On 04/21/2012 09:50 PM, Doug Lea wrote:
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
Given that fences is really low level, I'm not sure it's a good idea
to have a public API for them.
I would prefer to have a class sun.misc.Fences (or in sun.misc.Unsafe)
i.e. a class that is part of the VM API but not part of the JDK API.
Rémi
