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Bug #54120 Impossible to freed memory complety after call to Xpath() method.
Submitted: 2011-02-28 15:58 UTC Modified: 2011-03-01 22:15 UTC
From: ikaos at gmx dot fr Assigned:
Status: Not a bug Package: SimpleXML related
PHP Version: 5.3.5 OS: Linux CentOS 5.5 x86_64
Private report: No CVE-ID: None
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 [2011-02-28 15:58 UTC] ikaos at gmx dot fr
Description:
------------
---
From manual page: http://www.php.net/simplexmlelement.xpath
---


Test script:
---------------
<?php
$sxe = NULL; $nodes = NULL;
print("Without call to xpath method : " . PHP_EOL);
print("Memory before SXE  : " . memory_get_usage() . PHP_EOL);
$sxe = new SimpleXMLElement("large_file.xml", 0, TRUE);
$sxe = NULL; $nodes = NULL;
print("Memory after freed memory : " . memory_get_usage() . PHP_EOL . PHP_EOL);

$sxe = NULL; $nodes = NULL;
print("Calling xpath method : " . PHP_EOL);
print("Memory before SXE  : " . memory_get_usage() . PHP_EOL);
$sxe = new SimpleXMLElement("large_file.xml", 0, TRUE);
$nodes = $sxe->xpath('/ITEMS/ITEM');
$sxe = NULL; $nodes = NULL;
print("Memory after freed memory : " . memory_get_usage() . PHP_EOL . PHP_EOL);

// short sleep, time to launch 'ps aux'
sleep(10);
?>


Expected result:
----------------
When I unset (or set to NULL) the result of the Xpath() method ($nodes in the example), I expect memory will be completly freed. It's not the case. 

I tried, to recursively unset each element from $nodes and I got the same result. 



Actual result:
--------------
Result of the test script :

Without call to xpath method : 
Memory before SXE  : 625736
Memory after freed memory : 625904

Calling xpath method : 
Memory before SXE  : 625904
Memory after freed memory : 1608944


memory_get_usage says that few kilos octets of memory are still used, but in reality several thousand mega octets are still used.

# ps aux | grep test.php

me  28859 83.4 48.3 *1119908* *996056* pts/3  S+   15:30   0:08 /bin/php test.php

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 [2011-03-01 20:54 UTC] rrichards@php.net
-Status: Open +Status: Bogus
 [2011-03-01 20:54 UTC] rrichards@php.net
Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

Memory is freed. http://xmlsoft.org/xmlmem.html you can read how memory is managed 
and might not be returned immediately to the kernel.
 [2011-03-01 22:15 UTC] ikaos at gmx dot fr
Thank you for this quick answer. I red these informations on http://xmlsoft.org/xmlmem.html before.

But there is something strange (perhaps I don't understand how kernel release memory), after freeing memory I could understand the kernel doesn't release immediatly the memory, but in that case there is a big problem for me, for exemple I use pcntl and try to fork my script after using xpath and releasing memory, and it fails. If I don't use xpath() it works.

I think I understand there is a command malloc_trim, but it doesn't exists directly from PHP. Is there a workaround I could use ?

Second question, It seems that memory_get_usage returns the memory used by PHP, after freeing memory (after xpath()) I still have more memory used than before using xpath(). Is it normal or still related to the way the kernel works ?

Thanks.
 
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