| Re: Exhaustive memory allocation using arrays [message #181748 is a reply to message #181747] |
Wed, 29 May 2013 08:54   |
The Natural Philosoph
Messages: 993
Registered: September 2010
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Senior Member |
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On 29/05/13 09:22, Denis McMahon wrote:
> On Tue, 28 May 2013 23:27:13 -0700, tombert.at wrote:
>
>> 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??
> From: http://php.net/manual/en/language.types.array.php
>
> "An array in PHP is actually an ordered map."
>
> You may be expecting arrays where an array of 1e6 floats is held in
> memory in 1e6 sequential 8 byte locations, for a total of 8e6 bytes, but
> I suspect that if you did, you'd be very very very wrong.
>
> It's possible that:
>
> $arr = array_values( $arr );
>
> May shrink your array, I don't know for sure.
>
> It's also possible that:
>
> $a = Array();
> for ( $i = 0; i < 1e6; i++ )
> $a[] = 23.4567890123;
>
> might produce a smaller array, simply because it's not assigning keys.
> Again, I don't know for sure.
>
> The reason I don't know for sure is that these implementation details
> are, to me, just that. If they don't affect me, I don't need to
> understand them in the level of detail you're looking for. There's plenty
> of things I do need to understand at that level of detail, and I have
> limited time to put into gaining such understandings, so I use it where
> it's most needed. :)
>
Exactly so. There are times when you realise that an 'excellent for some
tasks' tool is in fact a complete dogs breakfast when applied to another
task.
I found, for example, that on the fly generation of graphics using PHP
was massively CPU and RAM hungry and appeared to suffer re-rentrancy
problems when used in multiple concurrent access: the solution was to
generate the image data asynchronously using a C program.
I can think of many ways in which an array element dynamically assigned
needing pointers to memory blocks and so on could easily have several
tens of bytes overhead.
That of course is an issue with an interpreted language: it has to more
or less avoid statically allocated memory especially if there is no way
to declare it.
--
Ineptocracy
(in-ep-toc’-ra-cy) – a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers.
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