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Hello. I am trying to test out the nowait clause but I'm having trouble with catching when the offload actually completes. I need to sync between the host and card before writing to global memory.
without the nowait clause everything runs fine. With it, nothing seems to be happening - offload does not complete. Any ideas? Thanks
PROGRAM ASYNC_TEST USE OMP_LIB USE IFPORT IMPLICIT NONE INTEGER :: X,Y,IE REAL(16), ALLOCATABLE :: x_arr(:), y_arr(:) REAL(16) t1a,t1b,t1c,t2a,t2b,t2c t1a = omp_get_wtime() allocate(x_arr(1000000)) allocate(y_arr(1000000)) t1b = omp_get_wtime()-t1a WRITE(*,*) 'Allocation time on HOST: ',t1b !on host DO IE = 1,1000 x_arr(IE) = RAND() y_arr(IE) = RAND() END DO ! !$omp target nowait depend(out:Y) map(to:x_arr,y_arr) !$omp target nowait t1a = omp_get_wtime() DO X =1,100 DO IE = 1,500000 x_arr(IE) = x_arr(IE)*x_arr(IE)+y_arr(IE) END DO END DO t1b = omp_get_wtime()-t1a WRITE(*,*) 'MIC COMPUTE: ',t1b !$omp end target t2a = omp_get_wtime() DO X = 1,100 DO IE = 500001,1000000 x_arr(IE) = x_arr(IE)*x_arr(IE)+y_arr(IE) END DO END DO t2b = omp_get_wtime()-t2a WRITE(*,*) 'HOST_COMPUTE: ',t2b WRITE(*,*) 'MIC DONE' ! !$omp task depend(in:t2b) ! WRITE(*,*) 'HOST DONE' ! !$omp end task END PROGRAM
Also, do I have to encase the target directive in a task region? I will only have 1 thread on the host offloading. How do tasks work when there's only 1 tread?
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Hi,
the pattern is almost correct. If you want to synchronize the host execution with the async offload this is what you'd need to do:
integer :: sync_var ! offloaded code section !$omp target depend(out:sync_var) nowait call offloaded_stuff() !$omp end target ! this part here executes concurrently with the target device call stuff() ! now synchronize host and offload !$omp task depend(in:sync_var) if(0) !$omp end task
The empty task is not really executed, it is just there to have a way to express the dependency of the offloaded region with the host execution. All code that follows the empty task will only execute when the async offload has finished.
If there's only one thread, the OpenMP runtime does the magic to still have an async offload.
Hope that helps!
Cheers,
-michael
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Michael Klemm (Intel) wrote:
Hi,
the pattern is almost correct. If you want to synchronize the host execution with the async offload this is what you'd need to do:
integer :: sync_var ! offloaded code section !$omp target depend(out:sync_var) nowait call offloaded_stuff() !$omp end target ! this part here executes concurrently with the target device call stuff() ! now synchronize host and offload !$omp task depend(in:sync_var) if(0) !$omp end taskThe empty task is not really executed, it is just there to have a way to express the dependency of the offloaded region with the host execution. All code that follows the empty task will only execute when the async offload has finished.
If there's only one thread, the OpenMP runtime does the magic to still have an async offload.
Hope that helps!
Cheers,
-michael
Hi, Thanks that makes sense. I've tried a similar configuration but the problem persists. The offload just never seems to end. The last thing offload reports show is the target--> host copy.
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As you can see in the terminal, it never reaches done.
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Added taskwait before MIC DONE as shown below.
!$omp taskwait
WRITE(*,*) 'MIC DONE'
And this is the result I got
ifort -qopenmp nowait.f90
./a.out
Allocation time on HOST: 9.059906005859375000000000000000000E-0006
HOST_COMPUTE: 1.38332700729370117187500000000000
MIC COMPUTE: 10.8623330593109130859375000000000
MIC DONE
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