On 04/04/2011 09:26 PM, mark florisson wrote:
On 4 April 2011 19:18, Dag Sverre Seljebotn<d.s.seljeb...@astro.uio.no> wrote:
On 04/04/2011 05:22 PM, mark florisson wrote:
On 4 April 2011 13:53, Dag Sverre Seljebotn<d.s.seljeb...@astro.uio.no>
wrote:
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).
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...
Also there's the else:-block, although we could make that part of the
scope.
And the "lastprivate" functionality, although that could be dropped
without
much loss.
Also, in the example, the local variable declaration of "tmp" outside of
the loop looks somewhat misplaced, although it's precedented by
comprehensions (which also have their own local scope in Cython).
Well, depending on the decision of lastprivate, the declaration would
need
to be outside; I really like the idea of moving "cdef", and am prepared
to
drop lastprivate for this.
Being explicit about thread-local variables does make things a lot safer
to
use.
(One problem is that switching between serial and parallel one needs to
move
variable declarations. But that only happens once, and one can use
"nthreads=1" to disable parallel after that.)
An example would then be:
def f(np.ndarray[double] x, double alpha):
cdef double s = 0, globtmp
with nogil:
for i in prange(x.shape[0]):
cdef double tmp # thread-private
tmp = alpha * i # alpha available from global scope
s += x[i] * tmp # still automatic reduction for inplace
operators
# printf(...s) -> now leads to error, since s is not declared
thread-private but is read
else:
# tmp still available here...looks a bit strange, but useful
s += tmp * 10
globtmp = tmp # we save tmp for later
# tmp not available here, globtmp is
return s
Or, we just drop support for the else block on these loops.
I think since we are disallowing break (yet) we shouldn't support the
else clause. Basically, I think we can make the CEP a tad more simple.
I think we could declare everything outside of the prange body. Then,
in the prange loop body:
if a variable is assigned to anywhere -> make it lastprivate
- if a variable is read before assigned to -> make it
firstprivate in addition to lastprivate (raise compiler error if the
variable is not initialized outside of the loop body)
if a variable is only ever read -> make it shared (the default for
OpenMP)
if a variable has an inplace operator -> make it a reduction
There is really no reason to disallow reading of the reduction
variable (in e.g. a printf). The reduction should also be initialized
outside of the prange body.
The reason for disallowing reading the reduction variable is that otherwise
you have a contradiction above, since a reduction variable may also be a
thread-local variable. Or, you disable inplace operators for thread-local
variables? (ugh)
Yes, an inplace operator would make it a reduction variable, just like
assigning something makes it lastprivate, only reading makes it shared
and reading before writing makes it firstprivate in addition to
lastprivate. This is all implicit.
Alternatively, if you want it more explicit, then instead of the
inplace operator you could allow something like
sum = cython.parallel.reduction('+', sum) + var1 * var2
instead of
sum += var1 * var2
That's the main reason I'm leaning towards explicit declaring local
variables using "cdef".
If we're reducing complexity BTW, I'd rather remove firstprivate/lastprivate
alltogether, see below.
Then prange() could be implemented in pure mode as simply the
sequential version, i.e. range() which some more arguments.
For any scratch space buffers etc, I'd prefer something like
with cython.parallel:
cdef char *buf = malloc(100)
for i in prange(n):
use buf
free(buf)
At least it fits my brain pretty well :) (this code does however
assume that malloc is thread-safe).
Yes...perhaps a cython.parellel block will make everybody happy:
- It's more obvious that we create a new scope, which at least answers some
of Stefan's complaints
- We can use normal "for i in range", and put scheduling params on
parallel(), which makes Nathaniel happy
That doesn't sound intuitive, as the scheduling pertains to the
worksharing 'for' construct, and not the entire parallel region. So
scheduling parameters should be provided to e.g.
cython.parallel.range() (or cython.prange, cython.parallel_range,
whatever).
Then if cython.parallel.range() is in a 'with cython.parallel' block,
it would have '#pragma omp for' semantics (considering OpenMP),
whereas it would be a '#pragma omp parallel for' if not closely nested
in such a block.
In this case I'd say we simply do not support firstprivate, all thread-local
variables must be declared in the block, and for firstprivate behaviour you
just initialize them yourself which is more explicit and Pythonic. The
"else:"-block on loops is still useful for lastprivate behaviour -- the
point of executing the else block in one of the threads is that you can then
copy thread-local variables of the "last" thread into shared variables to
get lastprivate behaviour (again, more explicit and Python).
Why? They are entirely implicit in my proposal, and intuitively so.
Having the parallel range match the sequential range semantics in this
way feel much more Pythonic than having to copy things over in an else
block and having to declare and define simple variables in a special
place.
I'm just afraid the risk of creating bugs is too high with inplace
operators being as magic as you propose. We must not only judge the
convenience when done right, but also the chance of using it in the
wrong way.
"import this" says "Explicit is better than implicit". I'll likely draft
a different CEP tomorrow just to explore more -- in the end I may still
well favour your approach.
Dag Sverre
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