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Date: Fri, 08 Apr 2011 23:29:55 +0200
From: Heiko Wundram <modelnine@modelnine.org>
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Subject: Re: Copy-on-write when forking a python process
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:2876

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.