[Cython] cython.parallel tasks, single, master, critical, barriers

[mark florisson]

On 9 October 2011 22:27, mark florisson <markflorisson88 at gmail.com> wrote:
>
> On 9 October 2011 21:48, Jon Olav Vik <jonovik at gmail.com> wrote:
> > On Sun, Oct 9, 2011 at 9:01 PM, mark florisson
> > <markflorisson88 at gmail.com> wrote:
> >> On 9 October 2011 19:54, Jon Olav Vik <jonovik at gmail.com> wrote:
> >>> Personally, I think I'd prefer context managers as a very
> >>> readable way to deal with parallelism
> >>
> >> Yeah it makes a lot of sense for mutual exclusion, but 'master' really
> >> means "only the master thread executes this peace of code, even though
> >> other threads encounter the same code", which is more akin to 'if'
> >> than 'with'.
> >
> > I see your point. However, another similarity with "with" statements
> > as an encapsulated "try..finally" is when there's a barrier at the end
> > of the block. I can live with some magic if it saves me from having a
> > boilerplate line of "barrier" everywhere 8-)
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> >
>
> Hm, indeed. I just noticed that unlike single constructs, master
> constructs don't have barriers. Both are also not allowed to be
> closely nested in worksharing constructs. I think the single directive
> is more useful with respect to tasks, e.g. have a single thread
> generate tasks and have other threads waiting at the barrier execute
> them. In that sense I suppose 'if parallel.is_master():' makes sense
> (no barrier, master thread) and 'with single():' (with barrier, any
> thread).
>
> We could still support single in prange though, if we simply have the
> master thread execute it ('if (omp_get_thread_num() == 0)') and put a
> barrier after the block. This makes me wonder what the point of master
> was supposed to be...

Scratch that last part about master/single in parallel sections, it
doesn't make sense. It only makes sense if you think of those sections
as tasks you submit that would be immediately taken up by a (certain)
thread. But that's not quite what it means. I do like 'if is_master()'
and 'with single', though.

Another thing we could support is arbitrary reductions. In OpenMP 3.1
you get reduction operators 'and', 'max' and 'min', but it wouldn't be
hard to support arbitrary user functions. e.g.

@cython.reduction
cdef int func(int a, int b):
    ...

for i in prange(...):
    a = func(a, b)

I'm not sure how common this is though. You probably have your
reduction data in an array so you're already using numpy so you'll
likely already have your functionality.