why my own AtomicLong are slower than the one provide in the JDK?

Question

I was writing my own AtomicLong class and I just found that the function I had is much slower than the one provided in the Unsafe class. I am wondering why?

Below are the codes I have:

public interface Counter {
    void increment();
    long get();
}


public class PrimitiveUnsafeSupportCounter implements Counter{

    private volatile long count = 0;
    private Unsafe unsafe;
    private long offset;

    public PrimitiveUnsafeSupportCounter() throws IllegalAccessException, NoSuchFieldException {
        Field f = Unsafe.class.getDeclaredField("theUnsafe");
        f.setAccessible(true);
        this.unsafe = (Unsafe) f.get(null);
        this.offset = this.unsafe.objectFieldOffset(PrimitiveUnsafeSupportCounter.class.getDeclaredField("count"));
    }

    @Override
    public void increment() {

        this.unsafe.getAndAddLong(this, this.offset, 1);
    }

    @Override
    public long get() {
        return this.count;
    }
}

public class CounterThread implements Runnable {

    private Counter counter;

    public CounterThread(Counter counter){
        this.counter = counter;
    }
    @Override
    public void run() {

        for (int i = 0; i < 100000; i ++){
            this.counter.increment();
        }
    }
}

class Test{

    public static void test(Counter counter) throws NoSuchFieldException, IllegalAccessException, InterruptedException {

        ExecutorService executor = Executors.newFixedThreadPool(1000);

        long start = System.currentTimeMillis();
        for (int i = 0 ; i < 1000; i++){
            executor.submit(new CounterThread(counter));
        }

        executor.shutdown();
        executor.awaitTermination(1, TimeUnit.MINUTES);
        long stop = System.currentTimeMillis();

        System.out.println(counter.get());
        System.out.println(stop - start);
    }

}

public class Main {
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException, InterruptedException {

        Counter primitiveUnsafeSupportCounter = new PrimitiveUnsafeSupportCounter();
        Test.test(primitiveUnsafeSupportCounter);

    }

}

it takes about 3000ms to finish the above codes. however, it takes about even 7000ms if I used the below codes instead of this.unsafe.getAndAddLong(this, this.offset, 1);.

long before;
do {
     before = this.unsafe.getLongVolatile(this, this.offset);
} while (!this.unsafe.compareAndSwapLong(this, this.offset, before, before + 1));

I went through the source codes of getAndAddLong and found it does nearly the same thing as the above codes, so what should I miss?

Answer 1

That's JVM intrinsic and hand-written loop version has highly inefficient compiled code for the purpose. On x86 you can have atomic version of such read-modify-write operations via lock prefix. See Intel Manual 8.1.2.2 Software Controlled Bus Locking :

To explicitly force the LOCK semantics, software can use the LOCK prefix with the following instructions when they are used to modify a memory location.

In particular you can have something like lock add op1 op2. In your example you test the result of cmpxchg and do some jump which is obviously slower. Also as far as I remember on x86 volatile access requires some sort of mfence or lock to ensure memory ordering.