VisualVM

GraalVM provides support for VisualVM, the all-in-one Java (and polyglot) monitoring and troubleshooting tool. VisualVM enables powerful yet easy-to-use Java tooling which includes heap analysis for the supported guest languages. The following languages and features are currently available:

Java: Heap Summary, Objects View, Threads View, OQL Console
JavaScript: Heap Summary, Objects View, Thread View
Python: Heap Summary, Objects View
Ruby: Heap Summary, Objects View, Threads View
R: Heap Summary, Objects View

Starting VisualVM #

VisualVM is shipped as an installable component and can be added to GraalVM using GraalVM Updater:

gu install visualvm

This installs VisualVM in the $JAVA_HOME/bin directory. To start VisualVM, execute jvisualvm:

$JAVA_HOME/bin/jvisualvm

Immediately after startup, the tool shows all locally running Java processes in the Applications area, including the VisualVM process, itself.

Capture a Heap Dump #

To capture a heap dump of, for example, a Ruby application for later analysis, start your application and let it run for a few seconds to warm up. Then right-click its process in VisualVM and invoke the Heap Dump action. A new heap viewer for the Ruby process opens.

Note: Heap dump support must be explicitly enabled when using Native Image. Add the --enable-monitoring=heapdump,jvmstat option when invoking the native-image tool to enable the heap dump feature and allow VisualVM to detect native executables via jvmstat. This way your application will handle signals and capture a heap dump when it receives the SIGUSR1 signal. See the Generating Native Heap Dumps page for details on capturing heap dumps from a native image process.

Analyzing Objects #

Initially the Summary view for the Java heap is displayed. To analyze the Ruby heap, click the leftmost (Summary) dropdown in the Heap Viewer toolbar, choose the Ruby Heap scope and select the Objects view. Now the heap viewer displays all Ruby heap objects, aggregated by their type.

Expand the Proc node in the Results view to see a list of objects of this type. Each object displays its logical value as provided by the underlying implementation. Expand the objects to access their variables and references, where available.

Now enable the Preview, Variables, and References details by clicking the buttons in the toolbar, and select the individual ProcType objects. Where available, the Preview view shows the corresponding source fragment, the Variables view shows variables of the object, and the References view shows objects referring to the selected object.

Last, use the Presets dropdown in the Heap Viewer toolbar to switch the view from All Objects to Dominators or GC Roots. To display the heap dominators, retained sizes must be computed first, which can take a few minutes for the server.rb example. Select the Objects aggregation in the toolbar to view the individual dominators or GC roots.

Analyzing Threads #

Click the leftmost dropdown in the Heap Viewer toolbar and select the Threads view for the Ruby heap. The heap viewer now displays the Ruby thread stack trace, including local objects. The stack trace can alternatively be displayed textually by clicking the HTML toolbar button.

Reading JFR Snapshots #

The VisualVM tool bundled with GraalVM 19.2.x and later has the ability to read JFR snapshots – snapshots taken with JDK Flight Recorder (previously Java Flight Recorder). JFR is a tool for collecting diagnostic and profiling data about a running Java application. It is integrated into the Java Virtual Machine (JVM) and causes almost no performance overhead, so it can be used even in heavily loaded production environments.

To install the JFR support, released as a plugin:

Run $JAVA_HOME/bin/jvisualvm to start VisualVM;
Navigate to Tools > Plugins > Available Plugins to list all available plugins, then install the VisualVM-JFR and VisualVM-JFR-Generic modules.

The JFR snapshots can be opened using either the File > Load action, or by double-clicking the JFR Snapshots node and adding the snapshot into the JFR repository, permanently. Please follow the documentation for your Java version to create JFR snapshots.

The JFR viewer reads all JFR snapshots created from Java 7 onward, and presents the data in typical VisualVM views familiar to the tool users.

These views and functionality tabs are currently available:

Overview tab - displays the basic information about the recorded process like its main class, arguments, JVM version and configuration, and system properties. This tab also provides access to the recorded thread dumps.
Monitor tab - shows the process uptime and basic telemetry: CPU usage, Heap and Metaspace utilization, number of loaded classes, and number of live & started threads.
Threads tab - reconstructs the threads timeline based on all events recorded in the snapshot as precisely as possible, based on the recording configuration.
Locks tab - allows the user to analyze threads synchronization.
File IO tab - presents information on read and write events to the filesystem.
Socket IO tab - presents information on read and write events to the network.
Sampler tab - shows per-thread CPU utilization and memory allocations, and a heap histogram. There is also an experimental feature, “CPU sampler,” for building a CPU snapshot from the recorded events. It does not provide an exact performance analysis but still helps to understand what was going on in the recorded application, and where the CPU bottleneck might be.
Browser tab - provides a generic browser of all events recorded in the snapshot.
Environment tab - gives an overview of the recording machine setup and conditions like CPU model, memory size, operating system version, CPU utilization, memory usage, etc.
Recording tab - lists the recording settings and basic snapshot telemetry like number of events, total recording time, etc.

Note: The support of JDK Flight Recorder is currently experimental. Some advanced features like analyzing JVM internals, showing event stack traces, or support for creating JFR snapshots from live processes are not available in this preview version and will be addressed incrementally in the following releases.