On 05/04/2011 12:00 PM, mark florisson wrote:
On 21 April 2011 20:13, Dag Sverre Seljebotn<d.s.seljeb...@astro.uio.no> wrote:
On 04/21/2011 10:37 AM, Robert Bradshaw wrote:
On Mon, Apr 18, 2011 at 7:51 AM, mark florisson
<markflorisso...@gmail.com> wrote:
On 18 April 2011 16:41, Dag Sverre Seljebotn<d.s.seljeb...@astro.uio.no>
wrote:
Excellent! Sounds great! (as I won't have my laptop for some days I
can't
have a look yet but I will later)
You're right about (the current) buffers and the gil. A testcase
explicitly
for them would be good.
Firstprivate etc: i think it'd be nice myself, but it is probably better
to
take a break from it at this point so that we can think more about that
and
not do anything rash; perhaps open up a specific thread on them and ask
for
more general input. Perhaps you want to take a break or task-switch to
something else (fused types?) until I can get around to review and merge
what you have so far? You'll know best what works for you though. If you
decide to implement explicit threadprivate variables because you've got
the
flow I certainly wom't object myself.
Ok, cool, I'll move on :) I already included a test with a prange and
a numpy buffer with indexing.
Wow, you're just plowing away at this. Very cool.
+1 to disallowing nested prange, that seems to get really messy with
little benefit.
In terms of the CEP, I'm still unconvinced that firstprivate is not
safe to infer, but lets leave the initial values undefined rather than
specifying them to be NaNs (we can do that as an implementation if you
want), which will give us flexibility to change later once we've had a
chance to play around with it.
I don't see any technical issues with inferring firstprivate, the question
is whether we want to. I suggest not inferring it in order to make this
safer: One should be able to just try to change a loop from "range" to
"prange", and either a) have things fail very hard, or b) just work
correctly and be able to trust the results.
Note that when I suggest using NaN, it is as initial values for EACH
ITERATION, not per-thread initialization. It is not about "firstprivate" or
not, but about disabling thread-private variables entirely in favor of
"per-iteration" variables.
I believe that by talking about "readonly" and "per-iteration" variables,
rather than "thread-shared" and "thread-private" variables, this can be used
much more safely and with virtually no knowledge of the details of
threading. Again, what's in my mind are scientific programmers with (too)
little training.
In the end it's a matter of taste and what is most convenient to more users.
But I believe the case of needing real thread-private variables that
preserves per-thread values across iterations (and thus also can possibly
benefit from firstprivate) is seldomly enough used that an explicit
declaration is OK, in particular when it buys us so much in safety in the
common case.
To be very precise,
cdef double x, z
for i in prange(n):
x = f(x)
z = f(i)
...
goes to
cdef double x, z
for i in prange(n):
x = z = nan
x = f(x)
z = f(i)
...
and we leave it to the C compiler to (trivially) optimize away "z = nan".
And, yes, it is a stopgap solution until we've got control flow analysis so
that we can outright disallow such uses of x (without threadprivate
declaration, which also gives firstprivate behaviour).
I think the preliminary OpenMP support is ready for review. It
supports 'with cython.parallel.parallel:' and 'for i in
cython.parallel.prange(...):'. It works in generators and closures and
the docs are updated. Support for break/continue/with gil isn't there
yet.
There are two remaining issue. The first is warnings for potentially
uninitialized variables for prange(). When you do
for i in prange(start, stop, step): ...
it generates code like
nsteps = (stop - start) / step;
#pragma omp parallel for lastprivate(i)
for (temp = 0; temp< nsteps; temp++) {
i = start + temp * step;
...
}
So here it will complain about 'i' being potentially uninitialized, as
it might not be assigned to in the loop. However, simply assigning 0
to 'i' can't work either, as you expect zero iterations not to touch
it. So for now, we have a bunch of warnings, as I don't see a
__attribute__ to suppress it selectively.
Isn't this is orthogonal to OpenMP -- even if it said "range", your
testcase could get such a warning? If so, the fix is simply to
initialize i in your testcase code.
The second is NaN-ing private variables, NaN isn't part of C. For gcc,
the docs ( http://www.delorie.com/gnu/docs/glibc/libc_407.html ) have
the following to say:
"You can use `#ifdef NAN' to test whether the machine supports NaN.
(Of course, you must arrange for GNU extensions to be visible, such as
by defining _GNU_SOURCE, and then you must include `math.h'.)"
So I'm thinking that if NaN is not available (or the compiler is not
GCC), we can use FLT_MAX, DBL_MAX and LDBL_MAX instead from float.h.
Would this be the proper way to handle this?
I think it is sufficient. A relatively portable way would be to
initialize a double variable to 0.0/0.0 at program startup; a problem is
that that would flag exceptions in the FPU though.
Here's some more compiler-specific stuff I found:
http://www.koders.com/c/fid6EF58B6683BCD810AE371607818952EB039CBC32.aspx
DS
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