PHP memory error with large array

Question

I am trying to create a 2D array in PHP of 2000x2000 size (4 million records). It seems like I'm running out of memory, but the way the error appears is confusing to me.

When I define an array and fill it first using the array_fill command, and initialize each position in the array (matrix) with 0, there is no problem.

However, if I try to iterate through an array and fill each position with 0, it runs out of memory.

I would suggest that when I run array_fill, it allocates memory at this point and should not exit memory in a loop.

Of course, this is just a simplified version of the code. In my actual application, I will use the X and Y coordinates to find the value from another table, process it, and then save it in my matrix. These will be floating point values.

Can someone help through some light on this please? Is there any other way I have to do this?

Thanks!

<?php // Set error reporting. error_reporting(E_ALL); ini_set('display_errors', TRUE); ini_set('display_startup_errors', TRUE); // Define Matrix dimensions. define("MATRIX_WIDTH", 2000+1); define("MATRIX_HEIGHT", 2000+1); // Setup array for matrix and initialize it. $matrix = array_fill(0,MATRIX_HEIGHT,array_fill(0,MATRIX_WIDTH,0)); // Populate each matrix point with calculated value. for($y_cood=0;$y_cood<MATRIX_HEIGHT;$y_cood++) { // Debugging statement to see where the script stops running. if( ($y_cood % 100) == 0 ) {print("Y=$y_cood<br>"); flush();} for($x_cood=0;$x_cood<MATRIX_WIDTH;$x_cood++) { $fill_value = 0; $matrix[$y_cood][$x_cood]=$fill_value; } } print("Matrix width: ".count($matrix)."<br>"); print("Matrix height: ".count($matrix[0])."<br>"); ?> 

Answer 1

I would suggest that when I run array_fill, it allocates memory at this point and should not exit memory in a loop.

Yes and no. Allocating memory and executing program code are two different shoes (usually).
The memory allocated for the program / process is usually divided into two parts - a heap and a stack. When you "allocate memory" (in the value that you used in your question), this happens on the heap. When you execute the program code, the stack is also used. Both are not completely separate, as you can click and / or pop links (pointers to a bunch) on and / or from the stack.

The fact is that the heap and the stack share part of the memory (allocated to this process), and usually it grows (populates) from higher addresses to lower ones, and the other - from low addresses to higher ones, and therefore you have a "floating" the border between both. Once both sides have reached this “border,” you “lost your memory.”

So, in your case, when you create and populate your array (matrix), you used memory for integers 2001 x 2001. If an integer requires 32 bits or 4 bytes, that is 2001 x 2001 x 4 bytes = 4004001 x 4 bytes = 16016004 bytes ~ 16 MB.
When the code is executed, the stack is filled with (local) variables - a loop condition variable, a loop counter, and all other variables.
You should also not forget that PHP (library) code must also be loaded into memory, so depending on the value you set as memory_limit in your configuration, you can quickly run out of memory.