Java Flight Recorder

Flight Recorder Info¶

The info below was testing specifically for Java version 8.

As of Java version 11, the Flight recorder is now part of the standard OpenJDK and does not require enabling of commerical features. This also means that it can be used in production.

As of OpenJDK 8u262 JFR is now also available.

Enabling JVM app for Flight Recorder¶

Oracle JVM¶

For Oracle there are specific flags that need to be set for the app.

Obviously the changes below are only allowed on non-production systems unless you have paid for licenses from Oracle.

-XX:+UnlockCommercialFeatures -XX:+FlightRecorder

It is also recommended to enable the two flags below.

Out of the box the method profiler will use safepoint boundaries when profiling methods, which may skew results. When enabling the flags below when necessary, you will get more accurate profiling results on where the time is spent in your app. This is why Flight recorder is preferred over other profilers such as YourKit, VisualVM.

-XX:+UnlockDiagnosticVMOptions -XX:+DebugNonSafepoints

Capturing a recording¶

Flight Recorder - Start with app¶

-XX:StartFlightRecording=duration=20s,delay=5s,settings=profile,filename=recording.jfr

Flight Recorder - Start Recording on running JVM¶

Start a recording with a time limit, specifying location to save to and compression.

jcmd <process id> JFR.start name=Test-01 filename=/app/record-test-01.jfr settings=/somelocation/default-with-alloc duration=30s stackdepth=1024

Flight Recorder - Start recording with no duration defined¶

Start a recording without a time limit, a future command will be executed to dump manually to a file.

jcmd <process id> JFR.start name=Test-07 settings=profile

Flight Recorder - Dump recording that is running¶

Dump the recording when enough data has been captured.

jcmd <process id> JFR.dump name=Test-08 filename=/app/test-08.jfr

Flight Recorder - Enable Allocations capture¶

Copy default.jfr and edit the file in /jre/lib/jfr/ Edit the file (or better yet a copy of it) and change the following to enable allocation tracking. The profile.jfr also includes allocation capture.

<flag name="allocation-profiling-enabled" label="Allocation Profiling">true</flag>

<event path="java/object_alloc_in_new_TLAB">
  <setting name="enabled" control="allocation-profiling-enabled">true</setting>
  <setting name="stackTrace">true</setting>
</event>

<event path="java/object_alloc_outside_TLAB">
  <setting name="enabled" control="allocation-profiling-enabled">true</setting>
  <setting name="stackTrace">true</setting>
</event>

Enable Physical Memory capture more frequently¶

As default it seems to be every chunk, if you want it more frequently override the default.

<!-- Default -->
<event name="jdk.PhysicalMemory">
  <setting name="enabled">true</setting>
  <setting name="period">everyChunk</setting>
</event>
<!-- every 15s -->
<event name="jdk.PhysicalMemory">
  <setting name="enabled">true</setting>
  <setting name="period">15s</setting>
</event>

Java Mission Control¶

Mission Control provides one avenue for reviewing the data captured by JFR.

Timezones¶

In order to coordinate with different timezones and to make the calculations easier, you can set the timezone on startup.

https://hirt.se/blog/?p=185

On OSX you need to drill down into the .App file to get to the executable.

# Start with PT timezone
./jmc -vmargs -Duser.timezone=PST8PDT

Links¶

https://developers.redhat.com/blog/2020/08/25/get-started-with-jdk-flight-recorder-in-openjdk-8u/ https://bell-sw.com/announcements/2020/07/22/Hunting-down-code-hotspots-with-JDK-Flight-Recorder/ https://devblogs.microsoft.com/devops/how-cpu-sampling-works/ https://sap.github.io/SapMachine/jfrevents/#executionsample