As your app gets increasingly more complex, you might run into Memory exhausted errors, and even though you can always increase the allowed memory usage — either by tweaking php.ini or locally with ini_set() — it should be a better option to find out what’s using so much memory in the first place.
First, you should install Xdebug, a debugger and profiler tool for PHP that’s quite popular and it’s probably already available on your OS repositories. In Ubuntu, you can install it with sudo apt-get install php5-xdebug
Check your php_info() to check if the extension is being loaded. If you’re using PHP-FPM, remember that you should restart the service with sudo service php5-fpm restart. Since loading this extension can affect performance, you should only enable it on your development server.
Then, we’ll need to enable function traces. Edit /etc/php5/mods-available/xdebug.ini and add the following:
xdebug.trace_enable_trigger = 1 xdebug.trace_format = 1
The first line will enable function traces but only when the current URL uses a special parameter: http://dev.lo/index.php?XDEBUG_TRACE=1 (check the extension docs to see how you can also enable the trigger with a POST parameter or a cookie).
The second line will set the trace format to a Tab-Separated-Values format, which we’ll use as input on a script that can summarize the generated data.
When you hit the URL you want to debug, the function trace report will be generated on your tmp files, for instance: /tmp/trace.2931297.xt
Now that you have your trace data, get the tracefile analyser from Derick Rethans’ (the php5-xdebug developer) blog or directly from his GitHub repo.
Call the script from the command line. You can run php tracefile-analyser.php with no arguments to see the available options.
For instance, running php tracefile-analyzer.php trace.2987318.xt memory-own 20 will show the 20 most costly calls sorted by memory-own
This will get you a solid starting point to begin analyzing and improving your code.
The output will be something like this:
Inclusive Own
function #calls time memory time memory
----------------------------------------------------------------------------------------
{main} 1 6.2529 20038368 0.0004 328
require 124 6.2525 20037984 0.0085 744
require_once 24 6.0223 18954720 0.0295 272
call_user_func_array 1068 3.6975 2314968 0.0170 91640
get_object_vars 1065 0.0163 1964720 0.0163 1964720
do_action 188 3.3480 1904872 0.0049 -19624
preg_replace 2943 0.0507 1778248 0.0507 1778248
array_keys 573 0.0081 1749608 0.0081 1749608
str_replace 2065 0.0288 1306552 0.0288 1306552
debug_backtrace 43 0.0024 1219280 0.0024 1219280
WP_Query->query 3 0.2543 1152352 0.0001 -8016
WP_Query->get_posts 3 0.2539 1149464 0.0017 -25720
wp_cache_get 2296 0.1765 1102160 0.0469 168
WP_Object_Cache->get 2296 0.1296 1101992 0.0694 991784
wp 1 0.2217 1082456 0.0000 88
WP->main 1 0.2216 1082368 0.0001 1904
wpdb->get_results 14 0.1221 1063720 0.0034 58024
WP->query_posts 1 0.2171 1040112 0.0000 216
update_post_caches 5 0.1718 1038048 0.0003 -15760
_prime_post_caches 2 0.1521 1006248 0.0001 -84472