Provided by: libdevel-size-perl_0.80-1build1_amd64 
NAME
Devel::Size - Perl extension for finding the memory usage of Perl variables
SYNOPSIS
use Devel::Size qw(size total_size);
my $size = size("A string");
my @foo = (1, 2, 3, 4, 5);
my $other_size = size(\@foo);
my $foo = {a => [1, 2, 3],
b => {a => [1, 3, 4]}
};
my $total_size = total_size($foo);
DESCRIPTION
This module figures out the real size of Perl variables in bytes, as accurately as
possible.
Call functions with a reference to the variable you want the size of. If the variable is
a plain scalar it returns the size of this scalar. If the variable is a hash or an array,
use a reference when calling.
FUNCTIONS
size($ref)
The "size" function returns the amount of memory the variable returns. If the variable is
a hash or an array, it only reports the amount used by the structure, not the contents.
total_size($ref)
The "total_size" function will traverse the variable and look at the sizes of contents.
Any references contained in the variable will also be followed, so this function can be
used to get the total size of a multidimensional data structure. At the moment there is
no way to get the size of an array or a hash and its elements without using this function.
EXPORT
None but default, but optionally "size" and "total_size".
UNDERSTANDING MEMORY ALLOCATION
Please note that the following discussion of memory allocation in perl is based on the
perl 5.8.0 sources. While this is generally applicable to all versions of perl, some of
the gory details are omitted. It also makes some presumptions on how your system memory
allocator works so, while it will be generally correct, it may not exactly reflect your
system. (Generally the only issue is the size of the constant values we'll talk about, not
their existence)
The C library
It's important first to understand how your OS and libraries handle memory. When the perl
interpreter needs some memory, it asks the C runtime library for it, using the "malloc()"
call. "malloc" has one parameter, the size of the memory allocation you want, and returns
a pointer to that memory. "malloc" also makes sure that the pointer it returns to you is
properly aligned. When you're done with the memory you hand it back to the library with
the "free()" call. "free" has one parameter, the pointer that "malloc" returned. There
are a couple of interesting ramifications to this.
Because malloc has to return an aligned pointer, it will round up the memory allocation to
make sure that the memory it returns is aligned right. What that alignment is depends on
your CPU, OS, and compiler settings, but things are generally aligned to either a 4 or 8
byte boundary. That means that if you ask for 1 byte, "malloc" will silently round up to
either 4 or 8 bytes, though it doesn't tell the program making the request, so the extra
memory can't be used.
Since "free" isn't given the size of the memory chunk you're freeing, it has to track it
another way. Most libraries do this by tacking on a length field just before the memory it
hands to your program. (It's put before the beginning rather than after the end because
it's less likely to get mangled by program bugs) This size field is the size of your
platform integer, Generally either 4 or 8 bytes.
So, if you asked for 1 byte, malloc would build something like this:
+------------------+
| 4 byte length |
+------------------+ <----- the pointer malloc returns
| your 1 byte |
+------------------+
| 3 bytes padding |
+------------------+
As you can see, you asked for 1 byte but "malloc" used 8. If your integers were 8 bytes
rather than 4, "malloc" would have used 16 bytes to satisfy your 1 byte request.
The C memory allocation system also keeps a list of free memory chunks, so it can recycle
freed memory. For performance reasons, some C memory allocation systems put a limit to the
number of free segments that are on the free list, or only search through a small number
of memory chunks waiting to be recycled before just allocating more memory from the
system.
The memory allocation system tries to keep as few chunks on the free list as possible. It
does this by trying to notice if there are two adjacent chunks of memory on the free list
and, if there are, coalescing them into a single larger chunk. This works pretty well, but
there are ways to have a lot of memory on the free list yet still not have anything that
can be allocated. If a program allocates one million eight-byte chunks, for example, then
frees every other chunk, there will be four million bytes of memory on the free list, but
none of that memory can be handed out to satisfy a request for 10 bytes. This is what's
referred to as a fragmented free list, and can be one reason why your program could have a
lot of free memory yet still not be able to allocate more, or have a huge process size and
still have almost no memory actually allocated to the program running.
Perl
Perl's memory allocation scheme is a bit convoluted, and more complex than can really be
addressed here, but there is one common spot where Perl's memory allocation is
unintuitive, and that's for hash keys.
