managed memory : seg fault with pointer on host

Accelerated Computing HPC Compilers Legacy PGI Compilers
1

Hello .

I’m using pgi/16.5 ( or 16.1 & cuda7.5 or 8.0RC ) and testing managed memory in fortran on MESONH , a big Frecnh Fortran90 Weather Model .
A present time the code compile with managed memory , run incorrectly ( Nan ) on nvidia , and “seg fault” on host .

Here is a sample test case with fortran pointer showing/doing the “seg faut” on the host part , but not in the nvidia part ( Maxwell/K620 or Kepler/Titan or K40 )

I’ve no seen in the doc on managed memory any limitation on using pointer with managed memory .
One of the big advantage/goal of this one been precisely to simplify derive-type management & pointer usage !

PROGRAM MANAG_POINTER
  
  IMPLICIT NONE 

  REAL , DIMENSION(:), POINTER , CONTIGUOUS :: ZTAB 
  !REAL , DIMENSION(:), ALLOCATABLE          :: ZTAB 
  
  ALLOCATE(ZTAB(1000000))
  
  !$acc kernels copyout(ZTAB)
  ZTAB = 10.767867868
  !$acc end kernels
  
  print*,'ZTAB=',ZTAB(123456)
  
END PROGRAM MANAG_POINTER

Compiled with managed memory

pgf90 -Mcuda -ta=host,tesla:managed -Minfo=acc manag_pointer.f90 -o manag_pointer
manag_pointer:
     10, Generating copyout(ztab(:))
     11, Loop is parallelizable
         Accelerator kernel generated
         Generating Tesla code
         11, !$acc loop gang, vector(128) ! blockidx%x threadidx%x

Running it on nvidia kepler/maxwell , no problem

CUDA_VISIBLE_DEVICES=1 ACC_DEVICE=NVIDIA ./manag_pointer
 ZTAB=    10.76787

Running on the host = Seg Fault

CUDA_VISIBLE_DEVICES=1 ACC_DEVICE=HOST ./manag_pointer 
Erreur de segmentation

Changing the pointer to allocatable ( or removing the managed otion ) solve the problem but it is not applicable for us ( big code with lot of derive type + pointer ) .

For our big code it very important to have in the same executable the double target host/tesla:managed at the same time to track ‘bit reproducible’ problems .

REM : with the allocatable , I got also a “seg fault” without the ‘-Mcuda’ option

Bye

Juan

2

Hi Juan,

I talked with our compiler engineers and this is something that think they can support in a future release. I’ve added TPR#22656 to track it.

Thanks!
Mat

3

Hello Mat .

Think you for the rapid answers .

→ So for the host part , it’s not supported yet , but will be .

→ But and the device part , are the fortran pointer actually working
like it the case for all the linked lists case found on the net in C or C++
( I’m testing it with the driver CUDA8.0RC a present time )

Bye

Juan

4

Should be fixed with release 19.7