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Using Java Flight Recorder

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The Java Flight Recorder (JFR) feature allows you to record JVM, system, and Java level events to monitor the behavior and troubleshoot problems in your Java applications. Keeping the overhead of the recording process low, Azul Zulu Prime Builds of OpenJDK (Azul Zulu Prime JVM) Java Flight Recorder records large amount of information about your system from command-line parameters and system properties to time spent executing a piece of code.

JFR is available in Azul Zulu Prime JVM without the need for command-line options to enable it.

To analyze the JVM or your application, you can either interactively connect to a Java process from the ZMC desktop application or start a JFR recording on the command line with the jcmd tool.

JFR Recording with jcmd

On the same host and userid as the java process under analysis, run the following command to start a recording:

 jcmd JVMID JFR.start filename=rec.jfr 

Where JVMID is either the process ID or the classname assigned with the java process as shown when running jcmd without parameters.

To stop a recording, use:

 jcmd JVMID JFR.stop name=1 

To start a recording with more detailed metrics, use the following instead:

 jcmd JVMID JFR.start filename=rec.jfr settings=profile 

Analyzing a JFR Recording

It is recommended to open and read Azul Zulu Prime JVM Java Flight recordings using the Zulu® JDK Mission Control (ZMC) tool available for download from the Azul Zulu® JDK Mission Control page.

Alternatively, a Java Flight recording can be viewed using the jfr command-line tool.

Example usage of the jfr command:

 $JAVA_HOME/bin/jfr print rec.jfr > file.txt 

Using the ZMC Desktop Application

With the ZMC application, you can also start and stop recording or interactively browse metrics of the java process under analysis.

By default, JFR uses a Unix socket for communication and therefore does not offer remote connections. To allow remote connection, you need to enable the JVM’s JMX connector.

See "Find JVM to connect" to in the Zulu® Mission Control Quick Start Guide for details.

Starting a Recording at JVM Start Time

For detailed performance analysis, you might prefer to start a JFR recording directly when launching a java process.

To enable recording, add either of the following command-line options to the java process under analysis:

  • -XX:StartFlightRecording=filename=rec.jfr - for an on-disk recording

  • -XX:StartFlightRecording=disk=false,filename=rec.jfr - for an in-memory recording

More Java Flight Recorder command-line options are listed in Using ZVM Command-Line Options.

Using Zing Live Objects

Zing Live Objects help determine the type of objects that are leaking while troubleshooting memory leaks. To identify and address a memory leak using information gathered in a Zing flight recording, complete the following steps.

To create a Zing flight recording for inspection, use the following arguments:

 java -XX:StartFlightRecording=disk=false,duration=<recording_time_duration>,settings=profile,filename=<recording_filename>.jfr 

The Flight Recorder has two configuration files: default and profile.

To enable the Live Objects feature, you need to enable ZingOldObjectsEvent in the configuration file. By default, ZingOldObjectsEvent is enabled only in the profile configuration file in versions of Zing prior to 19.12.0.0. Starting with ZVM 19.12.0.0, ZingOldObjectEvent is disabled by default in all configurations.

Note
  • The JVMInformation, GarbageCollection, and OldGarbageCollection events must be enabled in your configuration file in order to analyze Live Objects in ZMC. The listed events are enabled in the default and profile configuration files by default.

  • The profile settings may increase performance overhead due to collecting more data by the Java Flight Recorder. Customizing any settings file, either default or profile, may incur increased performance overhead.

By default, the amount of live set events recorded by Java Flight Recorder is set to 99%. To decrease the recording file size, you can set the value of the JFRDescribeLiveSetPercentage command-line option to a number less than 99:

 java -XX:JFRDescribeLiveSetPercentage=90 -XX:StartFlightRecording=disk=false,duration=<recording_time_duration>,settings=profile,filename=<recording_filename>.jfr 

When a recording is started, the output should look similar to the snippet below:

 $ /home/username/<JDK_installation_path>/bin/java ‑XX:StartFlightRecording=disk=false,settings=profile,filename=myrecording.jfr
Started recording 1.
Use jcmd 32747 JFR.dump name=1 to copy recording data to file. 

If just a filename without a path is specified (i.e., filename=<recording_filename>.jfr), the flight recording is saved to the directory from which you ran the command.

