Scattering context options

[Graham Kells]

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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
>>
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