> On Linux 3.16? 4.7?  What is difference between 3.16 and 4.7?

On 4.8 (which has the same issue as 4.7).
The difference is that on 4.7, the rlimit has to be set incredibly high for the child process not to hit the limit.

> Again, I doubt Python's memory consumption increased by Linux version.
> Isn't rlimit more strict for now?

That's what I think too, because I would have noticed if the interpreter actually started using 20MB for such a simple operation.

> Even if memory usage is really grow, I don't think it's a Python's issue.
> Maybe, environment issue or kernel issue.

I agree. But I don't know how to reproduce this issue without multiprocessing.Queue.