PHP MongoDB findOne Memory Leak - php

Question

I have found some old posts regarding the problem of a memory leak with the php mongodb driver. But none gave a final solution or explenation for older versions of the php mongodb driver.
My Version of the Driver is 1.4.5 (stable)
with PHP 5.3.10
Code with Debug echos:
echo memory_get_usage()." in MB : ";
echo memory_get_usage()/1024/1024;
echo "<br>";
unset($cursor);
$dt = new DateTime($day." 00:00:00", new DateTimeZone($this->timezone));
$mongodate = new MongoDate($dt->getTimestamp());
// print_r($mongodate);
$cursor = $dc->findOne(array('keyword' => $keyword, 'date' => $mongodate));
echo "Cursor loaded Doc (".$cursor['_id'].") : ";
echo memory_get_usage()." in MB : ";
echo memory_get_usage()/1024/1024;
echo "<br>";
** Echos True Memory Usage**
3932160 in MB : 3.75
Cursor geladen Doc (534cdee3c30fd1b8ee0bb641) : 218305980 in MB : 208.1928062439
Code with Debug echos True Memory Usage:
echo memory_get_usage(true)." in MB : ";
echo memory_get_peak_usage(true)/1024/1024;
echo "<br>";
unset($cursor);
$dt = new DateTime($day." 00:00:00", new DateTimeZone($this->timezone));
$mongodate = new MongoDate($dt->getTimestamp());
// print_r($mongodate);
$cursor = $dc->findOne(array('keyword' => $keyword, 'date' => $mongodate));
/*
echo "<pre>";
print_r($cursor);
echo "</pre>";
*/
echo "Cursor loaded Doc (".$cursor['_id'].") : ";
echo memory_get_usage(true)." in MB : ";
echo memory_get_peak_usage(true)/1024/1024;
echo "<br>";
** Echos True Memory Usage**
3932160 in MB : 3.75
Cursor loaded Doc (534cdee3c30fd1b8ee0bb641) : 218628096 in MB : 224.5
So only one Documents causes an encrease by over 200 MB of memory.
bitrs3:PRIMARY> var doc = db.dailies.findOne({"_id" : ObjectId("534cdee3c30fd1b8ee0bb641")})
bitrs3:PRIMARY> Object.bsonsize(doc)
16754823
The document loaded is truely not small, it has 16754823 Bytes so reaches the maximum Bson Size of 16 MB
Still I am wondering if it is normal that the findOne + cursor operation which creates an array out of the results needs so much memory.

Answer 1

You can verify if this is "the cost of doing business with PHP" or if you have found a bug in the driver by serializing the array (using serialize() or even json_encode()) and save it to a file.
Then you unserialize() (or json_decode()) the content of the file and check the memory usage.
If the memory usage is similar you see the actual overhead of PHP typing.
<?php
$mc = new MongoClient;
$collection = $mc->selectCollection("myDB", "myCollection");
$d = $collection->findOne(array("my" => "criteria"));
var_dump(memory_get_usage() / 1024 / 1024);
file_put_contents("serialized.bin", serialize($d));
?>
Then loading it again:
<?php
$val = unserialize(file_get_contents("serialized.bin"));
var_dump(memory_get_usage() / 1024 / 1024);
?>
EDIT: To preemptively clarify any misunderstanding.
A 16Mb MongoDB document does generally not need hundreds of megabytes of memory.
If you however have hundreds of thousands of elements inside that object, then individual element overhead starts to count times every element you have.