Why does the HotSpot ExecutionSample event always return STATE_RUNNABLE?

Question

I'm using the ExecutionSample event from HotSpot in OpenJDK11.

It has a field for thread state, but I only ever see one value for the field: STATE_RUNNABLE

How does HotSpot choose which threads to sample? Why are they always runnable?

Simple reproduction code in Kotlin:

import jdk.jfr.Recording
import jdk.jfr.consumer.RecordingFile
import java.nio.file.Path

object ExecutionSampleTest {

    private const val EXECUTION_SAMPLE = "jdk.ExecutionSample"
    private val RECORDING_PATH = Path.of("/tmp/recording.jfr")

    @JvmStatic
    fun main(args: Array<String>) {
        Recording().use { recording ->
            recording.enable(EXECUTION_SAMPLE)
            recording.settings = recording.settings.plus("$EXECUTION_SAMPLE#period" to "1 ms")

            recording.start()
            repeat(100) {
                // start some sleeping threads, just so we've got something to sample
                Thread { Thread.sleep(20_000) }.start()
            }
            Thread.sleep(20_000)
            recording.stop()
            recording.dump(RECORDING_PATH)

            RecordingFile.readAllEvents(RECORDING_PATH).forEach {
                println("Thread state: ${it.getString("state")}")
            }
        }
    }
}

Only ever prints: Thread state: STATE_RUNNABLE

Answer 1

Why does the HotSpot ExecutionSample event always return STATE_RUNNABLE?

By design.

JFR method profiler periodically samples Java threads and produces two types of events:

• ExecutionSample, when a thread state is in_java
• NativeMethodSample, when a thread state is in_native

See jfrThreadSampler.cpp source code:

  if (JAVA_SAMPLE == type) {
    if (thread_state_in_java(thread)) {
      ret = sample_thread_in_java(thread, frames, max_frames);
    }
  } else {
    assert(NATIVE_SAMPLE == type, "invariant");
    if (thread_state_in_native(thread)) {
      ret = sample_thread_in_native(thread, frames, max_frames);
    }
  }

Both in_java and in_native correspond to STATE_RUNNABLE in JFR format.

When a thread is not runnable for either reason (STATE_SLEEPING, STATE_PARKED etc.), its JVM state is thread_blocked. However, JFR method profiler does not sample blocked threads at all.

---

If you are interested in wall clock profiling (i.e. sampling all threads, whether they run on CPU or sleep), you may want to look at async-profiler. Async-profiler can also produce output in JFR compatible format, but unlike JFR, it generates ExecutionSample events both for RUNNABLE and IDLE threads.