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

[mark florisson]

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...