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Xdebug will skew your performance

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Xdebug is an absolutely invaluable tool when it comes to PHP programming. Due to portability reasons, I don't usually like to rely too much on extensions that have not been bundled with PHP, but Xdebug is the one extension I will always include in my development environment.

Some of the smaller convenience features provided by Xdebug, like changes to var_dump() output, can be annoying at times (though, all of them can be configured). However, the extension provides three crucial features for developing an application larger than a few thousand lines:

  • Remote debugger
  • Code Coverage
  • Profiler

These great features come at a rather high cost, though. Xdebug also imparts a great overhead, which not only reduces performance, but it does it in an unpredictable manner.

These are not the optimizations you are looking for

Whenever there's an argument on what kind of code is the fastest way to solve problems, people usually like to rely on naive performance tests. Unfortunately, one of the questions I always have to ask first is: Did you run with or without Xdebug? If the performance degradation caused by Xdebug was even, there would be fewer problems, but usually, a piece of code can perform wildly differently on production environment compared to a development environment.

Let us take a look at a simple example. Imagine you have a multi-level array of data that you want to output as JSON. However, you know the array contains DateTime instances and you want to convert those to Unix timestamps instead. On top of my head, I could come up with two different solutions.

The first solution would be to make a simple recursive function like so:

function recurseArray(array $array): array
    foreach ($array as $key => $value) {
        if (\is_array($value)) {
            $array[$key] = recurseArray($value);
        } if ($value instanceof DateTimeInterface) {
            $array[$key] = $value->getTimestamp();

    return $array;

In this case, PHP also provides a convenient function array_walk_recursive(), which could also do the job:

function walkArray(array $array): array
    array_walk_recursive($array, function (& $value) {
        if ($value instanceof DateTimeInterface) {
            $value = $value->getTimestamp();

    return $array;

However, I can't quickly tell which of these functions is faster. On one hand, the walkArray() function has a lot of overhead due to numerous function calls, but on the other, it delegates the array recursion to PHP internals.

To test this, one would typically do a quick and naive speed test like the following:

$times = 10;
$testData = array_fill(0, 10000, [[['foo']], 2, new DateTime(), [[[[new DateTime(), [1]]]]]]);

$timer = microtime(true);

for ($i = 0; $i < $times; $i++) {
    $result = walkArray($testData);

echo 'Walk:    ' . round((microtime(true) - $timer) * 1000) . "ms\n";

$timer = microtime(true);

for ($i = 0; $i < $times; $i++) {
    $result = recurseArray($testData);

echo 'Recurse: ' . round((microtime(true) - $timer) * 1000) . "ms\n";

I run the code and get the following results:

Walk:    1284ms
Recurse: 1285ms

Looks like they're about the same based on this speed test. But wait, did I run this code with Xdebug enabled or not? A quick command line call reveals that I did, in fact, have Xdebug enabled:

$ php -v
PHP 7.2.14 (cli) (built: Jan 12 2019 05:21:04) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.2.14, Copyright (c) 1999-2018, by Zend Technologies
    with Xdebug v2.6.1, Copyright (c) 2002-2018, by Derick Rethans

So, let's disable the extension and try running the speed test again. This time, we get the following results:

Walk:    550ms
Recurse: 284ms

Now, it looks like the recursion method is actually quite a bit faster.

While this is not the best example, it does illustrate that the performance profile of PHP is completely different based on whether you have Xdebug enabled or not.

You should not, however, stop at simple speed test like the one above. It can give you a good idea what might be faster, but running the code on production data may yield different results. The previous example is based real-world scenario, in which the array_walk_recursive() actually ended up being more efficient in production.

Dealing with performance woes

Despite the fact that Xdebug causes performance problems, it still provides great features, as I said in the beginning. There are several ways you could potentially deal with the problems depending on your use case and setup.

Simply don't enable Xdebug

The simplest solution is to just not enable the extension. Note that it is not enough to just set xdebug.default_enable = 0 in your config. In fact, all that it does is disable traces from errors. The performance issues are caused simply by loading the extension since it needs to hook into various places in the PHP core.

When you actually need to use one of the features from Xdebug, only load the extension on those occasions. To help with this, there are couple handy solutions.

Using a xdebug toggler script

If you're using PHP on a macOS from homebrew, one way to easily toggle Xdebug on and off is to use the xdebug-toggle script:

When you set up Xdebug via PECL, this script effectively just renames ext-xdebug.ini in your PHP installation's conf.d directory to enable and disable the Xdebug extension via command line. It can also handily reboot your apache at the same time.

Setting up the extension in PHPStorm

If you're using PHPStorm, you can also add a separate path to the Xdebug extension in your PHP cli interpreter configuration. This allows PHPStorm to include the extension via command line only when you run scripts via "Debug" or run PHPUnit with code coverage.

Running separate containers with Xdebug

If you happen to have a dockerized PHP development setup, you could alternatively setup a second container with Xdebug enabled. This could allow you to use the step by step remote debugging capabilities without the need of restarting your server. Nginx can be set up to simply redirect the request on the appropriate container based on whether you want to enable remote debugging or not.

You can read a good write up about setting up a setup like that from Juan Treminio in his article Developing at Full Speed with Xdebug.

Avoid opening remote debugging sessions on each request

If any of the aforementioned solutions seem like too much work to set up, I would at least recommend using some kind of tool to manage remote debugging sessions from the browser.

If you enable the configuration xdebug.remote_autostart, Xdebug will always start a remote debugging session and that will needlessly hurt the speed of your application. Managing a debugging session cookie manually isn't very handy either.

Using a browser addon like xdebug-helper can be a great boon in enabling and disabling debugging sessions. That particular extension is available for both Chrome and Firefox.

Whitelisting for code coverage

When you want to generate code coverage, there aren't many alternatives to Xdebug. While PHPUnit also supports code coverage by running your tests using phpdbg -qrr command, it tends to come with its own set of problems (like instability, bugs, and differences in coverage metrics).

Recently, however, a new feature has been added to Xdebug which allows setting up whitelists for code coverage gathering with xdebug_set_filter(). This makes it much faster to run tests with code coverage, as the metrics are only gathered for specific files.

If you're interested, you can read more about this solution from Faster Code Coverage by Sebastian Bergmann and Sebastian Heuer.

Profile with care

Given the performance impact, you might be quick to think that this makes the profiler provided by Xdebug a bit useless. However, it can still provide quite a bit of valuable information if you use it knowing its weaknesses.

The biggest mistake you can make with Xdebug is to rely on only its metrics for speed improvements. If you try to make your code faster only under the profiler, you tend to lean towards micro-optimizations which may not actually give any real-world benefit.

However, the profiler can still provide you with useful insight about which parts are actually taking more time than you expected.

In real-world scenarios, I've often found that profiler tends to highlight two different kinds of errors:

  • Code that was taking much more time than expected due to being called too often
  • Code that was doing something completely unintended and unnecessary.

Especially, if you are unfamiliar with a code base, a profiler can help you locate potential bottlenecks. Things like a function being called 100,000 times or a script making hundreds of database queries are quite easy to spot using a profiler.

As I discussed in my previous blog post, many performance issues tend to be structural in nature and a profiler is a good tool to find those kinds of issues.

Knowing your tools is important

Xdebug is an invaluable tool to any proficient PHP developer. It can still easily lead you astray if you don't fully understand it.

The way I see it:

  • The first step is to learn that Xdebug is a great tool
  • The second step is to understand that it has flaws
  • The third step is understanding how to use it effectively while fully knowing its flaws

Debug and test your performance responsibly.


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