Re: JavaMemoryModel: october JMM spec seems to break java's threading model

From: Doug Lea (
Date: Thu Nov 06 2003 - 19:34:43 EST

> > The best you can say is "over sufficiently long
> > observation periods, unblocked threads will progress"
> Will all unblocked threads make progress?

Thanks, I should have said: "over sufficiently long observation
periods, one or more unblocked threads will progress"

> A common fairness guarantee
> in model checking literature would be that if a thread can infinitely
> often make progress (e.g it is unblocked and perhaps has the highest
> priority), then it will infinitely often make progress or terminate.
> There isn't any guarantee in timing, just that eventually each
> unblocked thread will make progress.
> Do existing JVMs provide this guarantee?

Well, it's not in the spec. Generally, a given JVM will provide
approximately the same progress guarantees (or lack thereof) as the OS
it runs on. Almost definitionally, a JVM can do no better than that.

RTSJ (Real-time Java) makes much stronger guarantees, but is not
spec'ed as fair. Instead it is strictly priority based (and can only
run on priority-based real-time OS kernels.)

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