Why Unsafe.putOrderedObject is need in push of ForkJoinPool.WorkQueue instead of use array[index]

Question

    final void push(ForkJoinTask<?> task) {
        ForkJoinTask<?>[] a; ForkJoinPool p;
        int b = base, s = top, n;
        if ((a = array) != null) {    // ignore if queue removed
            int m = a.length - 1;     // fenced write for task visibility
            U.putOrderedObject(a, ((m & s) << ASHIFT) + ABASE, task);
            U.putOrderedInt(this, QTOP, s + 1);
            if ((n = s - b) <= 1) {
                if ((p = pool) != null)
                    p.signalWork(p.workQueues, this);
            }
            else if (n >= m)
                growArray();
        }
    }

U.putOrderedObject and U.putOrderedInt set the array and top of WorkQueue. So why don't just use array[i]=task and top=s+1.I’m reading the source of ForkJoinPool and meet this problem.

the source from oracle jdk 1.8(1.8.0_131).

UnSafe was replaced by VarHandles in Java9. What release are you talking about? Look at the code in Java9 or Java10. It's completely different. — edharned, Mar 29 '18 at 13:22

score 1 · Answer 1 · answered Mar 30 '18 at 14:21

Unsafe handled the problem of fences before VarHandle. See Rob Austin's blog for some detail. The F/J author does every trick in the book to gain speed. The code prior to VarHandle is most difficult to understand, now a little less so.

Rather than using standard Java arrays (like ConcurrentLinkedQueue with it's additional overhead) he uses his own raw array.

Why Unsafe.putOrderedObject is need in push of ForkJoinPool.WorkQueue instead of use array[index]

1 Answers1