Am 08.04.2011 20:34, schrieb jac: > I disagree with your statement that COW is an optimization for a > complete clone, it is an optimization that works at the memory page > level, not at the memory image level. In other words, if I write to a > copy-on-write page, only that page is copied into my process' address > space, not the entire parent image. To the best of my knowledge by > preventing the child process from altering an object's reference count > you can prevent the object from being copied (assuming the object is > not altered explicitly of course.)
As I said before: COW for "sharing" a processes forked memory is simply an implementation-detail, and an _optimization_ (and of course a sensible one at that) for fork; there is no provision in the semantics of fork that an operating system should use COW memory-pages for implementing the copying (and early UNIXes didn't do that; they explicitly copied the complete process image for the child). The only semantic that is specified for fork is that the parent and the child have independent process images, that are equivalent copies (except for some details) immediately after the fork call has returned successfully (see SUSv4). What you're thinking of (and what's generally useful in the context you're describing) is shared memory; Python supports putting objects into shared memory using e.g. POSH (which is an extension that allows you to place Python objects in shared memory, using the SysV IPC-featureset that most UNIXes implement today). -- --- Heiko. -- http://mail.python.org/mailman/listinfo/python-list
