Java Flight Recorder - Async I/O support

Question

Does JFR support Async I/O?

I am using Java Flight Recorder (on HotSpot 1.8.0_144) + Mission Control to profile an app.

On the File IO tabs of Mission Control, I see orders of magnitude less "activity" than I expected and that I know to be actually happening.
The IO "activity" that I am actually seeing seems to correspond to synchronous file IO, but I am not sure if this is just a coincidence, or a misconfiguration of the JFR on my part.

I did try this on a smaller example (see below), and it feels like the file async events are not being reported.

Sync File IO events are reported

val inputStream = new FileInputStream("/home/cmhteixeira/Desktop/output.txt")
val outputStream = new FileOutputStream("/home/cmhteixeira/Desktop/output-copy.txt")

inputStream.transferTo(outputStream)

Async File IO events are not reported

RxIo(used below) is a library claiming to use file async IO.

import org.javaync.io.AsyncFiles
import java.nio.file.Paths

val inputStream = Paths.get("/home/cmhteixeira/Desktop/output.txt")
val outputStream = Paths.get("/home/cmhteixeira/Desktop/output-copy.txt")

AsyncFiles
  .readAllBytes(inputStream)
  .thenCompose(bytes => AsyncFiles.writeBytes(outputStream, bytes))
  .thenAcceptAsync(index => println("done")/* invoked on completion */)

on both the cases above, the File IO Threshold is set to 0 ns when lunching via Java Mission Control.

Questions

• Is file async IO not supported?
• What about async socket IO (which I have not tested)?
• If the answer is no, how bad is this considered by the community that uses JFR?
  • It feels like its a big drawback, but I lack context to understand if in practise this matters much.

Answer 1

Is file async IO not supported?

There are no events for file async I/O.

What about async socket IO (which I have not tested)?

Not socket either.

If the answer is no, how bad is this considered by the community that uses JFR? It feels like its a big drawback, but I lack context to understand if in practise this matters much.

It's been a known issue for many years, but it has been tricky to implement. JFR does support Virtual Threads, which will hopefully reduce the need for using async APIs directly. It's still a preview feature (April 2022)

See this presentation from FOSDEM, 33:00 minutes in. https://archive.fosdem.org/2020/schedule/event/state_openjdk/