Draft j.u.c JEP (original) (raw)

Doug Lea dl at cs.oswego.edu
Sat Apr 21 19:50:08 UTC 2012

On 04/21/12 13:10, Brian Goetz wrote:

My only concern is the mention of a "fences" API; I would think that this might rise to the level of wanting its own JSR, since the memory model does not necessarily provide for the all various relaxed consistency modes that such an API would seem to imply, and might involve VM support.

Tying this to a JMM revision is a little scary: We know there are bugs (ranging from typos to unintended semantics to incompleteness) in the JMM. None of them are very interesting wrt 99.99999% of all java programs. Still, the sense is that any revision should fix these as well. But some of these are not amenable to simple bandaids, but would require a major re-spec effort. In the mean time, we have introduced some methods (like AtomicX.lazySet) as well as those in the JDK7 Fences proposal that can ride on top of flawed underlying spec, in a way that doesn't introduce any further flaws, and so doesn't make eventual JMM re-spec any harder.

When scoping out Fences for JDK7, I discussed the 3 minimal VM intrinsics methods to add with Hotspot and IBM J9 engineers. They were OK about doing it then (because it mainly entails just exposing some VM internals), so presumably still are, modulo scheduling effort to actually do it.

The main questions are as always whether the potential benefits of exposing these methods to people who need them outweigh their relatively high potential for errors and uninterpretable effects. People have very strong view on both sides. (This doesn't impact us much inside java.util.concurrent because we can for the most part get the intended effects by directly using existing intrinsics.)

Anyway, it's not clear we even want to have another discussion about introducing Fences. But I listed it because I think it is likely to come up again.

-Doug

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