memory_get_peak_usage() with “real usage”

Ok, lets test this using a simple script:

ini_set('memory_limit', '1M');
$x = '';
while(true) {
  echo "not real: ".(memory_get_peak_usage(false)/1024/1024)." MiB\n";
  echo "real: ".(memory_get_peak_usage(true)/1024/1024)." MiB\n\n";
  $x .= str_repeat(' ', 1024*25); //store 25kb more to string


not real: 0.73469543457031 MiB
real: 0.75 MiB

not real: 0.75910949707031 MiB
real: 1 MiB


not real: 0.95442199707031 MiB
real: 1 MiB

not real: 0.97883605957031 MiB
real: 1 MiB

PHP Fatal error:  Allowed memory size of 1048576 bytes exhausted (tried to allocate 793601 bytes) in /home/niko/test.php on line 7

Seems like real usage is the memory allocated from the system – which seems to get allocated in larger buckets than currently needed by the script. (I guess for performance reasons). This is also the memory the php process uses.

The $real_usage = false usage is the memory usage you actually used in your script, not the actual amount of memory allocated by Zend’s memory manager.

Read this question for more information.

In short: to get how close are you to the memory limit, use $real_usage = true

Leave a Comment