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Wall Time, Exclusive Time & Other Wonders

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We just made Blackfire profile our first page. One of the best things about Blackfire is that, instead of just... giving me some raw data-dump and saying:

Good luck navigating that black pit of data!

... they expose this treasure trove of info on their site with a beautiful interface. This is called the "call graph". The most challenging part of Blackfire for me was learning what all this stuff means... so I could really get the most out of it. If you stick with me for the next few minutes, your profiling game will get a huge boost.

By the way, throughout the tutorial, I'll give you links to view the exact profile on Blackfire that I'm navigating in the video. Feel free to open it up and play around. The first one is here: https://bit.ly/sfcasts-bf-profile1.

And yes, I know, the cool-looking graph in the middle is calling to us, but let's start by looking at the the left side: the list of function calls, ordered from the functions that took the longest to execute on top... down to the quickest on the bottom. Well actually, Blackfire "prunes" or "removes" function calls that took very little time... so you won't see everything here.

Viewing by Different Dimensions

The functions are ordered by "time" because we're viewing the call graph in the time "dimension". You can also look at all of this information ordered by several other dimensions - like which functions took the most memory. It's kind of like the process manager on your computer: you can see which applications are currently taking up the most CPU, the most memory, reading the most info from your disk or even using the most network. But more on these dimensions later.

Wall Time

In the profiling world, time is called "wall time". But, it's nothing fancy: wall time is the difference between the time at which a function was entered and the time at which the function was left. So... wall time is a fancy word for... um... time: the amount of "time" a function took to run.

Inclusive vs Exclusive

So... we just find the function with the highest wall time and optimize it, right? Well... what if a function is taking a really long time... but actually, 99% of that time is due to a function that it calls. In that case, the other function is probably the problem.

To help sort this all out, wall time is divided into two parts: exclusive time and inclusive time. If you hover over the red graph, you'll see this: exclusive time 37.9 milliseconds, inclusive time 101 milliseconds.

Inclusive time is the full time it took for the function to execute. Exclusive time is more interesting: it's the time a function took to execute excluding the time spent inside other functions it called: it's a pure measurement of the time that the code inside this function took.

Right now, we're actually ordering this list by exclusive time, because that usually shows you the biggest problems. You can also order by inclusive time... which is probably not very useful: the top item is where our script starts executing, the second is the next function call, and so on. Go back to exclusive.

So apparently, the biggest problem, according to exclusive time, is this UnitOfWork::createEntity function... whatever that is. If you use Doctrine, you might know what this is - but let's pretend we have no idea.

Before we dive further into the root cause behind this slow function, the other way to order the calls is by the number of times each is called. Wow! Apparently the function that's called the most times - over 6 thousand times - is ReflectionProperty::setValue. Huh. I wonder who calls that?

Deeper Function Details

Click to expand that function. I love this! Even though we're viewing the call graph in the "time" dimension, this gives us all the info about this function: the wall time, I/O wait time, CPU time, memory footprint and network.

Wall Time = I/O Time + CPU Time

This isn't a particularly time consuming function - its wall time is 9.13 milliseconds. Wall time itself is broken down into two pieces, and this is important: wall time = I/O time + CPU time. There is nothing else: either a function is using CPU or it's doing some I/O operation, like talking to the filesystem or making network calls. In this case, the 9.13 milliseconds wall time is all CPU time.

Finding Callers

Okay, but who actually calls this function so many times? Above this, see those 3 down arrow buttons? These represent the three other functions that call this one - the size is relative to how many times each calls this. Click the first one. Ah ha! It's UnitOfWork::createEntity! That's the function with the highest exclusive time - it calls this function 4,959 times. Wow. So... it's definitely a problem.

If you click the other two arrows, you can see the other two callers: one calls this 984 times and the other 216 times. Both are from Doctrine.

Viewing Callees

Close all of this up and go back to ordering by the highest exclusive time. Open up UnitOfWork::createEntity(). As I mentioned, even though we're currently viewing the call graph in the "time" dimension, we can see all this function's dimensions right here.

Hover over the time graph: even though the exclusive time is significant - 37.9 milliseconds - most of this function's time is still inclusive: it's taken up by other functions that it calls. That helps give us a hint as to if the problem is inside this function... or inside something it calls.

And actually, every dimension has inclusive and exclusive measurements: like CPU time and even memory. If any of these had a high inclusive value - meaning some function it calls is really taking up that resource - you can see what functions it calls by clicking one of the arrow buttons below this.

What I really want to know though is... what's happening in our code to cause this function - UnitOfWork::createEntity() - be called so many times? Click the biggest arrow above. Ah: ObjectHydrator::getEntity() is the main culprit.

But... honestly... I don't know what that function is either: this is still way too low-level in Doctrine - I have no idea what's really going on. So next, let's use the call graph - the pretty diagram on the right - to get a full picture of what's happening going on... and how to fix it.