Troubleshooting memory leaks

Question

I have set my PHP MAMP server to a maximum of "256M" in the php.ini but am suddenly getting a memory error. There are two things that are strange about this:

1. This is a shockingly high amount of memory for a script to use. Maybe I'm too easily shocked but should scripts be running over 256M? I wouldn't have thought so.
2. Even though I've now changed my setting to "512M" I still get the memory error and it still reports running out in/around the 256M range (as if my setting is being ignored). I did recycle the web server which I figured would be enough to get the new settings into play.

The exact message I'm getting is this:

[25-Oct-2012 14:27:53] PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 222300161 bytes) in [FILE_PATH]/wp-content/themes/lifegadget-pagelines/sections/lg_content/section.php on line 42
[25-Oct-2012 14:27:53] PHP Fatal error:  Call to a member function get() on a non-object in [FILE_PATH]/wp-content/object-cache.php on line 52

So my questions are as follows:

1. Troubleshooting . Is there any way to troubleshoot memory usage?
2. Scale . Is exceeding a 256M limit a clear problem or is this justified in some cases?
3. New Ceiling . Why is my new ceiling of 512M being ignored? Not that I'm happy to have it stay at 512M but I would have thought it would at least get me back up and running (I have 16GB on the machine so there's plenty of physical memory).

Answer 1

Infinite loop can cause this or if you have circular references with pre 5.3 PHP version. See: Garbage collection

Answer 2

Inject this function ini_set('memory_limit', -1); at the very beginning of your suspected memory leaking class/page, this is a temporary solution to get things working but it won't help you know where exactly your script is leaking memory.