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Memory needed for arrays? And major performance problems filling an array

php.internals

Matt W

21 years ago
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

Derick Rethans

21 years ago
On Thu, 9 Dec 2004, Matt W wrote:
> 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?
42 * number of array elements + size of keys + data
> 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?
Make a custom C extension.. I don't know why you needs arrays this large, but it sounds like a application design flaw. Derick

Matt W

21 years ago
----- Original Message ----- From: "Derick Rethans" Sent: Thursday, December 09, 2004
> On Thu, 9 Dec 2004, Matt W wrote: > > > 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? > > 42 * number of array elements + size of keys + data
Thanks Derick. Is that exact or just an estimate? So PHP needs 42 bytes for each array element, OK. For a multi-dimensional array, I assume the "number of elements" is the total from all dimensions added up. e.g. $foo[] = array(123, 456); is 3 elements (1 + 2)?
> > This is disappointing, especially being caused by an array that doesn't > > seem too outrageously large (and I'd like to go larger). :-( It's going
to
> > suck if I can't come up with a way to get it faster... Is there
anything
> > that would be different on a version newer than 4.3.6, as I didn't try
any
> > yet? Or if it's some Windows thing, then I don't care as much. > > > Comments, suggestions? > > Make a custom C extension.. I don't know why you needs arrays this > large, but it sounds like a application design flaw.
I need arrays this large because it's building a search index, which could have 100s of millions of entries; and I want to buffer many entries in arrays before doing writes... So even these "large" arrays are just working on a small chunk at a time. :-) I was playing with it more today trying to figure out why it was dramatically slowing down while filling the second large array ($foo). Now I understand more what's going own (thanks in part to that memory usage formula), but there's still a couple odd things. (And I realize that having a 40k element array with an 8 element array for each value takes a lot of memory!) Somewhere in the early part of filling the 40k element array, PHP seems to hit a "bump" where something (mem allocation?) takes much more time, and then it fills the array quickly again (e.g. 40k doesn't take much longer to create than 10k, for example). That I can understand I guess (though I always like to know exactly what's happening? :-)). The most puzzling part to me still remains: when I unset() the first large array ($big_arr), why does filling the second array take more than twice as long (5+ seconds vs. 2+)? If a smaller second array is used, it *does* go faster with the first array unset, which is what I'd expect. So why, when it gets to a certain size does having unset the first array make things slower?? :-/ Thanks if anyone has an explanation to help me better understand what's going on internally (especially the unset() thing). Matt

Wez Furlong

21 years ago
Sidestepping your original question, it sounds to me like you might benefit from using sqlite here, either direct to disk or using an in-memory sqlite database. --Wez. On Fri, 10 Dec 2004 05:02:16 -0600, Matt W <php_lists@realplain.com> wrote:

Andi Gutmans

21 years ago
Hi Matt, The problem is most probably because the Zend array hash table reallocs everytime it starts to hold too many elements. The larger it gets, the less likely it manages to reallocate in the same memory area and then it has to allocate a new big memory chunk and memcpy() all the old data to it. This is probably where you are feeling the hit. Andi At 05:02 AM 12/10/2004 -0600, Matt W wrote: