Sorry for the delay on this. It slipped by me in the ever growing
pile of things.
On Mar 12, 2006, at 2:05 AM, Soumya,Teena, Ranjana,Liss,Navya wrote:
> Hi
>
> We are working on the earlier problem. In the meantime, we have a
> few doubts regarding MPI_Alltoall.
> int MPI_Alltoall(void *sbuf, int scount, MPI_Datatype sdtype, void*
> rbuf, int rcount, MPI_Datatype rdtype, MPI_Comm comm)
>
>
>
> Does MPI_Alltoall help in acheiving synchronization between the
> processes?
>
How exactly do you mean? All of the processes in the communication
group ('comm') have to call this function in order for it to complete
in any process.
> In MPI_Alltoall, what does the receivecount stand for?----->
> maximum number of values to be got from each other process or exact
> number of the same? Should the amount of data to be received from
> each other process be the same?
>
MPI standard states "recvcount = number of elements received from any
process (integer)". The 'sendcount' from the sending process must
match the 'recvcount' on the receiving process. The receive buffer
need only be the size of 'recvcount'.
> How are data from the other processes organised in the receive
> buffer? Is it possible to retrieve data that has been received from
> say process 'n' from the receivebuffer?
>
They are organized by rank in the communication group. So if:
Rank 0 sends:
A A A A A A
Rank 1 sends:
B B B B B B
Rank 2 sends:
C C C C C C
All three ranks call:
MPI_Alltoall( send_buffer, 2, MPI_CHAR,
recv_buffer, 2, MPI_CHAR,
MPI_COMM_WORLD);
Where send_buffer and recv_buffer are both 6 characters long.
Then Rank 0,1,2 all receive:
A A B B C C
For more details on the standard specification for MPI_ALLTOALL[V]
read through the MPI1.1 specification which can be found linked from
the MPI Forum website, here:
http://www.mpi-forum.org/docs/docs.html
-- Josh
> Can somebody please throw some light on them as soon as possible?
>
> Thanks
>
> MEC
> --
> CE 2002-06
> _______________________________________________
> This list is archived at http://www.lam-mpi.org/MailArchives/lam/
----
Josh Hursey
jjhursey_at_[hidden]
http://www.lam-mpi.org/
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