On 5 April 2011 14:55, Pauli Virtanen <p...@iki.fi> wrote:
>
> Mon, 04 Apr 2011 21:26:34 +0200, mark florisson wrote:
> [clip]
> > For clarity, I'll add an example:
> [clip]
>
> How about making all the special declarations explicit? The automatic
> inference of variables has a problem in that a small change in a part of
> the code can have somewhat unintuitive non-local effects, as the private/
> shared/reduction status of the variable changes in the whole function
> scope (if Python scoping is retained).
>
> Like so with explicit declarations:
>
> def f(np.ndarray[double] x, double alpha):
> cdef double alpha = 6.6
> cdef char *ptr = something()
>
> # Parallel variables are declared beforehand;
> # the exact syntax could also be something else
> cdef cython.parallel.private[int] tmp = 2, tmp2
> cdef cython.parallel.reduction[int] s = 0
>
> # Act like ordinary cdef outside prange(); in the prange they are
> # firstprivate if initialized or written to outside the loop anywhere
> # in the scope. Or, they could be firstprivate always, if this
> # has a negligible performance impact.
> tmp = 3
The problem with firstprivate() is that it doesn't give you the same
semantics as in the sequential version. That's why I think it would be
best to forget about firstprivate entirely and allow reading of
private variables only after they are assigned to in the loop body.
>
> with nogil:
> s = 9
>
> for i in prange(x.shape[0]):
> if cython.parallel.first_iteration(i):
> # whatever initialization; Cython is in principle allowed
> # to move this outside the loop, at least if it is
> # the first thing here
> pass
For this I prefer the aforementioned 'with cython.parallel:' block.
>
> # tmp2 is not firstprivate, as it's not written to outside
> # the loop body; also, it's also not lastprivate as it's not
> # read outside the loop
> tmp2 = 99
>
> # Increment a private variable
> tmp += 2*tmp
>
> # Add stuff to reduction
> s += alpha*i
>
> # The following raise a compilation error -- the reduction
> # variable cannot be assigned to, and can be only operated on
> # with only a single reduction operation inside prange
> s *= 9
> s = 8
I think OpenMP allows arbitrary assignments and expressions to the
reduction variable, all the spec says "usually it will be of the form
'x <binop>= ...'".
>
> # It can be read, however, provided openmp supports this
> tmp = s
>
> # Assignment to non-private variables causes a compile-time
> # error; this avoids common mistakes, such as forgetting to
> # declare the reduction variable.
> alpha += 42
> alpha123 = 9
> ptr = 94
>
> # These, however, need to be allowed:
> # the users are on their own to make sure they don't clobber
> # non-local variables
> x[i] = 123
> (ptr + i)[0] = 123
> some_routine(x, ptr, i)
Indeed. They could be either shared or firstprivate (as the pointer
would be firstprivate, and not the entire array, unless it was
declared as a C array of certain size).
> else:
> # private variables are lastprivate if read outside the loop
> foo = tmp
>
> # The else: block can be added, but actually has no effect
> # as it is always executed --- the code here could as well
> # be written after the for loop
> foo = tmp # <- same result
>
> with nogil:
> # Suppose Cython allowed cdef inside blocks with usual scoping
> # rules
> cdef cython.parallel.reduction[double] r = 0
>
> # the same variables can be used again in a second parallel loop
> for i in prange(x.shape[0]):
> r += 1.5
> s -= i
> tmp = 9
>
> # also the iteration variable is available after the loop
> count = i
>
> # As per usual Cython scoping rules
> return r, s
>
> What did I miss here? As far as I see, the above would have the same
> semantics and scoping as a single-threaded Python implementation.
>
> The only change required to make things parallel is replacing range() by
> prange() and adding the variable declarations.
Basically, I like your approach. It's only slightly more verbose as
the implicit way, as you need to declare the type of each variable
anyway.
I also still like the implicit way, but it has a couple of problems:
- inplace operators suddenly declare a reduction
- assigning to a variable has implicit (last)private semantics,
whereas assigning to an element in a buffer has shared semantics
Your explicit version solves both these problems. So I'm +1.
> --
> Pauli Virtanen
>
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