GetAndDecrement Much Slower than GetAndIncrement.

Question

I have found that in Java using AtomicIntegers, GetAndDecrement is much slower than GetAndIncrement. Why does this happen?

Answer 1

Whichever one comes second is faster.

AtomicInteger a = new AtomicInteger(1);

long start1 = System.nanoTime();
a.decrementAndGet(); 
System.out.println("Time: " + (System.nanoTime()-start1));

AtomicInteger b = new AtomicInteger(1);

long start2 = System.nanoTime();
a.incrementAndGet();
System.out.println("Time: " + (System.nanoTime()-start2));

On my machine, this prints:

Time: 49264

Time: 4105

However, if we swap the order of the two operations:

AtomicInteger a = new AtomicInteger(1);

long start1 = System.nanoTime();
a.incrementAndGet();
System.out.println("Time: " + (System.nanoTime()-start1));

AtomicInteger b = new AtomicInteger(1);

long start2 = System.nanoTime();
a.decrementAndGet();
System.out.println("Time: " + (System.nanoTime()-start2));

Then we get:

Time: 43106

Time: 7697

Presumably the JVM or processor or something is doing some runtime optimizations.

Answer 2

They look pretty much the same to me --

public final int getAndIncrement() {
    for (;;) {
        int current = get();
        int next = current + 1;
        if (compareAndSet(current, next))
            return current;
    }
}

public final int getAndDecrement() {
    for (;;) {
        int current = get();
        int next = current - 1;
        if (compareAndSet(current, next))
            return current;
    }
}