On my system (Linux 64bit, Perl 5.12), an integer takes at least 24 Bytes:
perl -MDevel::Size=size -e 'print size(1)'
24
And a string starts at 56 Bytes:
perl -MDevel::Size=size -e 'print size("1")'
56
Here is a comparison of a string and two arrays:
use Devel::Size qw/total_size/; my $scalar = '123456789'; my $flat = ['123', '456', '789']; my $full = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]; printf "scalar: %5d\n", total_size $scalar; printf "flat: %5d\n", total_size $flat; printf "full: %5d\n", total_size $full;
# Output: scalar: 64 flat: 368 full: 896
My (old) German Perl blog has a better example: Array-Overhead mit Devel::Size bestimmen.
But I want to make another point here. Sometimes you need something else:
use Devel::Size qw/total_size/; use XML::LibXML; my $html = <<HTML; <html><head><title>test</title></head> <body><h1>headline</h1> <b><i>just</i> <u>testing</u></b> </body></html> HTML my $xml = XML::LibXML->new; my $doc = $xml->parse_html_string($html); printf "HTML length: %5d\n", length $html; printf "total_size: %5d\n", total_size $doc;
# Output: HTML length: 112 total_size: 72
Oh, the parsed document (which is a DOM tree), is smaller than the HTML string? Data::Dumper shows the problem:
$VAR1 = bless( do{\(my $o = 42302096)}, 'XML::LibXML::Document' );
XML::LibXML is a XS module, the DOM tree is not stored in Perl objects, but in C. So we need to ask the system how much memory our process is consuming. I could not find a good way in Perl. The best I found was ps:
ps -o rss --no-heading -p PID
Lets try this (I leave the first part out):
my $start = get_rss();
my $doc2 = $xml->parse_html_string($html);
my $final = get_rss();
printf "real size: %d KB\n", $final - $start;
sub get_rss {
my $rss = qx/ps -o rss --no-heading -p $$/;
$rss =~ s/\D//g;
return $rss;
}
This reveals a size of 8 KB. I tried this with bigger documents and it grows. :)But is measuring RSS (resident set size) really the right figure? Please comment, if you know it. Thanks.
Links:
A string is even bigger in 5.13!
ReplyDelete$ perlbrew switch perl-5.12.2
$ perl -MDevel::Size=size -E 'say size "1"'
56
$ perlbrew switch perl-5.13.7
$ perl -MDevel::Size=size -E 'say size "1"'
72