[m-rev.] for review: thread-local mutables

[Peter Wang]

On 2007-01-10, Julien Fischer <juliensf at csse.unimelb.edu.au> wrote:
> 
> Hi Peter,
> 
> A question ...
> 
> On Wed, 10 Jan 2007, Peter Wang wrote:
> 
> >	Add a `thread_local_mutables' field to MR_Spark and update the 
> >	parallel
> >	conjunction implementation to take into account thread-locals.
> >	Note that setting the value of a thread-local mutable inside a 
> >	parallel
> >	conjunction is not supported.
> 
> Isn't it just a matter of ensuring that when you create a new context
> as the result of a parallel conjunction that it points to the parent's
> thread_local_mutable array rather than creating a copy of it?  i.e.
> we should only make a copy of the array when the user explicitly requests
> the creation of a separate thread via spawn/3 rather than as a result
> of threads created as a result of parallel conjunctions.

I think you're right.

I was thinking, if a parallel conjunct could read from an array while
another parallel conjunct was updating the same array, we'd need to
protect the array with a mutex.  But it's a rather contrived situation,
as impure goals are not allowed in parallel conjuncts:

    set_var("foo", !IO),
    (
	set_var("bar", !IO)
    &
	semipure get_var(Var)
    )

Var could be "foo" or "bar" here, so nobody should be writing this code
anyway :-)

Peter

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