Property Caching
Now that we've got our application in production mode and we've dumped the autoloader, it's easier to see what the biggest performance problem is on this page: https://bit.ly/sf-bf-profile4
And actually, there might not be any more problems worth solving. I mean, it's loading in 104 milliseconds... even with the Probe doing all the profiling work.
But... let's see for sure. The function with the highest exclusive time now is PDOStatement::execute()... which is a low-level function that executes SQL queries.
Tip
The SQL Query information requires a Profiler plan or higher.
If we hover over the query info, these are only taking 12.5 milliseconds... but we are making 43 SQL calls on this page. Is that a problem? It's not ideal, but is it worth fixing? I guess it depends on how much you care... and whether the fix would be easy or if it would add a lot of complexity to our app.
Navigating the Call Graph: Top to Bottom, Bottom to Top
When you're trying to identify where the problem is, there are two ways to look at the call graph - and I often do both to help me understand what's going on. First, you can read from top to bottom - trace through your whole application flow to figure out what's going on down the hot path. Or, you can do the opposite: start at the bottom - start where the problem is... and trace up to find where your code starts.
Let's start from the top: handleRaw() is the framework booting up... and as we trace down... it renders our controller, renders our template... and we're once again inside the body block. This is really the same as last time! Our AppExtension::getUserActivityText() calls the countRecentCommentsForUser() function 23 times. That makes sense: we probably have 23 comments on the page... and for each comment, we need to count all the author's comments to print out this label.
Navigating Dimensions
Before we think about if, and how we might fix this, let's back up and look at other dimensions of this profile. In addition to wall time, we can completely re-draw the call graph based on only I/O time or CPU time. Remember, wall time is I/O time + CPU time. Or we could do something totally different: look at which functions are using the most memory... or even the most network bandwidth.
When we look at this in the network dimension, PDOStatement::execute() - the function that makes SQL calls - shows up here as a big problem. That's because SQL queries are technically network requests.
Re-draw the call graph for the I/O Wait time dimension. We see the same problem here because network calls - and so SQL calls - are part of I/O wait time.
The point is: while "wall time" is typically the most useful dimension, don't forget about these other ones: they can give us more information about what's going on. Is a function slow because of inefficient code inside? Or is it, for example, because of a network call?
Click back to I/O wait time - PDOStatement::execute() is definitely the issue according to this - and the critical path is pretty clear. This one function is taking over half the I/O wait time... but that's only 6 milliseconds. Optimizing this might not be worth it... but let's at least see if we can figure out how to call it less times.
As we already discovered, the problem is coming from CommentRepository::countForUser() which is called by AppExtension::getUserActivityText().
Over in src/Twig/AppExtension.php, each time we render a comment, it calls countForUser() and passes the User object attached to this comment:
| // ... lines 1 - 10 | |
| class AppExtension extends AbstractExtension | |
| { | |
| // ... lines 13 - 26 | |
| public function getUserActivityText(User $user): string | |
| { | |
| $commentCount = $this->commentHelper->countRecentCommentsForUser($user); | |
| // ... lines 30 - 43 | |
| } | |
| } |
Property Caching
Can we optimize this? Well... sometimes, the same user will comment many times on the same sighting - like this vborer user. When that happens, we're making a query to count that user's comments for every comment. That's wasteful!
So here's one idea: leverage "property caching". Basically, we'll keep track of the "status" strings for each user and use that to avoid calculating the status more than once for a given user.
Start by moving most of the logic into a private function called calculateUserActivityText(): this will have a User argument and return a string:
| // ... lines 1 - 10 | |
| class AppExtension extends AbstractExtension | |
| { | |
| // ... lines 13 - 37 | |
| private function calculateUserActivityText(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'; | |
| } | |
| } |
Next, add a new property to the top of the file: private $userStatuses = []:
| // ... lines 1 - 10 | |
| class AppExtension extends AbstractExtension | |
| { | |
| // ... lines 13 - 14 | |
| private $userStatuses = []; | |
| // ... lines 16 - 55 | |
| } |
Back in the public function, here's the magic: if not isset($this->userStatuses[$user->getId()]), then set it by saying $this->userStatuses[$user->getId()] = $this->calculateUserActivityText($user). At the bottom of the function, return $this->userStatuses[$user->getId()]:
| // ... lines 1 - 10 | |
| class AppExtension extends AbstractExtension | |
| { | |
| // ... lines 13 - 28 | |
| public function getUserActivityText(User $user): string | |
| { | |
| if (!isset($this->userStatuses[$user->getId()])) { | |
| $this->userStatuses[$user->getId()] = $this->calculateUserActivityText($user); | |
| } | |
| return $this->userStatuses[$user->getId()]; | |
| } | |
| // ... lines 37 - 55 | |
| } |
This is one of my favorite performance tricks because it has no downside, except for some extra code. If getUserActivityText() is called and passed the same User multiple times within a single request, we won't duplicate any work.
So... we probably made our site faster, right? Let's find out! Since we're in Symfony's prod environment, just to be safe, let's clear the cache:
php bin/console cache:clear
and warm it up:
php bin/console cache:warmup
Back in the browser, refresh the page and... let's profile! I'll name this one [Recording] show page try property caching. View the call graph: https://bit.ly/sf-bf-profile-prop-caching.
Ok - PDOStatement still looks like a main problem... but I think we're a little faster. You know what? Let's just compare the two profiles. Go back to the dashboard and compare the previous profile to this one. https://bit.ly/sf-bf-compare-prop-caching. I'll close the old profile.
Ok, so it did help - lower time in each dimension... and we saved 5 queries. So, this is a win, right? Maybe. If you profiled other Big foot sighting pages, which I did, you would find that this often did not help... or helped very little. In fact, this is the first time I've seen it help nearly this much.
So, does the improvement justify the added complexity in our code? If we can repeat this 13% improvement consistently, yea, it is. But if it's more like 1%, probably not.
And even 13% is not that much... and PDOStatement::execute() is still the biggest problem. I feel like the profile is trying to ask us: is there a better way to optimize this?
Next, let's try another approach: using a real cache layer. Truly caching things has its own downside: added complexity in your code and possibly - depending on what you're caching - the need to worry about invalidating cache. We'll want to be sure it's worth it.
