On 04/04/2011 03:04 PM, Stefan Behnel wrote:
Dag Sverre Seljebotn, 04.04.2011 13:53:
On 04/04/2011 01:23 PM, Stefan Behnel wrote:
Dag Sverre Seljebotn, 04.04.2011 12:17:
CEP up at http://wiki.cython.org/enhancements/prange
"""
Variable handling
Rather than explicit declaration of shared/private variables we rely on
conventions:
* Thread-shared: Variables that are only read and not written in the
loop
body are shared across threads. Variables that are only used in the
else
block are considered shared as well.
* Thread-private: Variables that are assigned to in the loop body are
thread-private. Obviously, the iteration counter is thread-private
as well.
* Reduction: Variables that only used on the LHS of an inplace
operator,
such as s above, are marked as targets for reduction. If the
variable is
also used in other ways (LHS of assignment or in an expression) it does
instead turn into a thread-private variable. Note: This means that if
one, e.g., inserts printf(... s) above, s is turned into a thread-local
variable. OTOH, there is simply no way to correctly emulate the effect
printf(... s) would have in a sequential loop, so such code must be
discouraged anyway.
"""
What about simply (ab-)using Python semantics and creating a new inner
scope for the prange loop body? That would basically make the loop
behave
like a closure function, but with the looping header at the 'right'
place
rather than after the closure.
I'm not quite sure what the concrete changes to the CEP this would
lead to
(assuming you mean this as a proposal for alternative semantics, and
not an
implementation detail).
What I would like to avoid is having to tell users "and now for
something completely different". It looks like a loop, but then
there's a whole page of new semantics for it. And this also cannot be
used in plain Python code due to the differing scoping behaviour.
Well, at least it's better than the 300 pages of semantics for OpenMP :-)
How would we treat reduction variables? They need to be supported, and
there's nothing in Python semantics to support reduction variables, they
are a rather special case everywhere. I suppose keeping the reduction
clause above, or use the "nonlocal" keyword in the loop body...
That's what I thought, yes. It looks unexpected, sure. That's the
clear advantage of using inner functions, which do not add anything
new at all. But if we want to add something that looks more like a
loop, we should at least make it behave like something that's easy to
explain.
Sorry for not taking the opportunity to articulate my scepticism in
the workshop discussion.
I like the idea of considering cdef/nonlocal in the prange blocks. But,
yes, I do feel that opposing a parallel loop construct in general is
rather late, or at least could have been done at a more convenient time...
All I know and care about is that a decorator-and-closure solution will
be a lot more obscure among non-CS people who have no clue what a
closure or decorator is, and those are exactly the people who need this
kind of simple 80%-solution. You and me don't really need any support
from Cython at all to write multithreaded apps (leaving aesthetics and
number of keystrokes to the side).
It'd be good to hear Robert's and Mark's opinions before going further,
let's economise this thread a bit.
Dag Sverre
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