php memory limit test - php

Question

It seems this is an ever unsolved question: I did a simple test to the memory limits in my local machine (from command line):
<?php
for ($i = 0; $i < 4000*4000; $i ++) {
$R[$i] = 1.00001;
}
?>
and I have memory limit set at 128M. But PHP still sends off "Allowed memory exhausted" message. Why?

Answer 1

Well I wouldn't say ever unsolved question. There are a few reasons for it-PHP is a very insufficient language in terms of memory management-it's no secret. Now the code you provided could be optimized a little bit, but not enough to make a difference. For example take the multiplication in the for loop outside and store the value in a variable. Otherwise you are performing that mathematical operation on each loop. But that will not make any significant difference - 2310451248 bytes as it is and 2310451144 bytes if you do it as I proposed. But the point remains - PHP is not a low level language so you can't expect it to have the same efficiency as C for example. In your particular case, the required memory to perform all this is a little over 2 GB(2.15 gb)
<?php
ini_set('memory_limit', '4096M');
$ii = 4000*4000;
//$R = new SplFixedArray($ii);
$R = array();
for ($i = 0; $i < $ii; $i ++) {
$R[$i] = 1.00001;
}
echo humanize(memory_get_usage())."\n";
function humanize($size)
{
$unit=array('b','kb','mb','gb','tb','pb');
return round($size/pow(1024,($i=floor(log($size,1024)))),2).' '.$unit[$i];
}
?>
But using SplFixedArray things change a lot:
<?php
ini_set('memory_limit', '4096M');
$ii = 4000*4000;
$R = new SplFixedArray($ii);
for ($i = 0; $i < $ii; $i ++) {
$R[$i] = 1.00001;
}
echo humanize(memory_get_usage())."\n";
function humanize($size)
{
$unit=array('b','kb','mb','gb','tb','pb');
return round($size/pow(1024,($i=floor(log($size,1024)))),2).' '.$unit[$i];
}
?>
Which requires "only" 854.72 mb.
This is one of the main reasons why companies who deal with larger amounts of data in general avoid using PHP and go for languages such as python instead. There is a great article describing all of the problems and causes around this topic, found here. Hope that helps.