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Get Started With Java Profiling in IntelliJ IDEA
Sometimes your app works, but you want to increase performance by boosting its throughput or reducing latency. Other times, you just want to know how your Java code behaves at runtime, determine where the hot spots are, or figure out how a framework operates under the hood.
Some of these details can be provided at design time, as IntelliJ IDEA provides you with all sorts of clues that can be produced statically. Of course, you cannot learn about all the bugs and inefficiencies by just looking at the code, because things get more complicated at runtime. Verifying any guesses you have can prove to be a tough task without the right tools at hand.
Two particularly helpful tools for examining the program at runtime are the debugger and profilers.
The debugger is very precise and gives you full and granular control over the execution of the program, which allows you to reproduce intricate failure conditions.
The profiler, on the other hand, offers a bird’s-eye view of arbitrarily large execution chunks. It does not interfere with the running program, nor does it provide granular data like the debugger does. However, it can collect valuable information no other tool can, which is why in this post we are going to take a look at the capabilities of IntelliJ IDEA’s Java profiler.
Start profiling with IntelliJ IDEA
IntelliJ IDEA provides integration with the following profilers:
- Java Flight Recorder – the standard profiling tool shipped as part of the JDK.
- Async Profiler – a very accurate profiler that can also collect native call and memory allocation data.
Both of these profilers’ functionality is available in the built-in IntelliJ Profiler. IntelliJ Profiler uses sampling, so the overhead is very low – close to running the program without the profiler. You can attach it to a Java process and view the profiling report directly from IntelliJ IDEA.
Where can I apply this?
Many people believe that they don’t need to learn how to profile as long as they don’t write high-load applications. In the example below, we’ll see how we can benefit from profiling even when dealing with very simple apps.
Let’s say we have the following application: https://github.com/flounder4130/profiler-example.
It repeatedly tries to create a path in the file system (we use createDirectories() from NIO2 for that). If an attempt fails, no exception is raised. We measure the throughput using an improvised benchmark.
Every time a task runs, the benchmark logic stores the current timestamp to a collection and removes all timestamps that point to a time earlier than the current time minus some interval. This makes it possible to find out how many events occurred during this time interval by just querying the collection. This workflow is supposed to help us understand how well or poorly our application performs.
When we run the program, we find that it doesn’t perform up to our expectations:
Average count: 4607 op Spent time: 2066 ms
Let’s profile it and see what’s wrong.
Starting the app with a profiler is simple: In the gutter, click the Run icon near the entry point of your application and select Profile with ‘IntelliJ Profiler’. This function is also available by clicking the three-dot menu in the upper-right corner.
When the app has finished running, a green popup will appear, prompting us to open the report. If we dismiss the popup by mistake, the report will still be available in the Profiler tool window.
The first thing we see after opening the report is the flame graph. The data on the flame graph is essentially a summary of all sampled stacks. The more samples with the same stack the profiler collects, the wider this stack grows on the flame graph. So, the width of the frame is roughly equivalent to the share of time spent in this state. Colors help to distinguish between native, project, and libraries.
To our surprise, the createDirectories() method did not account for the most execution time. The update() method from our homemade benchmark took about the same amount of time to execute!
Furthermore, if we look one frame above, we see that this is primarily because of the removeIf() method, which accounts for almost all the time of its parent method, update().
This clearly needs some looking into.
public static int update(Deque<Long> events, long nanos, long interval) {
events.add(nanos);
events.removeIf(aTime -> aTime < nanos - interval);
return events.size();
}
Apparently, removeIf() takes so long to execute because it iterates over the entire collection, even though it doesn’t really need to. Since we’re using an ordered collection, and events are added in chronological order, we can be sure that all elements subject for removal are always at the head of the queue.
So, if we replace removeIf() with a loop that breaks once it starts iterating over events that it is not going to remove, we can potentially improve performance:
while (events.peekFirst() < nanos - interval) {
events.removeFirst();
}
Let’s profile our app once again and look at the result:
If we search for the update() method, we find out that it has become a tiny little piece on the graph and doesn’t have monstrous overhead anymore. createDirectories() now occupies a more considerable share of application time.
The app also returns nice results figure-wise:
Average count: 17967 op Spent time: 515 ms
It’s much better now, isn’t it? Concise and neat as the functional style may be, we should always consider whether it really does what we need. Sometimes, the good old loop can save us tons of precious processor time.
We could stop now and pat ourselves on the back, but what’s going on with our createDirectories() method? Should attempts to create a file really be that resource consuming? If Java code does not tell us much, let’s see what happens on the OS level. To do that, check Build, Execution, Deployment | Java Profiler | Collect native calls in Settings/Preferences, then profile the program once again.
If we now examine the top part of the stack where createDirectories() is invoked, we see a lot of native frames that seem to deal with exceptions. But our app didn’t crash and we didn’t handle any, so why is that happening?
A little more investigation and zooming in gives us the answer. It does not take much programming experience to read the method names on the stack and conclude that the exceptions have to do with attempts to create a file that already exists.
Let’s try to avoid this and wrap the call to createDirectories() in a Files.exists() check:
Path p = Paths.get("./a/b");
if (!Files.exists(p)) {
Files.createDirectories(p);
}
Whoa! Our code is lightning fast now:
Average count: 50000 op Spent time: 72 ms
It is now 29 times as fast as it was originally.
This example shows us that even a simple piece of code that uses a well-known API may have a real impact on the performance of your application. More importantly, it showed us how we can detect suboptimal code, should we encounter such a problem in real life.
This example is far from exhaustive: IntelliJ IDEA provides a lot of ways to analyze the profiler report and the flame graph is only one of them. The range of cases where the profiler may be useful is also immense.
If you’re interested in profiling, make sure to check out the documentation and the dedicated page. Also, let us know in the comments if you’d like us to make tutorials for any specific profiling cases.
Happy developing!
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