[concurrency-interest] Java Memory Model and ParallelStream

Aleksey Shipilev shade at redhat.com
Fri Mar 6 06:44:57 EST 2020


On 3/6/20 12:11 PM, Luke Hutchison wrote:
> ...which is why I specifically excluded inlining in my original question (or said consider the state
> after all inlining has taken place). I realize that inlining doesn't just happen at compiletime, and
> the JIT could decide at any point to inline a function, but I want to ignore that (very real)
> possibility to understand whether reordering can take place across method boundaries _if inlining
> never happens_. 

That is an odd exclusion. Inlining is the mother of all optimizations: it expands the optimization
scope. But even "if" formal inlining does not happen, you can devise the closed-world/speculatve
optimizations that peek into method implementations and use that knowledge to optimize. Coming up
with the concrete example is counter-productive, IMO, because it plays into caring about
implementation specifics, rather than the high-level guarantees.


>     > There's no "element-wise volatile" array unless you resort to using an AtomicReferenceArray,
>     > which creates a wrapper object per array element, which is wasteful on computation and space.
> 
>     Not really related to this question, but: VarHandles provide "use-site" volatility without
>     "def-site" volatility. In other words, you can access any non-volatile element as if it is volatile.
> 
> Thanks for the pointer, although if you need to create one VarHandle per array element to guarantee
> this behavior, 

No, you don't need a VH per array element, you can have one that accepts the array and the index:
https://docs.oracle.com/javase/9/docs/api/java/lang/invoke/MethodHandles.html#arrayElementVarHandle-java.lang.Class-

> then that's logically no different than wrapping each array element in a wrapper
> object with AtomicReferenceArray.

No, it is not the same. AtomicReferenceArray gives you one additional indirection to its own array.
VarHandle can do the operation _on the array you give it_.


> I guess fundamentally I was asking if any memory reordering (or cache staleness) can happen across
> synchronization barriers. It sounds like that is not the case, due to synchronization barriers
> implementing a computational "happens-before" guarantee, which enforces the same "happens-before"
> total ordering on memory operations across the barrier.

Tell me what do you mean by "Synchronization barrier" (and how it relates to the what you are
asking, to avoid XY Problem), and then we can talk about what properties does it have. Loosely
defined things can have whatever properties :)

Otherwise, look, high-level guarantees are the king. They do not force you to know the low-level
details. In just about every parallel implementation everything that worker threads do
happens-before their thread/task termination/publication, and thread/result termination/publication
happens-before the detection/acquisition of the result.

It is not really relevant how that detection/acquisition happens:
 - successful Thread.join() for a terminating worker; (guaranteed by JLS)
 - successful Future.get() from executor; (guaranteed by package spec in java.util.concurrent.*)
 - successful forEach for a parallel stream; (provided by extension?)

-- 
Thanks,
-Aleksey

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