1Devel::Size(3) User Contributed Perl Documentation Devel::Size(3)
2
6 Devel::Size - Perl extension for finding the memory usage of Perl
7 variables
8
10 use Devel::Size qw(size total_size);
11 my $size = size("A string");
13 my @foo = (1, 2, 3, 4, 5);
15 my $other_size = size(\@foo);
16 my $foo = {a => [1, 2, 3],
18 b => {a => [1, 3, 4]}
19 };
20 my $total_size = total_size($foo);
21
23 This module figures out the real size of Perl variables in bytes, as
24 accurately as possible.
25 Call functions with a reference to the variable you want the size of.
27 If the variable is a plain scalar it returns the size of this scalar.
28 If the variable is a hash or an array, use a reference when calling.
29
31 size($ref)
32 The "size" function returns the amount of memory the variable returns.
33 If the variable is a hash or an array, it only reports the amount used
34 by the structure, not the contents.
35 total_size($ref)
37 The "total_size" function will traverse the variable and look at the
38 sizes of contents. Any references contained in the variable will also
39 be followed, so this function can be used to get the total size of a
40 multidimensional data structure. At the moment there is no way to get
41 the size of an array or a hash and its elements without using this
42 function.
43
45 None but default, but optionally "size" and "total_size".
46
48 Please note that the following discussion of memory allocation in perl
49 is based on the perl 5.8.0 sources. While this is generally applicable
50 to all versions of perl, some of the gory details are omitted. It also
51 makes some presumptions on how your system memory allocator works so,
52 while it will be generally correct, it may not exactly reflect your
53 system. (Generally the only issue is the size of the constant values
54 we'll talk about, not their existence)
55 The C library
57 It's important first to understand how your OS and libraries handle
58 memory. When the perl interpreter needs some memory, it asks the C
59 runtime library for it, using the "malloc()" call. "malloc" has one
60 parameter, the size of the memory allocation you want, and returns a
61 pointer to that memory. "malloc" also makes sure that the pointer it
62 returns to you is properly aligned. When you're done with the memory
63 you hand it back to the library with the "free()" call. "free" has one
64 parameter, the pointer that "malloc" returned. There are a couple of
65 interesting ramifications to this.
66 Because malloc has to return an aligned pointer, it will round up the
68 memory allocation to make sure that the memory it returns is aligned
69 right. What that alignment is depends on your CPU, OS, and compiler
70 settings, but things are generally aligned to either a 4 or 8 byte
71 boundary. That means that if you ask for 1 byte, "malloc" will silently
72 round up to either 4 or 8 bytes, though it doesn't tell the program
73 making the request, so the extra memory can't be used.
74 Since "free" isn't given the size of the memory chunk you're freeing,
76 it has to track it another way. Most libraries do this by tacking on a
77 length field just before the memory it hands to your program. (It's put
78 before the beginning rather than after the end because it's less likely
79 to get mangled by program bugs) This size field is the size of your
80 platform integer, Generally either 4 or 8 bytes.
81 So, if you asked for 1 byte, malloc would build something like this:
83 +------------------+
85 | 4 byte length |
86 +------------------+ <----- the pointer malloc returns
87 | your 1 byte |
88 +------------------+
89 | 3 bytes padding |
90 +------------------+
91 As you can see, you asked for 1 byte but "malloc" used 8. If your
93 integers were 8 bytes rather than 4, "malloc" would have used 16 bytes
94 to satisfy your 1 byte request.
95 The C memory allocation system also keeps a list of free memory chunks,
97 so it can recycle freed memory. For performance reasons, some C memory
98 allocation systems put a limit to the number of free segments that are
99 on the free list, or only search through a small number of memory
100 chunks waiting to be recycled before just allocating more memory from
101 the system.
102 The memory allocation system tries to keep as few chunks on the free
104 list as possible. It does this by trying to notice if there are two
105 adjacent chunks of memory on the free list and, if there are,
106 coalescing them into a single larger chunk. This works pretty well, but
107 there are ways to have a lot of memory on the free list yet still not
108 have anything that can be allocated. If a program allocates one million
109 eight-byte chunks, for example, then frees every other chunk, there
110 will be four million bytes of memory on the free list, but none of that
111 memory can be handed out to satisfy a request for 10 bytes. This is
112 what's referred to as a fragmented free list, and can be one reason why
113 your program could have a lot of free memory yet still not be able to
114 allocate more, or have a huge process size and still have almost no
115 memory actually allocated to the program running.
