Developer Area/Php profiling

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What is profiling

Profiling is a method that is used to detect performace issues. For this we use tools to record stack traces of the running program to later be analysed

The install and configuration instructions work on Ubuntu 18.04


The first thing we need to do is installed Xdebug php extension to get the data of Mahara's php scripts. This extension can also be used for debugging php code well, but I will only add the instructions fro profiling here.

install extension

sudo apt-get install php-xdebug

Check the extension is there

php -m | grep xdebug

On php.ini you need to add the following lines

xdebug.remote_enable = 1
xdebug.remote_autostart = 1
zend_extension = /usr/lib/php/20170718/

you should check where is the in your machine

Add the setting for xdebug profiling

Here you can choose if you want to trace all the php code that is run or only a few scripts by passing an argument to the selected ones. more information on this here

To trace all scripts

Set to 1 to turn it on for every request

xdebug.profiler_enable = 0

To trace only the scripts with the parameter, let's use a GET/POST parameter to turn on the profiler

xdebug.profiler_enable_trigger = 0

The GET/POST value we will pass; empty for any value

xdebug.profiler_enable_trigger_value = ""

Set up the output file name and location
Output cachegrind files to /tmp so our system cleans them up later

xdebug.profiler_output_dir = "/tmp/profiling"
xdebug.profiler_output_name = "cachegrind.out.%p"

Don't forget to restart the server

sudo service apache2 restart

If you made it work, you'll get files named cachegrind.out.* in the directory /tmp/profiling. These will contain data that looks like this

1409 2 0
calls=1 0 0
1410 0 160

So we will need another application to display this information in human friendly way.


Kcachegrind is a tool to display the profiling data in a graphical way. All those numbers and codes that we captured with the xdebug profiling tool can be seen as function calls and graphs.

How to in install

sudo apt-get install kcachegrind

Open the profiling files

Once you have the kcachegrind installed, you should be able to open one of the xdebug data generated files. On the kcachegrind application, open one of the 'cachegrind.out' files located in the directory set on xdebug.profiler_output_dir property (php.ini). You should be seeing something like

Kcachegrind example

How to use it to identify code to refactor

Here you can see all the function call and the resources consumed by it. In this case I've selected the resource to be 'Time' but you can change that to be 'Memory' at the top bar.

On the left table, the functions can be order by self, and incl. where

  • self: cost of the function itself ('Self Cost')
  • incl.: cost including all called functions ('Inclusive Cost')

You can play with this to identify which functions have the higher cost and should be looked into. Take into consideration that this method will not differentiate between Mahara code and third party library functions. There's not much we can do about the cost of the third party function calls but we can check how many times they are being called by our code.

On the right side we have the graph view, here we have the sequence of function calls and how many times they are executed. We should be paying attention to the call that have great cost (% of the total cost) and are repeated many times.

More information of how to use this application and what the settings mean can be found here

Common error

  • I've set up the Xdebug tool for profiling but the 'cachegrind.out' are not generated.

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