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Re: Scattering context options


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Ok, thanks to you both. It looks like I will have to rethink this one :-/

Regards,
Graham

On Fri, 27 Jul 2007, Barry Smith wrote:


  Graham,

  It seems unlike the matrix was preallocated; this will
definitely slow things down a great deal. Pluse you have a
scatter for each row, this is fundamentally not scalable.

 Barry


On Fri, 27 Jul 2007, Graham Kells wrote:

Hi ,

I'm scattering from a global vector (used as a kind of  look up table) to a
bunch of local vectors and then using the values in the local vectors as
indices to populate a global matrix.
The basic population routine goes something like this

 do myrow=Istart,Iend-1

     ! Returns column indices of full matrix for rowindex list(myrow+1)
   call getrow(list(myrow+1),rowCOO,colCOO,valCOO)
       call ISCreateGeneral(PETSC_COMM_WORLD,n,(colCOO-1),from,ierr)
   call ISCreateGeneral(PETSC_COMM_SELF,n,idx_to-1,to,ierr)    call
VecScatterCreate(vlist,from,outvec,to,scatter,ierr)
   call
VecScatterBegin(scatter,vlist,outvec,INSERT_VALUES,SCATTER_FORWARD,ierr)
   call VecScatterEnd(scatter,vlist,outvec,INSERT_VALUES,SCATTER_FORWARD,ierr)
   call VecScatterDestroy(scatter,ierr)
   call ISDestroy(from,ierr)
   call ISDestroy(to,ierr)

   call VecGetValues(outvec,n,idx_to-1,idx_real,ierr)
   idx_from=int(idx_real)


   call MatSetValues(A,1,myrow,n,idx_from-1,valCOO,INSERT_VALUES,ierr)

end do

While this works, it is prohibitively slow. Any ideas on why this is?  Of
course if you can suggest a better way of doing this that would be great.

Supposing you can't and I want to experiment with different MPI communication
modes within the scatter context. How do specify the  MPI_Ssend option, for
example?

Thanks in advance,

Graham