Finding Issues via the Call Graph

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There are two different ways to optimize any function: either optimize the code inside that function or you can try to call the function less times. In our case, we found that the most problematic function is UnitOfWork::createEntity. But this is a vendor function: it's not our code. So it's not something that we can optimize. And honestly, it's probably already super-optimized anyways.

But we could try to call it less times... if we can understand what in our app is causing so many calls! The call graph - the big diagram in the center of this page - holds the answer.

Call Graph: Visual Function List

Start by clicking on the magnifying glass next to createEntity. Woh! That zoomed us straight to that "node" on the right. Let's zoom out a little.

The first thing to notice is that the call graph is basically a visual representation of the information from the function list. On the left, it says this function has two "callers". On the right, we can see those two callers. But when you're trying to figure out the big picture of what's going on, the call graph is way nicer.

The Critical Path

Let's zoom out a bunch further. Now we can see a clear red path... that eventually leads to the dark red node down here. This is called the critical path. One of Blackfire's main jobs is to help us make sense out of all this data. One way it does that is exactly this: by highlighting the "path" to the biggest problem in our app.

I'm going to hit this little "home" icon - that will reset the call graph, instead of centering it around the createEntity node. In this view, Blackfire does hide some less-important information around the createEntity node, but it gives us the best overall summary of what's going on: we can clearly see the critical path. The critical thing to understand is: why is that path in our app so slow?

Let's trace up from the problem node... to find where our code starts. Ah, here's our controller being rendered... and then it renders a template. That's interesting: it means the problem is coming from inside a template... from inside the body block apparently. Then it jumps to a Twig extension called getUserActivityText()... that calls something else CommentHelper::countRecentCommentsForUser(). That's the last function before it jumps into Doctrine.

Finding the Problem

So the problem in our code is something around this getUserActivityText() stuff. Let's open up this template: main/sighting_show.html.twig - at templates/main/sighting_show.html.twig.

If you look at the site itself, each commenter has a label next to them - like "hobbyist" or "bigfoot fanatic" - that tells us how active they are in the great and noble quest of finding BigFoot. Over in the Twig template, we get this text via a custom Twig filter called user_activity_text:

... lines 1 - 4
{% block body %}
<div class="col">
... lines 7 - 19
{% for comment in sighting.comments %}
<div class="comment-container mb-3">
<div class="row">
... lines 23 - 25
<div class="col">
... line 27
<span>({{ comment.owner|user_activity_text }})</span>
... lines 29 - 33
</div>
</div>
</div>
{% endfor %}
</div>
{% endblock %}
... lines 41 - 42

If you're not familiar with Twig, no problem. The important piece is that whenever this filter code is hit, a function inside src/Twig/AppExtension.php is called... it's this getUserActivityText() method:

... lines 1 - 10
class AppExtension extends AbstractExtension
{
... lines 13 - 26
public function getUserActivityText(User $user): string
{
$commentCount = $this->commentHelper->countRecentCommentsForUser($user);
if ($commentCount > 50) {
return 'bigfoot fanatic';
}
if ($commentCount > 30) {
return 'believer';
}
if ($commentCount > 20) {
return 'hobbyist';
}
return 'skeptic';
}
}

This counts how many "recent" comments this user has made... and via our complex & proprietary algorithm, it prints the correct label.

Back over in Blackfire, it told us that the last call before Doctrine was CommentHelper::countRecentCommentsForUser() - that's this function call right here!

... lines 1 - 10
class AppExtension extends AbstractExtension
{
... lines 13 - 26
public function getUserActivityText(User $user): string
{
$commentCount = $this->commentHelper->countRecentCommentsForUser($user);
... lines 30 - 43
}
}

Let's go open that up - it's in the src/Service directory:

... lines 1 - 6
class CommentHelper
{
public function countRecentCommentsForUser(User $user): int
{
$comments = $user->getComments();
$commentCount = 0;
$recentDate = new \DateTimeImmutable('-3 months');
foreach ($comments as $comment) {
if ($comment->getCreatedAt() > $recentDate) {
$commentCount++;
}
}
return $commentCount;
}
}

