How to profile your PHP code with Xdebug

Photo by Jek in the Box

I was adding some functionality to my mail system, and noticed it seemed to be running more slowly. I didn’t have any way to be sure though, and I didn’t know how to get performance information on my PHP code. After a bit of research, I discovered the xdebug PHP extension can gather profile data, and you can then use kcachegrind to view it.

On Fedora linux, you can install xdebug by running the following line in superuser mode:
yum update php-pecl-xdebug

To enable capturing of performance data, you’ll then need to edit your php.ini file, adding the following lines:

; xdebug settings
xdebug.profiler_enable = 1
xdebug.profiler_append = 1
xdebug.profiler_output_name = cachegrind.out.%s

This will output raw profile data to files named cachegrind.out.<your php script name> in /tmp. There’s a some options you may want to tweak, for example not appending repeated calls, or naming them after something other than the script name.

Once you’ve made those changes, restart apache with
/sbin/service httpd restart

Now navigate to the pages you want to profile, and the data should appear as files in /tmp. Once you’ve done the operations you’re interested in, possibly repeatedly to generate a larger sample, edit the php.ini xdebug.profile_enable to 0 and restart apache again.

I’d now got a nice collection of data, but that wasn’t much use without a way to understand what it meant. Kcachegrind is the most popular tool for viewing the output files, but it doesn’t have a native OS X version. I tried the darwin ports approach, but as always at least one of the dozens of dependencies failed to compile automatically, so I resorted to my Fedora Linux installation running in Parallels. If you’re on Windows, WinCacheGrind is a native version that’s had some good reviews. I couldn’t find a separate Linux binary, but it’s part of the kdevelop suite, so I was able to install that easily through the package manager.

Once that’s installed, copy over the cachegrind data files from your server, and then open them up in the application. You should see a list of function calls, and  if you’re used to desktop profiling, a lot of the options for drilling down through the data should seem familiar. The kcachegrind team has some tips if you’re looking for a good way to get started.

For my case, I found that most of the time was spent inside IMAP, which is actually good news since it means I’m running close to the maximum download speed and my parsing code isn’t getting in the way too much.

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