Why is PHP Doctine's free() not working?

Question

This is one is for any of you Doctrine users out there. I have a PHP CLI daemon process that checks a table every n seconds to find entries that haven't been processed. It's basically a FIFO. Anyways, I always exceed the memory allocated to PHP becuase Doctrine does not free it's resources. To combat this problem it provides free for the query object. I can't seem to make it work though. Here's the code:

 22     print "You are using " . (memory_get_usage() / 1024). "\n";
 23     $query = Doctrine_Query::create()
 24                 ->from('SubmissionQueue s')
 25                 ->where('s.time_acted_on IS NULL')
 26                 ->orderby('s.time_queued')
 27                 ->limit(1)
 28                 ->execute();
 29     print $query[0]->time_queued . "\n";
 30     $query->free();

Any ideas what I am doing wrong?

EDIT: I am using 1.0.3

EDIT: I have tried all of the below suggestions. I was very hopeful for unset() as I had it in there before I found free().

Here is more of the code to possibly aid in any help. Before you question the opening and closing of the connection it will be daemon process that spawns children and from I have experienced the connection has to be unique.

  1 <?php
  2
  3 require_once('/usr/local/lib/php/Doctrine/lib/Doctrine.php');
  4
  5 spl_autoload_register(array('Doctrine', 'autoload'));
  6
  7 $manager = Doctrine_Manager::getInstance();
  8 $manager->setAttribute('model_loading','conservative');
  9 Doctrine::loadModels('lib/model/master');
 10
 11 while(1){
 12     print "You are using " . intval(memory_get_usage() / 1024) . "\n";
 13     $manager->connection('********************************************','master');
 14     $query = Doctrine_Query::create()
 15                 ->from('SubmissionQueue s')
 16                 ->where('s.time_acted_on IS NULL')
 17                 ->orderby('s.time_queued')
 18                 ->limit(1)
 19                 ->execute();
 20     print "[" . $query[0]->time_queued . "]\n";
 21     $query->free();
 22     unset($query);
 23     $query = null;
 24     $manager->closeConnection(Doctrine_Manager::getInstance()->getConnection('master'));
 25     sleep(5);
 26 }
 27

Some sample output:

You are using 14949KB
[2008-11-17 13:59:00]
You are using 14978KB
[2008-11-17 13:59:00]
You are using 15007KB
[2008-11-17 13:59:00]
You are using 15035KB
[2008-11-17 13:59:00]
You are using 15064KB
[2008-11-17 13:59:00]
You are using 15093KB
[2008-11-17 13:59:00]
You are using 15121KB
[2008-11-17 13:59:00]
You are using 15150KB
[2008-11-17 13:59:00]
You are using 15179KB
[2008-11-17 13:59:00]
You are using 15207KB
[2008-11-17 13:59:00]
You are using 15236KB
[2008-11-17 13:59:00]
You are using 15265KB
[2008-11-17 13:59:00]
You are using 15293KB
[2008-11-17 13:59:00]
You are using 15322KB

Answer 1

The problem is, that free() does not remove the Doctrine objects from memory but just eliminates the circular references on those objects, making it possible for the garbage collector to cleanup those objects. Please see 23.6 Free Objects in the Doctrine manual:

As of version 5.2.5, PHP is not able to garbage collect object graphs that have circular references, e.g. Parent has a reference to Child which has a reference to Parent. Since many doctrine model objects have such relations, PHP will not free their memory even when the objects go out of scope.

For most PHP applications, this problem is of little consequence, since PHP scripts tend to be short-lived. Longer-lived scripts, e.g. bulk data importers and exporters, can run out of memory unless you manually break the circular reference chains. Doctrine provides a free() function on Doctrine_Record, Doctrine_Collection, and Doctrine_Query which eliminates the circular references on those objects, freeing them up for garbage collection.

The solution should be to unset() the $query object after using free():

$query = Doctrine_Query::create()
            ->from('SubmissionQueue s')
            ->where('s.time_acted_on IS NULL')
            ->orderby('s.time_queued')
            ->limit(1);
$query->execute();
print $query[0]->time_queued . "\n";
$query->free();
unset($query); // perhaps $query = null; will also work