On 09/24/2017 08:15 AM, 'Carl Mastrangelo' via mechanical-sympathy wrote:
Tl;Dr: does CMPXCHG assume it will fail or succeed?
I am on Java 8, and need an atomic boolean. I don't want to pay
higher memory cost, so I am using an AtomicIntegerFieldUpdater.
Right now I have some code that looks
queue.add(work);
if (running.compareAndSet(this, stopped, running)) {
executor.execute(queueHandler);
}
I was looking at optimizing this code and noticed that compareAndSet()
could be changed to a getAndSet() pretty easily. Looking at the
hotspot code there are three atomic instructions that are used:
cmpxchg, xadd, and xchg.
Here is my question: the 2nd and 3rd instructions know for sure they
are going to modify the value, so they can bring in the cacheline as
modified. But, cmpxchg could potentially bring the cacheline in as
shared/exclusive/owned and not invalidate all the other caches. If I
have special knowledge that I will likely fail to exchange values,
then wouldn't it make sense to call get() just before compareAndSet()
? I wouldn't have to do this is cmpxchg automatically did that for
me, but then how could it guess whether it will win or lose?
It can vary by processor, but Intels are optimistic. They assume that
either the operation will succeed, or that you will loop until it does.
If you know it will usually fail, and it's important to you, you can do
your own compare first.
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