In last month's update I explained that I was investigating whether Xdebug can make use of PHP's Observer API. It turns out that it can be used to intercept function calls, but only include or require calls if the included files contain code, and not just class definitions. As Xdebug treats include and friends as actual function calls, I can unfortunately not solely rely on the Observer API.

In the wake of checking out the Observer API, I also thought I should have a look at some performance improvements. For example, I noticed that Xdebug would always collect local variables with each function call. This is only really needed when showing local variables, in stack traces, or through the step debugger.

Another optimisation that I worked on was to optimise the way how function breakpoints are checked against. These breakpoints trigger when a function gets called, or returned from. This is not a feature that many people often use, but Xdebug would always do some work to be able to compare the configured breakpoints against a normalised function name reference.

These two optimisations together resulted in a 20% reduction in CPU instructions (roughly equivalent to execution time) with the front page of WordPress' demo site.

The third optimisation that I worked is related to file/line breakpoints. Xdebug would evaluate whether an IDE has set a line breakpoint on the current line. For this, it had to loop over all the existing breakpoints and compare them. Each additional breakpoint would be checked after every statement, meaning that the number of breakpoints affected the running time of the script.

My optimisation alleviates this by moving the check on whether line breakpoints exist for a function or method to the function call itself. If no breakpoints are set in the whole function, then Xdebug skips the check for line breakpoints after each statement. This shifts the factor of performance loss for having line breakpoints from the number of statements to the number of function calls. This shift results in a roughly 25% performance boost with only four line breakpoints enabled.

After attending IPC and speaking to fellow Xdebug users, a question came up about long running scripts. Right now, Xdebug's step debugger can only be activated when the script starts or by calling xdebug_connect_to_client(). Breakpoints can also only be configured when Xdebug is waiting for a command to continue a script (after a step, an existing breakpoint, or at the start of the script). While a script is running, you can not interrupt the execution to break, or add new breakpoints.

This let me to experiment with a control socket, currently only available on Linux. Through this socket you can then ask Xdebug for information, or request a breakpoint so that you can then use your IDE to add more breakpoints, or inspect the current state.

At the moment, I have implemented the "show me some information" feature, which allows me to show the running PHP scripts, with PID, memory usage (in kb), running time, and Xdebug version. The xdebug command line tool allows you to control Xdebug through the socket:

I have not merged this feature yet, but I hope to do so once I have the step debugger interruption feature as well.

Beyond this, I will continue to work on the features and issues on the 3.3 roadmap, without any guarantees these tickets will be implemented.

I have not published any new videos in the last month, but I did record one. It is set to premiere on November 7th. To get notified, you can subscribe to my YouTube channel, or follow me or Xdebug on Mastodon.

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