PGI User Forum

[paokara]

Hello,

I have a problem with my code in N-body problem in calculation of accelerations.

Here is my code
!$acc kernels
!$acc loop
do i=2,nbod
axhli = 0
ayhli = 0
azhli = 0
!$acc loop
do j=2,nbod

*some calculations

axhl(j) = axhl(j) + something
ayhl(j) = ayhl(j) + something
azhl(j) = azhl(j) + something

axhli = axhli + something
ayhli = ayhli + something
azhli = azhli + something

enddo

axhl(i) = axhl(i) + axhli
ayhl(i) = ayhl(i) + ayhli
azhl(i) = azhl(i) + azhli
enddo

Of course I get "loop carried dependence of "axhl" prevents parallelization" error. I try to understand what i must do to achive 2D parallelizaton in my GPU with the correct results of course. If i use private clause i get 2D parallelization but i with wrong results

Thank you,
Sotiris

[mkcolg]

Hi Sotiris,

This is a tough one since the algorithm isn't parallel. You might be able to create a 2-D temp array for each of the arrays, but you'd need to initialize them before the loop and then perform another reduction after. I'm not sure if the extra overhead would off-set any gains you achieve in paralleling the code. You'll need to experiment.

- Mat

[paokara]

Thank you very much Mat for your quick reply. I want to ask you one more thing. My program is written in Fortran. Can I implement this double loop in cuda Fortran and then call this cuda function inside an OpenACC DATA REGION?

Thank you,
Sotiris

[mkcolg]

Hi Sotiris,

[paokara]

Hi Mat,

I want to ask you for the best way to program the following loop(OpenAcc)