[Omp] A question about threadprivate

[Dieter an Mey]

The interesting point here is not related to threadprivate, but to the 
way omp_get_thread_num() works with nesting.

Look at that:

source:

program main
use omp_lib
   print *, "case 1"
	call omp_set_nested(.true.)
!$omp parallel num_threads(2)
!$omp parallel num_threads(1)
                print *, omp_get_thread_num()
!$omp end parallel
!$omp end parallel

   print *, "case 2"
	call omp_set_nested(.false.)
!$omp parallel num_threads(2)
!$omp parallel num_threads(1)
                print *, omp_get_thread_num()
!$omp end parallel
!$omp end parallel

   print *, "case 3"
!$omp parallel num_threads(2)
                print *, omp_get_thread_num()
!$omp end parallel

end program


output:
  case 1
  0
  0
  case 2
  0
  0
  case 3
  0
  1

regards,
Dieter

Marina Kraeva schrieb:
> Larry,
> 
> As I understood, Dieter suggested that the inner region modifying 'i' IS 
> relevant, because the master threads of the inner regions will be 
> modifying the parent copies of 'i', so after the nested parallel region 
> all four copies of 'i' will have value '-0'.
> 
> Anyway, I emailed a request to feedback at openmp.org asking to specify the 
> behavior in various situations, including these two examples.
> 
> Thanks again for your input!
> 
> Marina.
> 
> 
> Meadows, Lawrence F wrote:
>> Now that I re-read the second example, I agree with Dieter that
>> the result is well-defined by the 2.0 standard:
>>
>>
>>>>> int i,array(0:3)
>>>>> threadprivate(i)
>>>>> !$omp parallel num_threads(4)
>>>>>    i=omp_get_thread_num()
>>>>>    !$omp parallel num_threads(2)
>>>>>                i=-omp_get_thread_num()
>>>>>    !$omp end parallel
>>>>> !$omp end parallel
>>>>>
>>>>> !$omp parallel num_threads(4) shared(array)
>>>>>    array(omp_get_thread_num())=i
>>>>> !$omp end parallel
>>>>>
>>
>>
>> The two outer parallel regions are consecutive, have the
>> same number of threads, dyn-var is false, and nthreads-var
>> hasn't changed. The inner region modifying 'i' is not
>> relevant here, it is a different 'i'.
>>
>> So I believe that the value of 'i' for the threads in the second
>> out parallel region is retained in both examples, and that this
>> behavior is clearly specified by the standard.
>>
>>
> 

-- 
--------------------------------------------------------------------
Dieter an Mey
High Performance Computing               Hochleistungsrechnen
RWTH Aachen University                   Rechen- und Kommunikations-
Center for Computing and Communication   zentrum der RWTH Aachen
phone: ++49-(0)241-80-24377              Seffenter Weg 23
fax:   ++49-(0)241-80-22134              52074 Aachen, Germany
email: anmey at rz.rwth-aachen.de
--------------------------------------------------------------------