result-wallclock.log

$ java WallclockTest /path/to/test2.bin
PID=[114109]: Waiting for a while...
Start processing
Profiling started
write time: 22509508631, sleep time: 10004730457

WallclockTest.java

import java.io.IOException;
import java.io.UncheckedIOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.concurrent.atomic.AtomicReference;

public class WallclockTest {
    static class HandMeasuredProfile {
        long writeTimeNanos = 0;
        long sleepTimeNanos = 0;
    }

    public static void main(String[] args) throws Exception {
        Path filePath = Paths.get(args[0]);

        final AtomicReference<HandMeasuredProfile> result = new AtomicReference<>();
        Thread th = new Thread(() -> {
            HandMeasuredProfile profile = new HandMeasuredProfile();
            try (FileChannel channel = FileChannel.open(
                    filePath,
                    StandardOpenOption.CREATE,
                    StandardOpenOption.APPEND,
                    StandardOpenOption.SYNC)) {
                System.out.printf("PID=[%d]: Waiting for a while...\n", ProcessHandle.current().pid());
                Thread.sleep(3000L);
                System.out.println("Start processing");

                ByteBuffer buf = ByteBuffer.allocate(4);
                buf.putInt(0);
                for (int i = 0; i < 20; i++) {
                    buf.flip();

                    long t0 = System.nanoTime();
                    channel.write(buf);
                    long t = System.nanoTime();
                    profile.writeTimeNanos += t - t0;
                    t0 = t;

                    Thread.sleep(500L);
                    t = System.nanoTime();
                    profile.sleepTimeNanos += t - t0;
                }
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            } finally {
                result.set(profile);
            }
        });
        th.setName("test-thread");
        th.start();
        th.join();

        System.out.printf("write time: %d, sleep time: %d\n",
                          result.get().writeTimeNanos,
                          result.get().sleepTimeNanos);
    }
}