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Subject: Memory needed for arrays? And major performance problems filling an array
From: php_lists@realplain.com ("Matt W")

Hi all,

This is my first time posting to these lists, so I apologize if the
internals list is wrong. :-)

Before I describe my issue, I had been wondering how I can tell/estimate
how much memory PHP will use for arrays?  Is there at least a rough formula
someone can tell me?

My environment: 4.3.6 as an Apache 1.3 module on Windows 2000.

OK, now the array performance problem -- I have a simple script that
basically just has a 2 dimensional array with this structure: [4-10
character key][int key] = int value. For my examples, the array, $big_arr,
has ~40k elements, and ~100k total at the second level. I fill this array
in a loop. Then I have this code:

$list = implode("','", array_keys($big_arr));
$r = mysql_query(<query using each $big_arr key>);

while ($tmp = mysql_fetch_assoc($r))
{
    $foo[$tmp['key for string value']] = $tmp; // just 8 small columns in
$tmp
}


The problem is that it takes over 2 seconds to fill the $foo array with
~40k array elements. Ugh! (Still ~1.4s if I use the same 123 scalar value
for each.) That's not counting any time to do the query or fetch the rows.

I'm thinking it has something to do with $big_arr using a lot of memory
(how much??), because a script with just this code:

for ($i = 0; $i < 40000; ++$i) { $test["test$i"] = $i; }


... creates 40k elements in 0.3s, *including* the loop's running time.

Finally, I tried to unset($big_arr) before filling $foo, but that
*increased* running time by ~3.5s! :-( (The unset() took about 0.3s.) Why
is that causing $foo to be filled slower?! Is PHP searching for free memory
for $foo that had been used by $big_arr?

This is disappointing, especially being caused by an array that doesn't
seem too outrageously large (and I'd like to go larger). :-( It's going to
suck if I can't come up with a way to get it faster... Is there anything
that would be different on a version newer than 4.3.6, as I didn't try any
yet? Or if it's some Windows thing, then I don't care as much.

Comments, suggestions?

Thanks!
Matt