Why does a PHP script constantly take up more memory?

Question

See this example:

echo memory_get_usage() . "\n"; // 36640
$a = str_repeat("Hello", 4242);
echo memory_get_usage() . "\n"; // 57960
unset($a);
echo memory_get_usage() . "\n"; // 36744

Can anyone explain why after un-setting the variable the memory usage does not return to 36640

No, if you are running script loop, it not always allocated max memory. — azat
– azat, Commented Mar 23, 2011 at 15:27

powtac · Accepted Answer · 2011-03-23 15:26:13Z

5

If you do it twice the memory will stay at 36744...

echo memory_get_usage() . "\n"; // 36640
$a = str_repeat("Hello", 4242);
echo memory_get_usage() . "\n"; // 57960
unset($a);
echo memory_get_usage() . "\n"; // 36744
$a = str_repeat("Hello", 4242);
unset($a);
echo memory_get_usage() . "\n"; // -> 36744

answered Mar 23, 2011 at 15:26

powtac

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4 Comments

KingCrunch Over a year ago

It implies, that a long-running script is not "doomed"

Jon Over a year ago

I have the same question: How is this answer useful?

powtac Over a year ago

It shows you how the interpreter works. There is a small overhead even after unsetting something. But when doing it again there is no more memory overhead.

Jon Over a year ago

The first conclusion is obvious already from the original question. The second simply does not follow. And certainly after unsetting a variable that has already been unset we would expect nothing to change.

Marc B · Accepted Answer · 2011-03-23 15:35:09Z

4

Garbage collection is an expensive operation, even if there's only a single variable to unset. PHP won't run the collector each time you unset a var, as that'd waste a huge amount of CPU time.

PHP will only run the collector when it has to, as in when something wants more memory than is available.

answered Mar 23, 2011 at 15:35

Marc B

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Comments

Jean-Philippe Leclerc · Accepted Answer · 2011-03-23 15:27:29Z

3

What is your PHP version? The garbage collector in versions less than 5.3 is not really good. Please read this link to understand why:

Garbage collector

answered Mar 23, 2011 at 15:27

Jean-Philippe Leclerc

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4 Comments

Jon Over a year ago

He's freeing up the memory himself. Where does the GC come into it?

Jean-Philippe Leclerc Over a year ago

Try with 5.3 and post the results

Chris Over a year ago

Philippe: powtac has clarified that there is an overhead, but I am interested to see the difference between 5.2 and 5.3 and your response suggests you use 5.3 - Without having to install PHP on this PC, and without have to reinstall PHP on my server (which I am not prepaired to do today) can you enlighten us of the improvements between the old and new versions of the PHP GC by posting the results of this script?

Jean-Philippe Leclerc Over a year ago

I will favorite this thread and come back later because I can't right now.

Jim · Accepted Answer · 2011-03-23 15:38:30Z

1

Just posting this.

I just ran it as a test for fun on PHP 5.3, the results are pretty clear to what powtac said:

So yea, after the initial unset it appears to be consistent throughout. Code tested with:

while (1) {
        echo memory_get_usage() . "\n"; // 36640
        $a = str_repeat("Hello", 4242);
        echo memory_get_usage() . "\n"; // 57960
        unset($a);
        echo memory_get_usage() . "\n"; // 36744
}

Caution: that is an infinite loop :)

answered Mar 23, 2011 at 15:38

Jim

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Comments

NikiC · Accepted Answer · 2011-03-23 15:46:10Z

0

I'll try to give one possible explanation, but I cannot claim that it is the right one.

PHP stores variables in a hash table (because of it's dynamic nature). This hash table consists of "buckets" (linked lists of elements). As the number of elements grows the number of buckets is increased, too (to be precise: The number of buckets is doubled as soon as the limit is reached).

Thus it could be the case, that the creation of your variable results in an increase of buckets. As these buckets aren't removed again, the memory usage stays.

But again: Only guessing.

answered Mar 23, 2011 at 15:46

NikiC

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1 Comment

Narcissus Over a year ago

I would think the same thing: the 'test' is to define $a = '' first and run the memory_get_usage() function before assigning the huge string. The funny thing is, when I run the original code, my third result is exactly the same as my first.

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