[Tim]
In that case, it's because Python _does_ mutate the objects' refcount members under the covers, and so the OS ends up making fresh copies of the memory anyway.
[Greg Ewing <greg.ewing@canterbury.ac.nz>]
Has anyone ever considered addressing that by moving the refcounts out of the objects and keeping them somewhere else?
Not that I know of. I know Larry Hastings was considering doing it as part of his experiments with removing the GIL, but that had nothing to do with reducing cross-process copy-on-write surprises (it had to do with "batching" refcount operations to eliminate a need for fine-grained locking). As-is, I'd say it's "a feature" that the refcount is part of the object header. Ref count manipulations are very frequent, and as part of the object header a refcount tends to show up in cache lines "for free" as a side effect of accessing the object's type pointer.