To specify the path of your choice, use:

 filename=<path>/<recording_filename>.jfr 

Once you create a Zing Flight recording, start the Zulu Mission Control tool and open your Azul Zulu Prime JVM Flight recording from the File menu.

If you do not have the ZMC on your machine, download the ZMC installer file from Download Zulu Mission Control and see Downloading and Starting Zulu Mission Control for installation instructions if needed.

In the Outline navigation tree, select Memory > Zing Live Objects to display the set of live objects captured in the recording.

Study and compare GC cycles in the table with garbage collection identifiers (GC ID) for a live set suddenly increasing in size which indicates a memory leak.

Live objects types marked during each selected GC cycle display in the table to the right when a specific GC cycle is chosen.

compare gc cycles

Sort out live objects in a GC cycle by the total size to track the live object type with the biggest size and select a potentially leaking object type in the Referent column.

All items that refer to a potentially leaking object type display in the Referrer column in the bottom table. The latter helps identify the source of the leaking object by reference count.

(Optional) To retrieve more information, repeat step 5 if needed.

When a potentially leaking class is identified on the Zing Live Objects sub-tab, use the Memory tab to get information about the allocation site. The Memory tab contains allocation sites, which lead to allocation in a new thread local allocation buffer (TLAB) or allocation outside a TLAB, sorted by type. Checking stack traces for a potentially leaking class can help to stop the real place in code where the leak occurs.

In case the Memory tab does not help to find the leak, but the leak is still suspected, the Method Profiling tab might be used for this purpose. It is less convenient compared to the Memory tab, as it shows method calls of concrete types, not allocation sites.

Usually, the Zing Live Objects sub-tab and the Memory tab are all you need to find a leak.

Inspect the Stack Trace of your candidates to track where most memory allocation pressure comes from.

Using TTSP JFR Events

Azul Platform Prime 20.07.0.0 introduces support for profiling Time To Safepoint (TTSP) issues based on data about the Azul Zulu Prime JVM events collected in a Java Flight Recording (JFR).

To profile an existing TTSP issue using information gathered in a flight recording, complete the following steps:

  1. Create a flight recording for inspection using the following arguments:

     java -XX:StartFlightRecording=name=<recording_name>,filename=<recording_filename>.jfr,settings=profile 

    The Flight Recorder has default and profile configuration files. By default, the TimeToSafepointIssue event, which activates TTSP issue profiling, is enabled in all configurations. Use the following options to configure the event settings file for the Java Flight Recorder:

    Configuring Option Description Default

    -XX:[+/-]SafepointWaitTimeProfiler

    Generates asynchronous signals to log safepoint issues.

    true

    -XX:SafepointProfilerRealTimeSignalNumber=<number>

    Specifies the number of real time signals used for safepoint profiling.

    4

    -XX:SafepointProfilerThresholdMS=<number>

    Specifies the number of milliseconds to start safepoint profiling.

    0

    -XX:SafepointProfilerThreshold=<number>

    Specifies the threshold percentage value to start safepoint profiling.

    2

    -XX:SafepointProfilerInterval=<number>

    Specifies the interval percentage value for safepoint profiling.

    10

    -XX:SafepointProfilerMaxBuffers=<number>

    Specifies the number of safepoint profiler buffers to track.

    100

    -XX:SafepointWaitTimeProfilerLog=<log file>

    Specifies the log file to write safepoint profiling data to.

    stderr

    Note
    The profile settings may increase performance overhead due to collecting more data by the Java Flight Recorder. Customizing any settings file, either default or profile, may incur increased performance overhead.
  2. Once you create a Zing Flight recording, start the Zulu Mission Control tool and open your Zing Flight recording from the File menu.

    If you do not have the ZMC on your machine, download the ZMC installer file from Download Zulu Mission Control and see Downloading and Starting Zulu Mission Control for installation instructions if needed.

  3. In the Outline navigation tree, choose Event Browser.

  4. In the Event Types Tree, select Java Virtual Machine > Runtime > Safepoint to display the set of Safepoint event types captured in the recording.

    ttsp issue
  5. In case of a TTSP issue, check the Event Browser table.

    The contents of the Stack Trace window may help you understand the cause of the issue.