4 use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS $warn);
10 @ISA = qw(Exporter DynaLoader);
12 # This allows declaration use Devel::Size ':all';
13 %EXPORT_TAGS = ( 'all' => [ qw(
17 @EXPORT_OK = ( @{ $EXPORT_TAGS{'all'} } );
22 bootstrap Devel::Size $VERSION;
33 Devel::Size - Perl extension for finding the memory usage of Perl variables
37 use Devel::Size qw(size total_size);
39 my $size = size("A string");
41 my @foo = (1, 2, 3, 4, 5);
42 my $other_size = size(\@foo);
44 my $foo = {a => [1, 2, 3],
47 my $total_size = total_size($foo);
51 This module figures out the real size of Perl variables in bytes, as
52 accurately as possible.
54 Call functions with a reference to the variable you want the size
55 of. If the variable is a plain scalar it returns the size of
56 this scalar. If the variable is a hash or an array, use a reference
63 The C<size> function returns the amount of memory the variable
64 returns. If the variable is a hash or an array, it only reports
65 the amount used by the structure, I<not> the contents.
67 =head2 total_size($ref)
69 The C<total_size> function will traverse the variable and look
70 at the sizes of contents. Any references contained in the variable
71 will also be followed, so this function can be used to get the
72 total size of a multidimensional data structure. At the moment
73 there is no way to get the size of an array or a hash and its
74 elements without using this function.
78 None but default, but optionally C<size> and C<total_size>.
80 =head1 UNDERSTANDING MEMORY ALLOCATION
82 Please note that the following discussion of memory allocation in perl
83 is based on the perl 5.8.0 sources. While this is generally
84 applicable to all versions of perl, some of the gory details are
85 omitted. It also makes some presumptions on how your system memory
86 allocator works so, while it will be generally correct, it may not
87 exactly reflect your system. (Generally the only issue is the size of
88 the constant values we'll talk about, not their existence)
92 It's important firtst to understand how your OS and libraries handle
93 memory. When the perl interpreter needs some memory, it asks the C
94 runtime library for it, using the C<malloc()> call. C<malloc> has one
95 parameter, the size of the memory allocation you want, and returns a
96 pointer to that memory. C<malloc> also makes sure that the pointer it
97 returns to you is properly aligned. When you're done with the memory
98 you hand it back to the library with the C<free()> call. C<free> has
99 one parameter, the pointer that C<malloc> returned. There are a couple of interesting ramifications to this.
101 Because malloc has to return an aligned pointer, it will round up the
102 memory allocation to make sure that the memory it returns is aligned
103 right. What that alignment is depends on your CPU, OS, and compiler
104 settings, but things are generally aligned to either a 4 or 8 byte
105 boundary. That means that if you ask for 1 byte, C<malloc> will
106 silently round up to either 4 or 8 bytes, though it doesn't tell the
107 program making the request, so the extra memory can't be used.
109 Since C<free> isn't given the size of the memory chunk you're
110 freeing, it has to track it another way. Most libraries do this by
111 tacking on a length field just before the memory it hands to your
112 program. (It's put before the beginning rather than after the end
113 because it's less likely to get mangled by program bugs) This size
114 field is the size of your platform integer, Generally either 4 or 8
117 So, if you asked for 1 byte, malloc would build something like this:
121 +------------------+ <----- the pointer malloc returns
127 As you can see, you asked for 1 byte but C<malloc> used 8. If your
128 integers were 8 bytes rather than 4, C<malloc> would have used 16 bytes
129 to satisfy your 1 byte request.
131 The C memory allocation system also keeps a list of free memory
132 chunks, so it can recycle freed memory. For performance reasons, some
133 C memory allocation systems put a limit to the number of free
134 segments that are on the free list, or only search through a small
135 number of memory chunks waiting to be recycled before just
136 allocating more memory from the system.
