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

[Dag Sverre Seljebotn]

On 10/12/2011 10:36 AM, Dag Sverre Seljebotn wrote:
> On 10/12/2011 09:55 AM, Robert Bradshaw wrote:
>> On Sun, Oct 9, 2011 at 5:57 AM, Dag Sverre Seljebotn
>> <d.s.seljebotn at astro.uio.no> wrote:
>>> On 10/09/2011 02:18 PM, Dag Sverre Seljebotn wrote:
>>>>
>>>> On 10/09/2011 02:11 PM, mark florisson wrote:
>>>>> with parallel.critical():
>>>>> this section of code is mutually exclusive with other critical
>>>>> sections
>>>>> optional keyword argument 'name' specifies a name for the critical
>>>>> section,
>>>>> which means all sections with that name will exclude each other,
>>>>> but not
>>>>> critical sections with different names
>>>>>
>>>>> Note: all threads that encounter the section will execute it, just
>>>>> not at the same time
>>>

On critical sections, I do feel string naming is rather un-Pythonic. I'd 
rather have

lock_a = parallel.Mutex()
lock_b = parallel.Mutex()
with cython.parallel:
     with lock_a:
         ...
     with lock_b:
         ...

This maps well to pthread mutexes, though much harder to map it to OpenMP...

So my proposal is:

  a) parallel.Mutex() can take a string argument and then returns the 
same mutex each time for the same string, meaning you can do

with parallel.Mutex("somename"):

which maps directly to OpenMP.

  b) However, this does not make sense:

with parallel.Mutex():

because each thread would instantiate a *seperate* mutex. So raise 
compiler error ("Redundant code, thread will never block on fresh mutex")

  c) However, one can use a default global Mutex instance:

with parallel.global_mutex

(mapping to an un-named critical in OpenMP)

This seems to be simple enough to implement, and allows generalizing to 
the advanced case above later (probably using pthreads/Windows directly).

Dag Sverre