| Re: Exhaustive memory allocation using arrays [message #181754 is a reply to message #181744] |
Wed, 29 May 2013 13:30  |
Thomas 'PointedEars'
Messages: 701
Registered: October 2010
Karma:
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Senior Member |
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tombert(dot)at(at)gmail(dot)com wrote:
^^^^^^^^^^^^^^^^^^^^
Please get a real name.
> Why is the following code using > 100MB?
>
> for ($i=0; $i < 1000000; $i++) {
> $value[$i] = 23.4567890123;
> }
> echo (memory_get_usage(TRUE) / (1024 * 1024)) . "MB <br>";
>
> It looks like that each item takes 100 Byte for allocation??
First of all, 1 MB (megabyte) is 1'000'000 (one million, 10⁶) bytes. What
you call “MB” here actually is 1 _MiB_ (mega-binary byte; short: mebibyte)
or 2²⁰ bytes = 1'048'576 bytes (as you divided by 2¹⁰ × 2¹⁰ = 2²⁰ to arrive
at your “MB” value):
<http://en.wikipedia.org/wiki/Mebibyte>
So your calculation
100 “MB” ∕ 1'000'000 items = 100 bytes∕item
is wrong to begin with. Assuming that you got something close to 100 as a
result of
memory_get_usage(TRUE) / (1024 * 1024)
each item would take about
104'857'600 bytes / 1'000'000 = 105 bytes
<http://php.net/memory_get_usage>
Second, the value you assigned is a value of the float type. In PHP, the
precision of the float type, thus the number of bits required to store such
a value, is platform-dependent:
<http://php.net/manual/en/language.types.float.php>
The manual says that IEEE-754 “double precision” floating-point values are a
common implementation (as they are in other equally dynamic programming
languages), but we cannot assume that to be the case on your platform. And
you have not said which platform and PHP version you are using, so we cannot
look into the pertinent source code.
Third, you are not just storing the float value. You are storing a key-
value relationship:
,-<http://www.php.net/manual/en/language.types.array.php>
|
| An array in PHP is actually an ordered map. A map is a type that
| associates values to keys. […]
Fourth, that PHP arrays can be associative, i. e. can have string keys, and
that using the string representation of a numeric key accesses the same item
as the number (ibid.), suggests implementations in which all keys are
actually string values (as it is in other equally dynamic programming
languages, like ECMAScript implementations; if not like that, you would have
to store the type of the key which is even more memory-expensive), and
,-<http://www.php.net/manual/en/language.types.string.php>
|
| The string in PHP is implemented as an array of bytes and an integer
| indicating the length of the buffer. […]
Fast access to array items further suggests that the keys are not only
stored as-is (for array_keys()), but that the implementation includes a hash
table.
Fifth, PHP arrays do have an internal pointer – <http://php.net/current> –
that can be advanced: <http://php.net/next>. Combined with the dynamic size
of PHP arrays this suggests that the items of an array are at least arranged
in a linked list. The existence of a function to rewind the internal
pointer – <http://php.net/prev> – further suggests that they are arranged in
a double-linked list.
Finally, you are not just measuring the memory that is needed for the array,
but for executing the preceding program. There is *at least* the memory for
$i variable to consider. So “your math does not add up”.
Probably there are even more factors to consider. See also the
memory_get_usage() documentation and user comments.
So consider this: A PHP array could be implemented *at least* as a hash
table (for quick item access) in which the string keys are stored in a
linked list (so that array_keys() works), and the stored values could be in
a double-linked list (so that internal pointer movements work). In this
hash table you are storing items whose plain values require *at least* 64
bits = 8 bytes of memory each. Add to that the memory required to run the
program that fills the array.
I know of no native way to determine the memory it takes to store the PHP
array; it can perhaps be obtained with a PHP profiler. In any case, I
presume it will be smaller than what you measured with memory_get_usage().
HTH
PointedEars
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