Profiling with Java Flight Recorder

Java Profiling can help you to identify asses the performance of your program, improve your code and identify any defects such as memory leaks, high CPU usages, etc. Here I will discuss on how to profile your code using the java inbuilt utility JCMD and Java Mission Control.


Getting a Performance Profile

A profile can be obtained using both jcmd as well as mission control tools. jcmd is a command line based tool where as mission control comes with a UI. But since jcmd is lightweight when compared to mission control and hence has lesser effect to the performance of program/code which you are going to profile. Therefore jcmd is preferable for taking a profile. In order to get a profile:

First need to find the process id of the running program you want to profile. 

Then, unlock commercial features for that process:
jcmd <pid> VM.unlock_commercial_features


Once the comercial features are unlocked, start the recording.
jcmd <pid> JFR.start delay=20s duration=1200s name=rec_1 filename=./rec_1.jfr settings=profile


Here 'delay', 'name' and 'filename' all are optional. To check the status of the recording:
jcmd <pid> JFR.check


Here I have set the recording for 20 mins (1200 sec.). But you can take a snapshot of the recording at any point within that duration, without stopping the recording. To do that:
jcmd <pid> JFR.dump recording=rec_1 filename=rec_1_dump_1.jfr


Once the recording is finished, it will automatically write the output jfr to the file we gave at the start. But  if you want to stop the recording in the middle and get the profile, you can do that by:
jcmd <pid> JFR.stop recording=rec_1 filename=rec_1.jfr  


Analyzing the Profile

Now that we  have the profile, we need to analyze it. For that jcmd itslef not going to be enough. We are going to need Java Mission Control. You can simply open Mission Control and then open your .jfr file using it. (drag and drop the jfr file to mission control UI). Once the file is open, it will navigate you to the overview page, which usually looks like follows:


Here you can find various options to analyze your code. You can drill down to thread level, class level and method level, and see how the code have performed during the time we record the profile. In the next blog I will discuss in detail how we can identify any defects of the code using the profile we just obtained.

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