Question

I am doing currently a sample exercise and i found one weird observation that if I replace AutomicInteger with volatile program runs faster. Note : I am only doing read operations.

code:

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

public class Main {

AtomicInteger integer  = new AtomicInteger(100000000);
// volatile int integer= 100000000;

public static void main(String[] args) {
// We will store the threads so that we can check if they are done
List<Thread> threads = new ArrayList<Thread>();
 long start = System.currentTimeMillis();
 Main main = new Main();
// We will create 500 threads

for (int i = 0; i < 500; i++) {
  Runnable task = new MyRunnable(main.integer);
  Thread worker = new Thread(task);
  // We can set the name of the thread
  worker.setName(String.valueOf(i));
  // Start the thread, never call method run() direct
  worker.start();
  // Remember the thread for later usage
  threads.add(worker);
}
int running = 0;
do {
  running = 0;
  for (Thread thread : threads) {
    if (thread.isAlive()) {
      running++;
    }
  }
  System.out.println("We have " + running + " running threads. ");
} while (running > 0);

System.out.println("Total Time Required :" +(System.currentTimeMillis()- start));
}
} 

MyRunnable class:

 import java.util.concurrent.atomic.AtomicInteger;

public class MyRunnable implements Runnable {
private final AtomicInteger countUntil;

MyRunnable(AtomicInteger countUntil) {
    this.countUntil = countUntil;
}

@Override
public void run() {
    long sum = 0;
    for (long i = 1; i < countUntil.intValue(); i++) {
        sum += i;
    }
    System.out.println(sum);
}
}

Time Required for this program to run using AutomicInteger on my machine.

Total Time Required :102169

Total Time Required :90375

Time required for this program to run using volatile on my machine

Total Time Required :66760

Total Time Required :71773

Does this mean volatile is faster than AutomicInteger for read operation also?

Answer 1

AtomicInteger in read context is basically a thin wrapper around volatile int:

private volatile int value;

public final int get() {
    return value;
}

public int intValue() {
    return get();
}

Don’t expect the wrapper to be faster then the wrapped value taken alone. It can only be as fast as the volatile int if inlining is employed by the JVM.

---

And hint: if you are “only doing read operations“, this will be even faster:

static final int integer= 100000000;