Ah. If you don't use Doctrine, you might not see the problem - but it's one that can easily happen no matter how you talk to a database. Hold Command or Ctrl and click the getComments() method to jump inside:

... lines 1 - 15
class User implements UserInterface
{
... lines 18 - 50
/**
* @ORM\OneToMany(targetEntity="App\Entity\Comment", mappedBy="owner")
*/
private $comments;
... lines 55 - 187
/**
* @return Collection|Comment[]
*/
public function getComments(): Collection
{
return $this->comments;
}
... lines 195 - 206
}

Here's the story: each User on our site has a database relationship to the comment table: every user can have many comments. The way our code is written, Doctrine is querying for all the data for every comment that a User has ever made... simply to then loop over them, and count how many were created within the last 3 months:

... lines 1 - 6
class CommentHelper
{
public function countRecentCommentsForUser(User $user): int
{
$comments = $user->getComments();
$commentCount = 0;
$recentDate = new \DateTimeImmutable('-3 months');
foreach ($comments as $comment) {
if ($comment->getCreatedAt() > $recentDate) {
$commentCount++;
}
}
return $commentCount;
}
}

It's a massively inefficient way to get a simple count. This is problem number one.

It seems obvious now that I'm looking at it. But the nice thing is that... it's not a huge deal that I did this wrong originally - Blackfire points it out. And not over-obsessing about performance during development helps prevent premature optimization.

Attempting the Performance Bug Fix

So let's fix this performance bug. Open up src/Repository/CommentRepository.php. I've already created a function that will use a direct COUNT query to get the number of recent comments since a certain date:

... lines 1 - 7
use App\Entity\User;
... lines 9 - 15
class CommentRepository extends ServiceEntityRepository
{
... lines 18 - 22
public function countForUser(User $user, \DateTimeImmutable $sinceDate): int
{
return (int) $this->createQueryBuilder('comment')
->select('COUNT(comment.id)')
->andWhere('comment.owner = :user')
->andWhere('comment.createdAt >= :sinceDate')
->setParameter('user', $user)
->setParameter('sinceDate', $sinceDate)
->getQuery()
->getSingleScalarResult();
}
... lines 34 - 62
}

Let's use this... instead of my current, crazy logic.

To access CommentRepository inside CommentHelper - this is a bit specific to Symfony - create a public function __construct() and autowire it by adding a CommentRepository $commentRepository argument:

... lines 1 - 5
use App\Repository\CommentRepository;
class CommentHelper
{
... lines 10 - 11
public function __construct(CommentRepository $commentRepository)
{
... line 14
}
... lines 16 - 21
}

Add a private $commentRepository property... and set it in the constructor: $this->commentRepository = $commentRepository:

... lines 1 - 5
use App\Repository\CommentRepository;
class CommentHelper
{
private $commentRepository;
public function __construct(CommentRepository $commentRepository)
{
$this->commentRepository = $commentRepository;
}
... lines 16 - 21
}

Now... I don't need any of this logic. Just return $this->commentRepository->countForUser(). Pass this $user... and go steal the DateTimeImmutable from below and use that for the second argument. Celebrate by killing the rest of the code:

... lines 1 - 7
class CommentHelper
{
... lines 10 - 16
public function countRecentCommentsForUser(User $user): int
{
return $this->commentRepository
->countForUser($user, new \DateTimeImmutable('-3 months'));
}
}

If we've done a good job, we will hopefully be calling that UnitOfWork function many less times - the 23 calls into Doctrine from CommentHelper eventually caused many, many things to be called.

So... let's profile this and see the result! We'll do that next and use Blackfire's "comparison" feature to prove that this change was good... except for one small surprise.

Leave a comment!

  • 2019-11-27 Victor Bocharsky

    Hey Vasyl,

    FYI, the video was fixed, it should have HD quality now. Thanks again for reporting it!

    Cheers!

  • 2019-11-27 Victor Bocharsky

    Hey Vasyl,

    Thank you for letting us know! We will fix the video quality as soon as possible

    Cheers!

  • 2019-11-27 Vasyl Hebrian

    Hmm.. very bad quality of video and no button to change it to HD.
    Previous videos are OK.