Perl Best Practices [Electronic resources] نسخه متنی

19.9. Memory

Don't optimize data structuresmeasure them .

Intuitions about the relative space efficiency of different data structures aren't very reliable, either. If you are concerned about the memory footprint of a data structure that you are using, the Devel::Size module makes it easy to see how heavy the burden actually is:

# This look-up table is handy, but seems to be too bloated...

my %lookup = load_lookup_table($file);
# So let's look at how much memory it's using...

use Devel::Size qw( size total_size );
use Perl6::Form;
my $hash_mem  = size(\%lookup);           
# Storage overheads only

my $total_mem = total_size(\%lookup);     
# Overheads plus actual data

my $data_mem  = $total_mem - $hash_mem;   
# Data only

print form(
'hash alone: {>>>,>>>,>>} bytes',  $hash_mem,
'data alone: {>>>,>>>,>>} bytes',  $data_mem,
'============================',
'total:      {>>>,>>>,>>} bytes',  $total_mem,
); 

That might print something like:

hash alone:    8,704,075 bytes
data alone:    8,360,250 bytes
==============================
total:        17,064,325 bytes 

which indicates that storing your 8.36MB of data in a hash has incurred an overhead of an additional 8.70MB for buckets, hash tables, keys, and other internals.

The total_size( ) subroutine takes a reference to a variable and returns the total number of bytes of memory used by that variable. This includes both:

• The memory that the variable uses for its own implementation. For example, the buckets that are needed to implement a hash, or the flag bits that are used inside every scalar.

• The memory used by the data that the variable stores. For example, the space required for the keys and values in a hash, or for the value in a scalar.

The size( ) subroutine also takes a variable reference, but returns only the number of bytes that the variable uses for itself, excluding the memory required to store its data.