Re: java.util.concurrent.atomic

Eric Sosman <esosman@comcast-dot-net.invalid>
Tue, 29 Oct 2013 17:27:42 -0400
On 10/29/2013 4:35 PM, Andreas Leitgeb wrote:

Kevin McMurtrie <> wrote:

Why use code that can never block? [...]

That would be fine, if the code really didn't block, but if it
ends up doing the "do { ... } while (...)" idiom, then it really
IS blocking, just in a bad way.

What would be usecases of non-blocking synchronisation that don't
end up polling until success?

     Marcel M?ller described one elsethread. Yes, after a failure
the non-blocking algorithm goes back and tries again. What you may
have missed is that it tries again with a new "new value," and
doesn't simply try again with the same old failed one:

    while (true) {
        Thing oldThing = currentThing;
        Thing newThing = modifiedVersionOf(oldThing);

        // Done atomically by compare-and-swap:
        if (currentThing == oldThing) {
            // No other thread got there before me, so:
            currentThing = newThing;
        } else {
            // Rats! Somebody else changed currentThing, so
            // the newThing I computed is already obsolete.
            // Go back and try *everything* again.

The advantage of this approach is that you don't need to hold a
lock for however long modifiedVersionOf() takes. The disadvantage
is that currentThing might change during that time, meaning your
effort was wasted. In principle you could stay in the loop
"forever" if other threads are constantly updating currentThing --
but a blocking algorithm wouldn't remove that pain, it would
just distribute it differently (threads could wait "forever"
before acquiring a lock). If currentThing is constantly getting
updated, you at least know that *some* thread is making progress
even if it's not *your* thread ...

     On the other hand, blocking code is a good deal easier to
write and understand. I took a week-long course on non-blocking
algorithms from an ACM Fellow, and the main thing I learned is that
you pretty much *need* to be an ACM Fellow to avoid blunders ...

Eric Sosman

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