $x (1 .. 2_000_000) {
            $h{$x} = undef;
        }
    }

    my $s2 = Linux::Smaps->new($$)->all;
    my $after = $s2->shared_clean + $s2->shared_dirty;
    warn 'COPY-ON-WRITE: ' . ($before - $after) . ' KB';

    exit 0;
}

sleep 1000;
print "DONE\n";