138 The memory allocation system tries to keep as few chunks on the free
139 list as possible. It does this by trying to notice if there are two
140 adjacent chunks of memory on the free list and, if there are,
141 coalescing them into a single larger chunk. This works pretty well,
142 but there are ways to have a lot of memory on the free list yet still
143 not have anything that can be allocated. If a program allocates one
144 million eight-byte chunks, for example, then frees every other chunk,
145 there will be four million bytes of memory on the free list, but none
146 of that memory can be handed out to satisfy a request for 10
147 bytes. This is what's referred to as a fragmented free list, and can
148 be one reason why your program could have a lot of free memory yet
149 still not be able to allocate more, or have a huge process size and
150 still have almost no memory actually allocated to the program running.
154 Perl's memory allocation scheme is a bit convoluted, and more complex
155 than can really be addressed here, but there is one common spot where Perl's
156 memory allocation is unintuitive, and that's for hash keys.
158 When you have a hash, each entry has a structure that points to the
159 key and the value for that entry. The value is just a pointer to the
160 scalar in the entry, and doesn't take up any special amount of
161 memory. The key structure holds the hash value for the key, the key
162 length, and the key string. (The entry and key structures are
163 separate so perl can potentially share keys across multiple hashes)
165 The entry structure has three pointers in it, and takes up either 12
166 or 24 bytes, depending on whether you're on a 32 bit or 64 bit
167 system. Since these structures are of fixed size, perl can keep a big
168 pool of them internally (generally called an arena) so it doesn't
169 have to allocate memory for each one.
171 The key structure, though, is of variable length because the key
172 string is of variable length, so perl has to ask the system for a
173 memory allocation for each key. The base size of this structure is
174 8 or 16 bytes (once again, depending on whether you're on a 32 bit or
175 64 bit system) plus the string length plus two bytes.
177 Since this memory has to be allocated from the system there's the
178 malloc size-field overhead (4 or 8 bytes) plus the alignment bytes (0
179 to 7, depending on your system and the key length)
180 that get added on to the chunk perl requests. If the key is only 1
181 character, and you're on a 32 bit system, the allocation will be 16
182 bytes. If the key is 7 characters then the allocation is 24 bytes on
183 a 32 bit system. If you're on a 64 bit system the numbers get even
186 This does mean that hashes eat up a I<lot> of memory, both in memory
187 Devel::Size can track (the memory actually in the structures and
188 strings) and that it can't (the malloc alignment and length overhead).
192 Devel::Size, because of the way it works, can consume a
193 considerable amount of memory as it runs. It will use five
194 pointers, two integers, and two bytes worth of storage, plus
195 potential alignment and bucket overhead, per thing it looks at. This
196 memory is released at the end, but it may fragment your free pool,
197 and will definitely expand your process' memory footprint.
199 =head1 Messages: texts originating from this module.
205 =item "Devel::Size: Unknown variable type"
207 The thing (or something contained within it) that you gave to
208 total_size() was unrecognisable as a Perl entity.
214 These messages warn you that for some types, the sizes calculated may not include
215 everything that could be associated with those types. The differences are usually
216 insignificant for most uses of this module.
218 These may be disabled by setting
220 $Devel::Size::warn = 0
224 =item "Devel::Size: Calculated sizes for CVs are incomplete"
226 =item "Devel::Size: Calculated sizes for FMs are incomplete"
232 Doesn't currently walk all the bits for code refs, formats, and
233 IO. Those throw a warning, but a minimum size for them is returned.
235 Devel::Size only counts the memory that perl actually allocates. It
236 doesn't count 'dark' memory--memory that is lost due to fragmented free lists,
237 allocation alignments, or C library overhead.
241 Dan Sugalski dan@sidhe.org
243 Small portion taken from the B module as shipped with perl 5.6.2.
245 Maintained now by Tels <http://bloodgate.com>
249 Copyright (C) 2005 Dan Sugalski, Copyright (C) 2007-2008 Tels
251 This module is free software; you can redistribute it and/or modify it
252 under the same terms as Perl v5.8.8.
256 perl(1), L<Devel::Size::Report>.