When you have a hash, each entry has a structure that points to the key and the value for
that entry. The value is just a pointer to the scalar in the entry, and doesn't take up
any special amount of memory. The key structure holds the hash value for the key, the key
length, and the key string. (The entry and key structures are separate so perl can
potentially share keys across multiple hashes)
The entry structure has three pointers in it, and takes up either 12 or 24 bytes,
depending on whether you're on a 32 bit or 64 bit system. Since these structures are of
fixed size, perl can keep a big pool of them internally (generally called an arena) so it
doesn't have to allocate memory for each one.
The key structure, though, is of variable length because the key string is of variable
length, so perl has to ask the system for a memory allocation for each key. The base size
of this structure is 8 or 16 bytes (once again, depending on whether you're on a 32 bit or
64 bit system) plus the string length plus two bytes.
Since this memory has to be allocated from the system there's the malloc size-field
overhead (4 or 8 bytes) plus the alignment bytes (0 to 7, depending on your system and the
key length) that get added on to the chunk perl requests. If the key is only 1 character,
and you're on a 32 bit system, the allocation will be 16 bytes. If the key is 7 characters
then the allocation is 24 bytes on a 32 bit system. If you're on a 64 bit system the
numbers get even larger.
DANGERS
Since version 0.72, Devel::Size uses a new pointer tracking mechanism that consumes far
less memory than was previously the case. It does this by using a bit vector where 1 bit
represents each 4- or 8-byte aligned pointer (32- or 64-bit platform dependent) that could
exist. Further, it segments that bit vector and only allocates each chunk when an address
is seen within that chunk. Since version 0.73, chunks are allocated in blocks of 2**16
bits (ie 8K), accessed via a 256-way tree. The tree is 2 levels deep on a 32 bit system, 6
levels deep on a 64 bit system. This avoids having make any assumptions about address
layout on 64 bit systems or trade offs about sizes to allocate. It assumes that the
addresses of allocated pointers are reasonably contiguous, so that relevant parts of the
tree stay in the CPU cache.
Besides saving a lot of memory, this change means that Devel::Size runs significantly
faster than previous versions.
Messages: texts originating from this module.
Errors
"Devel::Size: Unknown variable type"
The thing (or something contained within it) that you gave to total_size() was
unrecognisable as a Perl entity.
warnings
These messages warn you that for some types, the sizes calculated may not include
everything that could be associated with those types. The differences are usually
insignificant for most uses of this module.
These may be disabled by setting
$Devel::Size::warn = 0
"Devel::Size: Calculated sizes for CVs are incomplete"
"Devel::Size: Calculated sizes for FMs are incomplete"
"Devel::Size: Calculated sizes for compiled regexes are incompatible, and probably always
will be"
New warnings since 0.72
Devel::Size has always been vulnerable to trapping when traversing Perl's internal data
structures, if it encounters uninitialised (dangling) pointers.
MSVC provides exception handling able to deal with this possibility, and when built with
MSVC Devel::Size will now attempt to ignore (or log) them and continue. These messages are
mainly of interest to Devel::Size and core developers, and so are disabled by default.
They may be enabled by setting
$Devel::Size::dangle = 0
"Devel::Size: Can't determine class of operator OPx_XXXX, assuming BASEOP\n"
"Devel::Size: Encountered bad magic at: 0xXXXXXXXX"
"Devel::Size: Encountered dangling pointer in opcode at: 0xXXXXXXXX"
"Devel::Size: Encountered invalid pointer: 0xXXXXXXXX"
BUGS
Doesn't currently walk all the bits for code refs, formats, and IO. Those throw a warning,
but a minimum size for them is returned.
Devel::Size only counts the memory that perl actually allocates. It doesn't count 'dark'
memory--memory that is lost due to fragmented free lists, allocation alignments, or C
library overhead.
AUTHOR
Dan Sugalski dan@sidhe.org
Small portion taken from the B module as shipped with perl 5.6.2.
Previously maintained by Tels <http://bloodgate.com>
New pointer tracking & exception handling for 0.72 by BrowserUK
Currently maintained by Nicholas Clark
COPYRIGHT
Copyright (C) 2005 Dan Sugalski, Copyright (C) 2007-2008 Tels
This module is free software; you can redistribute it and/or modify it under the same
terms as Perl v5.8.8.
SEE ALSO
perl(1), Devel::Size::Report.