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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.

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