Java Flight Recorder application method start and end time

Question

I'm interested in whether it is possible to get start and end time in nanoseconds for every or most of user methods in profiling with JFR program?

I know that it is possible to do with code instrumentation, but I would like to leverage advantage of sampling profiler to reduce impact on actual program execution.

I have a sense that it should be possible as I believe JFR need this info for hot methods percentage evaluation, but don't know how to get that time. Could someone help?

Answer 1

It's not possible to get that using JFR.

Since it's a sampling profiler, it will only collect what method that was on top of the stack when the sample was taken. You can get a high precision timestamp when it occurred, but it will not tell you for how long time the method executed.

Here is a program that shows how the histogram is built.

try (RecordingFile f = new RecordingFile(Paths.get("recording.jfr"))) {
   Map<String, SimpleEntry<String, Integer>> histogram = new HashMap<>();
   int total = 0;
   while (f.hasMoreEvents()) {
     RecordedEvent event = f.readEvent();
     if (event.getEventType().getName().endsWith(".ExecutionSample")) {
       RecordedStackTrace s = event.getStackTrace();
       if (s != null) {
         RecordedFrame topFrame= s.getFrames().get(0);
         if (topFrame.isJavaFrame())  {
           RecordedMethod method = topFrame.getMethod();
           String methodName = method.getType().getName() + "#" + method.getName() + " " + method.getDescriptor();
           Entry entry = histogram.computeIfAbsent(methodName, u -> new SimpleEntry<String, Integer>(methodName, 0));
           entry.setValue(entry.getValue() + 1);
           total++;
         }
       }
     }
   }
   List<SimpleEntry<String, Integer>> entries = new ArrayList<>(histogram.values());
   entries.sort((u, v) -> v.getValue().compareTo(u.getValue()));
   for (SimpleEntry<String, Integer> c : entries) {
     System.out.printf("%2.0f%% %s\n", 100 * (float) c.getValue() / total, c.getKey());
   }
   System.out.println("\nSample count: " + total);
 }