PHP Array Performance - Reasonable Memory

Question

I have a 5 MB array, I pass it to a function in a function (I do not pass it by reference )

foo( $arr ); function foo( $arr ) { .... bar( $arr ); .... } function bar( $arr ) { .... test( $arr ); .... } function test( $arr ) { .... test2( $arr ); .... } 

PHP passes array values by value (copy of value) by default to a function.

My question is: if this array value is passed to 100 function calls, will PHP consume 100 x 5 MB = 500 MB of memory?

How does PHP handle large arrays (from memory) for function calls?

Answer 1

Here is the code to check:

 <?php function testnochanges($arr1){ foreach($arr1 as $val){ // } return $arr1; } function testwithchanges($arr1){ $arr1[] = 1; return $arr1; } echo "Stage 0: Mem usage is: " . memory_get_usage() . "<br />"; for ($i = 0; $i < 100000; ++$i) { $arr[] = rand(); } echo "Stage 1 (Array Created): Mem usage is: " . memory_get_usage() . "<br />"; $arrtest1 = testnochanges($arr); echo "Stage 2 (Function did NO changes to array): Mem usage is: " . memory_get_usage() . "<br />"; $arrtest2 = testwithchanges($arr); echo "Stage 3 (Function DID changes to array): Mem usage is: " . memory_get_usage() . "<br />"; ?> 

and here is the result after launch:

 Stage 0: Mem usage is: 330656 Stage 1 (Array Created): Mem usage is: 8855296 Stage 2 (Function did NO changes to array): Mem usage is: 8855352 Stage 3 (Function DID changes to array): Mem usage is: 14179864 

At stage 0, we see that before creating the array, PHP already uses some space in memory. After creating the first array ( Stage 1 ), we can see big changes in memory usage, as expected. But after calling the testnochanges function and creating $arrtest1 in Stage 2, we see that the memory usage has not changed much. This is because we did not make changes to $arr , so $arrtest1 and $arr still point to the same array. But in Stage 3 , where we call the testwithchanges function and add the element to $arr PHP performs copy-on-write , and the returned array, which is assigned to $arrtest2 , now uses a different part of memory and again we see a big increase in memory usage.

Dry conclusion: if you copy an array to another array and do not change it, the memory usage remains the same as both arrays are pointed to the same. If you change the array, then PHP does copy-on-write and, of course, memory usage is increasing.

Good to read: Be careful with garbage collection, part 2 .

Answer 2

The only way to find out how your specific use case works is to complete the test (quick and dirty). If you are having performance issues, you can look in the SPL. For example, SplFixedArray can save memory when working with large arrays, if you know the size in advance. In addition, newer versions of PHP handle garbage collection much better than previous versions, so updating can also help.

If you decide to run tests, this will allow you to start (compares SPL with raw arrays): https://github.com/elazar/spl-benchmarks