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

[Robert Bradshaw]

On Mon, Oct 10, 2011 at 1:12 AM, mark florisson
<markflorisson88 at gmail.com> wrote:
> 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)

Interesting idea. An alternative syntax could be

a = cython.parallel.reduce(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.
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