I am testing sharing of pages between a parent and child process. However, my script is reporting that very little sharing is going on.

Basically, all it is doing is allocating a large string in the parent, forking a child and having the child report how many pages of memory are shared (by looking at /proc/self/statm).

When I run it, however, the space used by the string seems to be in the UNSHARED pages. Is that really correct?

Some typical output:

$ ./size-check
SIZE: 3792.000 SHARED: 324.000 UNSHARED: 3468.000 LENGTH($x): 0
$ ./size-check
Allocating a large string
SIZE: 23332.000 SHARED: 444.000 UNSHARED: 22888.000 LENGTH($x): 100000
+00
$
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#!/usr/bin/perl

our $x;
if (rand(10) < 5) {
  print "Allocating a large string\n";
  $x = ' ' x 10_000_000;
}

my $pid = fork();
die "unable to fork: $!" unless defined($pid);
if ($pid) {
  # parent
  wait;
} else {
  report_size();
}

sub report_size{
  my ($total, $shared) = _linux_size_check();
  my $unshared = $total - $shared;
  printf "SIZE: %.3f SHARED: %.3f UNSHARED: %.3f LENGTH(\$x): %d\n", $
+total, $shared, $unshared, length($x);
}

# borrowed rom Apache::SizeLimit
sub _linux_size_check {
    my ( $size, $share ) = ( 0, 0 );

    if ( open my $fh, '<', '/proc/self/statm' ) {
        ( $size, $share ) = ( split /\s/, scalar <$fh> )[0,2];
        close $fh;
    }
    else {
        die "Fatal Error: couldn't access /proc/self/status";
    }

    # linux on intel x86 has 4KB page size...
    return ( $size * 4, $share * 4 );
}
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