Object Memory Leakage...

> I posted this to php-general several days ago, and the only response > suggested that perhaps I should forward the question to PHP internals. > If what's below isn't expected behavior, I'd be glad to dedicate a few > days to helping track down the problem. > > I've tried what's below on the server cluster in the office, running > Debian Sarge i386 (with an x86-64 kernel), PHP 5.0.16, and Apache > 2.0.54. I've also tried at home on a Gentoo box (Athlon-MPs) running PHP > 5.0.16 and Apache 2.2. Both produce similar results. (If > compiled-in/shared module lists, compile options, or any other > information would help, I can provide those too.) > > What's below this sentence is the original text of the message sent to > php-general. > > Sorry folks, this is a long one...I've re-read a bunch of the > documentation, and I can't find a solution. > > In debugging an out of memory condition, I found that PHP doesn't seem > to garbage collect unreferenced objects. It seems to work fine for > anything other than objects. (Arrays, strings, integers, etc.) > > Here's a simple example: define a class, create 3 instances, destroy > them in reverse order. I would expect that after each unset and/or NULL, > the memory usage would go down, they don't. See the output below. > > <?php > > class Tester { > protected $test1; > protected $test2; > protected $test3; > protected $test4; > > public function __construct() { > $test1 = rand(); > $test2 = rand(); > $test3 = rand(); > $test4 = rand(); > } > } > > echo memory_get_usage() . "\n"; > $t1 = new Tester(); > echo memory_get_usage() . "\n"; > $t2 = new Tester(); > echo memory_get_usage() . "\n"; > $t3 = new Tester(); > echo memory_get_usage() . "\n"; > unset($t3); > $t3 = NULL; > echo memory_get_usage() . "\n"; > $t2 = NULL; > echo memory_get_usage() . "\n"; > unset($t1); > echo memory_get_usage() . "\n"; > > > This outputs: > > 43344 > 43928 > 44232 > 44568 > 44640 > 44640 > 44640 > > Note: the memory usage remains constant after the objects are allocated, > even when they're unset/NULL'ed. > > Some of the user contributed notes on the PHP documentation suggest that > the memory is actually freed and available for use by other > variables/objects even though memory_get_usage() says otherwise. Another > little test seems to disprove that too... The idea here is to allocate > close to 8 MB, then free it to show that it can allocate that data > within the memory limit. Then allocate enough class instances and free > them again such that the memory usage increases enough that the same > data allocated and freed initially doesn't fit anymore. If PHP did > indeed free the objects, the data would fit the second time too... > > <?php > > ini_set('memory_limit','8m'); > > class Tester { > protected $test; > > public function __construct() { > $this->test = rand(); > } > } > > echo "Initial Memory Usage: " . memory_get_usage() . "\n"; > > $mb = 6 * 1024 * 1024; > echo "Allocating 7 MB of data..."; > $dat = ""; > for ($x=0;$x<$mb;$x++) { > $dat .= "x"; > } > echo "Memory Usage: " . memory_get_usage() . "\n"; > unset($dat,$x,$mb); > echo "Unset Usage: " . memory_get_usage() . "\n"; > > $num = 32767; > $objects = array(); > > echo "Initial Memory Usage (2): " . memory_get_usage() . "\n"; > echo "Allocating $num objects..."; > for ($x=0;$x<$num;$x++) { > $objects[$x] = new Tester(); > } > echo "Done. Memory Usage: " . memory_get_usage() . "\n"; > echo "Freeing $num objects..."; > for ($x=0;$x<$num;$x++) { > unset($objects[$x]); $objects[$x] = NULL; > } > echo "Done. Memory Usage: " . memory_get_usage() . "\n"; > > $mb = 6 * 1024 * 1024; > echo "Allocating 7 MB of data..."; > $dat = ""; > for ($x=0;$x<$mb;$x++) { > $dat .= "x"; > } > > This gives: > > Initial Memory Usage: 44248 > Allocating 7 MB of data...Memory Usage: 6335888 > Unset Usage: 44448 > Initial Memory Usage (2): 44504 > Allocating 32767 objects...Done. Memory Usage: 7752920 > Freeing 32767 objects...Done. Memory Usage: 2807080 > Allocating 7 MB of data... > Fatal error: Allowed memory size of 8388608 bytes exhausted (tried to > allocate 1 bytes) in /home/janderson/svn/memtest.php on line 44 > > > Am I missing something? (Is there a force_garbage_collection() function > somewhere that I'm missing?) Any suggestions/workarounds/anything would > be most appreciated. > > Cheers, > > jon