[issue39511] [subinterpreters] Per-interpreter singletons (None, True, False, etc.)

[Mark Shannon]

Mark Shannon <mark at hotpy.org> added the comment:

Consider the case where a thread that doesn't hold the GIL attempts to get a reference on `None`.

The problem with having a single immortal `None`, is that it will cause data cache thrashing as two different CPUs modify the refcount on the shared `None` object.
Each subinterpreter needs its own distinct `None`.

`None` could be made immortal, it just can't be shared between sub-interpreters.

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nosy: +Mark.Shannon

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