Description:
------------
the following code fails with 

  Fatal error: Allowed memory size of 587896 bytes exhausted (tried to allocate 262145 bytes)

command:
  php.exe -n -f memuse.php

memuse.php:
  @ob_end_flush();
  @ob_implicit_flush(1);
  define('big_size',256*1024);
  define('saftey',4*1024);
  ini_set('memory_limit',memory_get_usage()+big_size*2+saftey);
  $big = str_repeat('x',big_size);

yet if I change big_size to 1024*1024, then it works?


and a simlar script
  @ob_end_flush();
  @ob_implicit_flush(1);
  define('big_size',200*1024);
  define('saftey',4*1024);
  ini_set('memory_limit',memory_get_usage()+big_size+saftey);
  echo str_repeat('x',big_size);

also fails, yet if I change big_size to 180*1024 then it works?


PS, note that in the first script I set mem limit to 2 times big_size, and in the 2nd script I did not.


Reproduce code:
---------------
see above

Expected result:
----------------
see above

Actual result:
--------------
see above