#46181 [Opn]: memory leak on garbage collecting function return values
ID: 46181
User updated by: xl269 at cam dot ac dot uk
Reported By: xl269 at cam dot ac dot uk
Status: Open
Bug Type: Performance problem
Operating System: debian gnu/linux lenny
PHP Version: 5.3CVS-2008-09-26 (CVS)
New Comment:
oh, i forgot to add, if you uncomment those lines in the code provided,
the bug doesn't appear. which odd, since it appears for the smaller
data.
Previous Comments:
------------------------------------------------------------------------
[2008-09-26 10:28:47] xl269 at cam dot ac dot uk
Description:
------------
I have no idea if my explanation of this bug is correct, but here's a
guess anyway.
PHP does not seem to garbage collect return values from functions, when
they are operated-assigned (eg. $v = func() + 5; or $v = func()." ";) to
a variable in the caller. but this is fine for direct assignments (eg.
$v = func();)
It would seem from this that PHP is storing functions' return values
internally as if they were pointed to by a variable (that doesn't exist
in the script), and using copy-on-write to avoid copying this data when
it's assigned to an actual script variable. However copy-on-write,
doesn't trigger when there is an operation on this data before this
assignment.
Reproduce code:
---------------
<?php
define('LF', chr(10));
// define('REALLY_LONG_STRING', str_repeat("really ", 65536)."long
string");
function generic($for, $title) {
mem('g+');
$data = '<abbr title="information for
'.$for.'">'.$title.'</abbr>';
// $data = '<abbr title="information for
'.$for.'">'.$title.'</abbr>'.REALLY_LONG_STRING;
mem('g-');
return $data;
}
function lol1() {
mem('l1+');
$data = " ".generic("name", "test")." ";
mem('l1-');
return $data;
}
function lol2() {
mem('l2+');
$data = generic("name", "test");
mem('l2-');
return $data;
}
echo memory_get_usage().LF;
for ($i=0; $i<8; $i++) {
$a[] = lol2();
echo memory_get_usage().LF;
$a[] = lol1();
echo memory_get_usage().LF;
}
?>
Expected result:
----------------
see below
Actual result:
--------------
635168
635240 l2+
635496 g+
635672 g-
635672 l2- (no memory leak here)
635928
636024 l1+
636072 g+
636184 g-
636248 l1- (this value should only be 2 bytes larger than the
previous)
636336
------------------------------------------------------------------------
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