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🚧 The profiler feature is experimental.

About the Java profiler

The splunk-otel-java agent includes a continuous profiler that can be enabled with a configuration setting. This profiler periodically captures the call stack state for all JVM threads and sends these to the Splunk Observability Cloud. You can then view a flamegraph of application call stacks and inspect individual code-level call stacks for relevant traces.

How does the profiler work?

The profiler leverages the JVM's JDK Flight Recorder (JFR) to perform periodic call stack sampling. For every recording period, a new JFR recording file is flushed to disk and events are replayed.

In order to associate jdk.ThreadDump events from JFR with OpenTelemetry spans, a custom OpenTelemetry ContextStorage implementation is used to emit otel.ContextAttached events each time a span context change occurs. These context changes are then tracked during replay so that a call stack can be associated with the appropriate span.

Stack trace data is embedded as a string inside of an OTLP logs payload. The Splunk OpenTelemetry Connector will detect this profiling data inside of OTLP logs and will help it along its ingest path.

Requirements

  • Java 8 above 8u262, or Java 11+
  • 100 MB of free disk space
  • Splunk OpenTelemetry Connector version 0.33.1 or higher. Sending profiling data directly to ingest is not supported at this time.
  • Profiler is enabled at startup (disabled by default, see the Configuration section)

Enable the profiler

To enable the profiler, pass the -Dsplunk.profiler.enabled=true argument to your JVM or set the SPLUNK_PROFILER_ENABLED environment variable to your JVM process.

Configuration settings

Like all other OpenTelemetry Java configurations, the following settings can be used as a system property or as environment variables (by making them uppercase and replacing dots with underscores).

We strongly recommend using defaults for the following settings.

Setting Default Description
splunk.profiler.enabled false set to true to enable the profiler
splunk.profiler.directory "." location of jfr files
splunk.profiler.recording.duration 20s recording unit duration
splunk.profiler.keep-files false leave JFR files on disk id true
splunk.profiler.logs-endpoint otel.exporter.otlp.endpoint or http://localhost:4317 where to send OTLP logs
splunk.profiler.period.{eventName} n/a customize period (in ms) for a specific jfr event. For example, to set the ThreadDump frequency to 1s (1000ms): -Dsplunk.profiler.period.threaddump=1000
splunk.profiler.tlab.enabled false set to false to disable TLAB memory events
splunk.profiler.include.agent.internals false set to true to include agent internal call stacks

Escape hatch

The profiler limits its own behavior under two conditions:

  • Free disk space is too low (< 100MB free)
  • More than 5 minutes worth of JFR files are backed up on disk

If a recording is already running when this condition is met, the recording stops. When the conditions are no longer applicable, the recording and the profiler resume operation.

FAQ / Troubleshooting

How do I know if it's working?

At startup, the agent will log the string "JFR profiler is active" at INFO. You can grep for this in your logs to see something like this:

[otel.javaagent 2021-09-28 18:17:04:246 +0000] [main] INFO com.splunk.opentelemetry.profiler.JfrActivator - JFR profiler is active.

How can I see the profiler configuration?

The agent logs the profiling configuration at INFO during startup. You can grep for the string com.splunk.opentelemetry.profiler.ConfigurationLogger to see entries like the following:

[otel.javaagent 2021-09-28 18:17:04:237 +0000] [main] INFO <snip> - -----------------------
[otel.javaagent 2021-09-28 18:17:04:237 +0000] [main] INFO <snip> - Profiler configuration:
[otel.javaagent 2021-09-28 18:17:04:238 +0000] [main] INFO <snip> -                 splunk.profiler.enabled : true
[otel.javaagent 2021-09-28 18:17:04:239 +0000] [main] INFO <snip> -               splunk.profiler.directory : .
[otel.javaagent 2021-09-28 18:17:04:244 +0000] [main] INFO <snip> -      splunk.profiler.recording.duration : 20s
[otel.javaagent 2021-09-28 18:17:04:244 +0000] [main] INFO <snip> -              splunk.profiler.keep-files : false
[otel.javaagent 2021-09-28 18:17:04:245 +0000] [main] INFO <snip> -           splunk.profiler.logs-endpoint : null
[otel.javaagent 2021-09-28 18:17:04:245 +0000] [main] INFO <snip> -             otel.exporter.otlp.endpoint : http://collector:4317
[otel.javaagent 2021-09-28 18:17:04:245 +0000] [main] INFO <snip> -            splunk.profiler.tlab.enabled : false
[otel.javaagent 2021-09-28 18:17:04:246 +0000] [main] INFO <snip> -   splunk.profiler.period.jdk.threaddump : null
[otel.javaagent 2021-09-28 18:17:04:246 +0000] [main] INFO <snip> - -----------------------

What about this escape hatch?

If the escape hatch becomes active, it will log with com.splunk.opentelemetry.profiler.RecordingEscapeHatch (you can grep for this in the logs). You may also look for "** THIS WILL RESULT IN LOSS OF PROFILING DATA **" as a big hint that things are not well.

You may need to free up some disk space and/or give the JVM more resources.

What if I'm on an unsupported JVM?

If your JVM does not support JFR, the profiler logs a warning at startup with the message Java Flight Recorder (JFR) is not available in this JVM. Profiling is disabled. If you want to use the profiler and see this in your logs, you must upgrade your JVM version to 8u262 or later.

Why is the OTLP/logs exporter complaining?

Collector configuration issues may prevent logs from being exported and profiling data from showing in Splunk Observability Cloud.

Check for the following common issues:

  • Look at the values of the agent's configuration: splunk.profiler.logs-endpoint and otel.exporter.otlp.endpoint. Hint: they are logged at startup (see above).
  • Verify that a collector is actually running at that endpoint and that the application host/container can resolve any hostnames and actually connect to the given OTLP port (default: 4317)
  • Make sure you are running the Splunk OpenTelemetry Connector and that the version is 0.33.1 or greater. Other collector distributions may not be able to route the log data containing profiles correctly.
  • Make sure that the collector is configured correctly to handle profiling data. By default, the Splunk OpenTelemetry Connector handles this, but a custom configuration might have overridden some settings. Make sure that an OTLP receiver is configured in the collector and that an exporter is configured for splunk_hec export. Ensure that the token and endpoint fields are correctly configured. Lastly, double check that the logs pipeline is configured to use the OTLP receiver and splunk_hec exporter.

Can I tell the agent to ignore some threads?

Not yet. Some JVM internal threads are automatically omitted, but there is no user-serviceable mechanism for omitting or filtering threads. If you need this, file an issue.