How to understand the different output of two UnsetShared?

Question

Update 2

In v11.0.0, situation seems getting worse. UnsetShared doesn't unshare at all. Even the code in help documentation doesn't work as expected.

Update

From technical support,

Thank you for contacting Wolfram Technical Support. I understand that you are reporting an issue with UnsetShared deleting the shared variable after a second evaluation. There seems to be an issue and I was able to reproduce it. I have forwarded an incident report to our developers and have included your contact information in my report.

Original Post

In[1]:= z = 1; SetSharedVariable[z];
ParallelEvaluate[z]
ParallelEvaluate[Print@z];

(==> Out[2]= {1, 1})

(during the evaluation)
(kernel 1) 1
(kernel 2) 1

Looks resonable. However,

In[4]:= UnsetShared[z];
ParallelEvaluate[Print@z];

(during the evaluation)
(kernel 1) 1
(kernel 2) 1

this is pretty strange since the z is not recognized by the subkernels anymore.

The most strange part is if you evaluate the In[4] again :

In[6]:= UnsetShared[z];
ParallelEvaluate[Print@z];

(during the evaluation)
(kernel 1) z
(kernel 2) z

This really disturbes me. Is this a bug or did I misunderstand something? (Windows 10, Mathematica v10.4)

Answer 1

This is an extended comment.

This behaviour is new in 10.4. I can reproduce it with 10.4.1 on OS X, but not with 10.3.1.

This may be related: DistributeDefinitions and synchronization in Mathematica 10

We can try to analyse what happens like this:

z := (Print[$KernelID]; 1);
SetSharedVariable[z];

ParallelEvaluate[z]
ParallelEvaluate[Print@Definition[z]]
ParallelEvaluate[Print@z];

This will let us figure out:

• Where does z get evaluated? Since it's a shared variable, it is supposed to evaluate on the main kernel, then the result must be passed back to the subkernel.

• What does z evaluate to?

• What is the definition of z on the subkernels?

At this point we get what I would expect, and everything is in agreement with previous versions:

• z is evaluated on the main kernel
• z evaluates to 1
• z has a special definition on the subkernel that triggers the callback to the main kernel (I won't quote the output because it is very large)

Now we UnsetShared ...

UnsetShared[z];
ParallelEvaluate[Print@Definition[z]];
ParallelEvaluate[Print[z]]

... and we find that

• z evaluates on the subkernels
• z evaluates to 1 (which I consider wrong, like you).
• z has the same definition on the subkernels as on the main kernel, which is wrong, and differs from previous versions. This definition should not have been distributed by ParallelEvaluate.

And we do it again:

UnsetShared[z];
ParallelEvaluate[Print@Definition[z]];
ParallelEvaluate[Print[z]]

Now everything is back to normal. z has no definition on the subkernel any longer.