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

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:

----- 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 -- PHP Internals - PHP Runtime Development Mailing List To unsubscribe, visit: http://www.php.net/unsub.php