[pypy-dev] Would the following shared memory model be possible?

William Leslie william.leslie.ttg at gmail.com
Thu Jul 29 07:18:57 CEST 2010

On 29 July 2010 11:33, Kevin Ar18 <kevinar18 at hotmail.com> wrote:
> In detail, here's what I mean:
> * unlike POSH, utilize OS threads and shared memory (not processes)
> * Create a special shared memory location where you can place Python objects
> * Each Python object you place into this location can only be accessed
> (modified) by 1 thread.
> * You must manually assign ownership of an object to a particular thread.
> * The thread that "owns" the object is the only one that can modify it.
> * You can transfer ownership to another thread (but, as always only the
> owner can modify it).

When an object is mutable, it must be visible to at most one thread.
This means it can participate in return values, arguments and queues,
but the sender cannot keep a reference to an object it sends, because
if the receiver mutates the object, this will need to be reflected in
the sender's thread to ensure internal consistency. Well, you could
ignore internal consistency, require explicit locking, and have it
segfault when the change to the length of your list has propogated but
not the element you have added, but that wouldn't be much fun. The
alternative, implicitly writing updates back to memory as soon as
possible and reading them out of memory every time, can be hundreds or
more times slower. So you really can't have two tasks sharing mutable
objects, ever.

William Leslie

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