116 Perl
118 Perl's memory allocation scheme is a bit convoluted, and more complex
119 than can really be addressed here, but there is one common spot where
120 Perl's memory allocation is unintuitive, and that's for hash keys.
121 When you have a hash, each entry has a structure that points to the key
123 and the value for that entry. The value is just a pointer to the scalar
124 in the entry, and doesn't take up any special amount of memory. The key
125 structure holds the hash value for the key, the key length, and the key
126 string. (The entry and key structures are separate so perl can
127 potentially share keys across multiple hashes)
128 The entry structure has three pointers in it, and takes up either 12 or
130 24 bytes, depending on whether you're on a 32 bit or 64 bit system.
131 Since these structures are of fixed size, perl can keep a big pool of
132 them internally (generally called an arena) so it doesn't have to
133 allocate memory for each one.
134 The key structure, though, is of variable length because the key string
136 is of variable length, so perl has to ask the system for a memory
137 allocation for each key. The base size of this structure is 8 or 16
138 bytes (once again, depending on whether you're on a 32 bit or 64 bit
139 system) plus the string length plus two bytes.
140 Since this memory has to be allocated from the system there's the
142 malloc size-field overhead (4 or 8 bytes) plus the alignment bytes (0
143 to 7, depending on your system and the key length) that get added on to
144 the chunk perl requests. If the key is only 1 character, and you're on
145 a 32 bit system, the allocation will be 16 bytes. If the key is 7
146 characters then the allocation is 24 bytes on a 32 bit system. If
147 you're on a 64 bit system the numbers get even larger.
148
150 Since version 0.72, Devel::Size uses a new pointer tracking mechanism
151 that consumes far less memory than was previously the case. It does
152 this by using a bit vector where 1 bit represents each 4- or 8-byte
153 aligned pointer (32- or 64-bit platform dependent) that could exist.
154 Further, it segments that bit vector and only allocates each chunk when
155 an address is seen within that chunk. Since version 0.73, chunks are
156 allocated in blocks of 2**16 bits (ie 8K), accessed via a 256-way tree.
157 The tree is 2 levels deep on a 32 bit system, 6 levels deep on a 64 bit
158 system. This avoids having make any assumptions about address layout on
159 64 bit systems or trade offs about sizes to allocate. It assumes that
160 the addresses of allocated pointers are reasonably contiguous, so that
161 relevant parts of the tree stay in the CPU cache.
162 Besides saving a lot of memory, this change means that Devel::Size runs
164 significantly faster than previous versions.
165
167 Errors
168 "Devel::Size: Unknown variable type"
169 The thing (or something contained within it) that you gave to
170 total_size() was unrecognisable as a Perl entity.
171 warnings
173 These messages warn you that for some types, the sizes calculated may
174 not include everything that could be associated with those types. The
175 differences are usually insignificant for most uses of this module.
176 These may be disabled by setting
178 $Devel::Size::warn = 0
180 "Devel::Size: Calculated sizes for CVs are incomplete"
182 "Devel::Size: Calculated sizes for FMs are incomplete"
183 "Devel::Size: Calculated sizes for compiled regexes are incompatible,
184 and probably always will be"
185 New warnings since 0.72
187 Devel::Size has always been vulnerable to trapping when traversing
188 Perl's internal data structures, if it encounters uninitialised
189 (dangling) pointers.
190 MSVC provides exception handling able to deal with this possibility,
192 and when built with MSVC Devel::Size will now attempt to ignore (or
193 log) them and continue. These messages are mainly of interest to
194 Devel::Size and core developers, and so are disabled by default.
195 They may be enabled by setting
197 $Devel::Size::dangle = 0
199 "Devel::Size: Can't determine class of operator OPx_XXXX, assuming
201 BASEOP\n"
202 "Devel::Size: Encountered bad magic at: 0xXXXXXXXX"
203 "Devel::Size: Encountered dangling pointer in opcode at: 0xXXXXXXXX"
204 "Devel::Size: Encountered invalid pointer: 0xXXXXXXXX"
205
207 Doesn't currently walk all the bits for code refs, formats, and IO.
208 Those throw a warning, but a minimum size for them is returned.
209 Devel::Size only counts the memory that perl actually allocates. It
211 doesn't count 'dark' memory--memory that is lost due to fragmented free
212 lists, allocation alignments, or C library overhead.
213
215 Dan Sugalski dan@sidhe.org
216 Small portion taken from the B module as shipped with perl 5.6.2.
218 Previously maintained by Tels <http://bloodgate.com>
220 New pointer tracking & exception handling for 0.72 by BrowserUK
222 Currently maintained by Nicholas Clark
224
226 Copyright (C) 2005 Dan Sugalski, Copyright (C) 2007-2008 Tels
227 This module is free software; you can redistribute it and/or modify it
229 under the same terms as Perl v5.8.8.
230
232 perl(1), Devel::Size::Report.
233perl v5.32.1 2021-01-27 Devel::Size(3)