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atomic min/max for real data
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mkcolg

Joined: 30 Jun 2004
Posts: 4996
Location: The Portland Group Inc.

Posted: Thu Feb 11, 2010 5:20 pm Post subject:

Hi Tuan,

For arrays, you want to use the reduction intrinsic "minval". Something like:

Code:

% cat minreduc.cuf

module minutil

real, device :: dMinval
real, device :: dMaxval

contains

attributes(global) subroutine foo (Ad,N)
use cudafor
implicit none

integer, value :: N
real, device, dimension(N,N) :: Ad
integer i, j, tx, ty

tx = threadidx%x
ty = threadidx%y
i = (blockidx%x-1)*16 + tx
j = (blockidx%y-1)*16 + ty

Ad(i,j) = (N*(i-1))+(j-1)

call syncthreads()
if (i .eq. 1 .and. j .eq. 1 ) then
dMinval = minval(Ad)
dMaxval = maxval(Ad)
endif
call syncthreads()

end subroutine foo

subroutine testmin ()
use cudafor
implicit none
integer :: N = 64
real, dimension(N,N) :: A
real, device, dimension(N,N) :: Ad
real :: minval, maxval
type(dim3) :: dimGrid, dimBlock
A=-1
dMinval = -1
dimGrid=dim3(N/16,N/16,1)
dimBlock=dim3(16,16,1)
Ad=0
call foo<<<dimGrid,dimBlock>>>(Ad,N)
A=Ad
minval = dMinval
maxval = dMaxval
print *, minval, maxval
print *, A(1,1), A(N,N)
end subroutine testmin

end module minutil

program testme

use minutil

call testmin

end program testme

% pgfortran -o minreduc.out minreduc.cuf
% minreduc.out
0.000000 4095.000
0.000000 4095.000

Tuan

Joined: 11 Jun 2009
Posts: 226

Posted: Wed Feb 17, 2010 4:47 pm Post subject:

mkcolg wrote:

Hi Tuan,

For arrays, you want to use the reduction intrinsic "minval". Something like:

Code:

Hi Mat,
Is minval support double precision also, the document just say it supports real, so I'm not sure if this includes double precision?
Please explain me the difference of using min and minval? Does min cannot be used on GPU?

Thanks,
Tuan

mkcolg

Joined: 30 Jun 2004
Posts: 4996
Location: The Portland Group Inc.

Posted: Wed Feb 17, 2010 8:14 pm Post subject:

Hi Tuan,

Quote:

Is minval support double precision also, the document just say it supports real, so I'm not sure if this includes double precision?

Yes, minval supports double precision. When they doc say 'real', they mean both kinds.

Quote:

Please explain me the difference of using min and minval?

min determines which of two or more scalar values is the minimum values. minval finds the minimum value of an array.

Code:

Does min cannot be used on GPU?

min can be used on the GPU. Though, your code is trying to find the minimum value of an array, hence the use of minval.

- Mat

Tuan

Joined: 11 Jun 2009
Posts: 226

Posted: Wed Feb 17, 2010 8:34 pm Post subject:

mkcolg wrote:

Hi Tuan,

Quote:

Is minval support double precision also, the document just say it supports real, so I'm not sure if this includes double precision?

Yes, minval supports double precision. When they doc say 'real', they mean both kinds.

Quote:

Please explain me the difference of using min and minval?

min determines which of two or more scalar values is the minimum values. minval finds the minimum value of an array.

Code:

Does min cannot be used on GPU?

min can be used on the GPU. Though, your code is trying to find the minimum value of an array, hence the use of minval.

- Mat

All clear. Thank Mat.

Tuan

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