Allocatable arrays in CUDA-Fortran device data structures

问题

I'm trying to use allocatable arrays inside "device" data structures that reside in GPU memory. Code (pasted below) compiles, but gives a segfault. Am I doing something obviously wrong?

Module file is called 'gpu_modules.F90', given below:

!=============
! This module contains definitions for data structures and the data
! stored on the device
!=============

   module GPU_variables
   use cudafor

   type :: data_str_def

!=============
! single number quantities
!=============

      integer                       :: i, j 
      real(kind=8)                  :: a 

!=============
! Arrays
!=============

      real(kind=8),   allocatable   :: b(:)
      real(kind=8),   allocatable   :: c(:,:)
      real(kind=8),   allocatable   :: d(:,:,:)
      real(kind=8),   allocatable   :: e(:,:,:,:)

   end type data_str_def

!=============
! Actual data is here
!=============

   type(data_str_def), device, allocatable   :: data_str(:)

   contains

!=============
! subroutine to allocate memory
!=============

      subroutine allocate_mem(n1)
      implicit none 
      integer, intent(in)  :: n1 

      call deallocate_mem()

      write(*,*) 'works here'
      allocate(data_str(n1))

      write(*,*) 'what about allocating memory?'
      allocate(data_str(n1) % b(10))
      write(*,*) 'success!'

      return
      end subroutine allocate_mem

!=============
! subroutine to deallocate memory
!=============

      subroutine deallocate_mem()
      implicit none
      if(allocated(data_str)) deallocate(data_str)
      return 
      end subroutine deallocate_mem

   end module GPU_variables

Main program is 'gpu_test.F90', given below:

!=============
! main program 
!=============

    program gpu_test
    use gpu_variables
    implicit none

!=============
! local variables
!=============

    integer             :: i, j, n

!=============
! allocate data
!=============

    n       = 2                 ! number of data structures

    call allocate_mem(n)

!=============
! dallocate device data structures and exit
!=============

    call deallocate_mem()
    end program

Compilation command (from current folder) is:

pgfortran -Mcuda=cc5x *.F90

Terminal output:

$ ./a.out 
 works here
 what about allocating memory?
Segmentation fault (core dumped)

Any help/insight and solution would be appreciated.. and no, use of pointers is not a viable option.

Edit: another detail that may be relevant: I'm using pgfortran version 16.10

回答1:

The reason for the segmentation fault is that you have to access the memory for data_str on the host in order to allocate data_str(n1)%b. Since data_str is in device memory, not host memory, you get the segmentation fault. In theory, the compiler could create a host temp, allocate it, and then copy it to descriptor for data_str(n1)%b, but that's not part of today's CUDA Fortran.

You can work around this case by creating the temp yourself:

      subroutine allocate_mem(n1)
      implicit none
      integer, intent(in)  :: n1
      type(data_str_def) :: data_str_h

      call deallocate_mem()

      write(*,*) 'works here'
      allocate(data_str(n1))

      write(*,*) 'what about allocating memory?'
      allocate(data_str_h% b(10))
      data_str(n1) = data_str_h
      write(*,*) 'success!'

      return
      end subroutine allocate_mem

BTW, is your intention that components b, c, d, and e are allocated in host memory or device memory? I don't see the device attribute on them, so in the above, they'd go to host memory.

回答2:

So I posted this question on the PGI forums, and a guy from PGI confirms that the feature is not supported as I'm trying to use it.

http://www.pgroup.com/userforum/viewtopic.php?t=5661

His recommendation was to use the "managed" attribute or use fixed-sized arrays inside the data structure.

来源：https://stackoverflow.com/questions/45233207/allocatable-arrays-in-cuda-fortran-device-data-structures

标签

cuda

fortran

allocatable-array