Scattering context options

[Graham Kells <gkells@xxxxxxxxxxxxxx>]

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