Memory is not free()'d after xcache_get() data is unset

[krogot]

I've tested EA vs XCache performance on cache reads and got the following problem:

Code:

<?php
    function ea_read ($max) # void
    {
        global $DATA;

        for($i = 0;$i<10;$i++)
        {
            $key[$i] = 'some_key_'.$i;
            eaccelerator_put($key[$i], $DATA);
        }

        for($i = 0;$i<$max;$i++)
            $val = eaccelerator_get($key[$i%10]);
    }

    function xcache_read ($max) # void
    {
        global $DATA;

        for($i = 0;$i<10;$i++)
        {
            $key[$i] = 'some_key_'.$i;
            xcache_set($key[$i], $DATA);
        }

        for($i = 0;$i<$max;$i++)
        {
            $val = xcache_get($key[$i%10]);
        }
    }

Result ($max = 10000):

<?php
    $DATA = str_repat('x',10000); 

Everything is ok.

<?php
    $DATA = array_map('uniqid', range(1,10));

Fatal error: Allowed memory size of 16777216 bytes exhausted (tried to allocate 15 bytes).

It seems that XCache somehow marks array items that they aren't released (may be it increases ref_count?). I tried to add "unset($val);" after "$val = xcache_get();" but it didn't help.

The only solution I've found is:

<?php
   xcache_set($key,serialize($data));
   $val = unserialize(xcache_get($key));

but it looks weird (and slows down xcache calls) ...

[moo]

i have a error in theory about this issue. it will take some time for me to find a correct way

[oli]

Hi,

I am not quite sure, but I think this was the problem I was facing earlier today. It could be, that report:9 is also the same. Consider the following code:

<?php
$data='Some Content';
//xdebug_debug_zval('data');
xcache_set('example', $data, 3600);

$data=xcache_get('example');
//xdebug_debug_zval('data');

for($i=0;$i<=1000000;$i++){
	$var=xcache_get('example');
	if(($i%10000)==0) echo "Memory consumption after $i iterations is " . memory_get_usage() . "\n";
}
?>

Running this under two distinct systems both having PHP 5.2.5 together with XCache 1.2.2 results in

PHP Fatal error:  Allowed memory size of 33554432 bytes exhausted (tried to allocate 16 bytes) in ... on line 10

after ~700000 iterations. Well, it was assumed that xcache_get holds a reference which makes it impossible to free the variable ($var). I tried to verify this by using XDebug on one of the two machines. If you enable the two xdebug_debug_zval statments you'll get something like

data: (refcount=1, is_ref=0)='Some Content'
data: (refcount=2, is_ref=0)='Some Content'

which is kind of surprising. It seams the reference counter of the variable retrieved from xcache equals one plus the reference counter when setting the variable (originally I had a variable having refcount=3 and after retrieving it from xcache it had refcount=4). Replacing the variable in xcache_set through a constant string gives you a variable with refcount=1, which is actually correct (no memory leak). I don't know much about the internals of PHP, but it seams understandable that the memory is not freed since the variable reference counter too big.

I also find a (temporary) solution for that problem. Replacing the xcache_set statement through

xcache_set('example', (string)$data, 3600);

seams to fix it. I assume the explicit cast to string lets PHP create a temporary variable with refcount=0 (similar to a constant).

Please contact me, if you need any further information.

[moo]

thanks. the problem is confirmed already. i was trying to figure out a fix

[oli]

Thank you for looking at the problem. I was aware this was already confirmed. However, I hope the additional input makes it easier to locate the issue, and if not at least affected user can use the explicit cast trick when setting their variables as long as the bug is not fixed.

[moo]

hrm... you're right. the workarround can be done inside xcache_set. i'll check if the workarround fix the problem correctly. referenced variable is the key point.

[moo]

sorry, it does not work

<?php
$b = "a";
$a = array(&$b, &$b);
$c = (array) $a;
$c[0] = 1;
var_dump($b);
?>

actual result: int(1), expected: string(1) "a"

and btw, the problem is located already.

i just want to figure out a most optimized way to do it by paying more time thinking about it. this problem is not urgency because no one want to do dead or huge loop in webserver and the leak will be free'ed by php GC anyway.