Hi,
let me see that it is still not clear to me why you want to
reimplement the MPI_Gather supplied by an MPI implementation with your
own version.
You will never be able to attain the same level of performance using
point to point communication, since MPI_Gather uses internally a
binomial tree or even better advanced data structures to improve its
overall performances.
From your code snippet, I guess the problem arises because you are
not considering the extent of the datatype you are trying to receive:
using int MPI_Type_get_extent(MPI_Datatype datatype, MPI_Aint *lb,
MPI_Aint *extent );
MPI_Aint lower_bound, extent;
int rc;
rc = MPI_Type_get_extent(recvtype, &lower_bound, &extent);
if(rc == MPI_SUCCESS)
MPI_Irecv(buff + (i-1) * recvcount * extent, recvcount, recvtype, i,
0, comm, &array_request[i-1]);
or if you prefer non pointer notation
MPI_Irecv(buff[(i-1) * recvcount * extent], recvcount, recvtype, i,
0, comm, &array_request[i-1]);
In practice, the extent of a datatype should be equal to the size as
reported by sizeof(datatype).
Using MPI_Type_get_extent() is the portable way of doing this using MPI.
Regards,
Massimo
On 31/mar/09, at 12:16, Gabriele Fatigati wrote:
Mm,
OpenMPI functions like MPI_Irecv, does pointer arithmetics over recv
buffer using type info in ompi_datatype_t i suppose. I'm trying to
write a wrapper to MPI_Gather using Irecv functions:
int MPI_FT_Gather(void*sendbuf, int sendcount, MPI_Datatype sendtype,
void*recvbuff,
int recvcount, MPI_Datatype recvtype,
int root, MPI_Comm comm){
...
for( nprocs..){
MPI_Irecv(&recvbuff[(i-1)], recvcount, recvtype, i, 0, comm,
&array_request[i-1]);
}
}
where every proc sends 1 double. It doesn't work,( received values are
wrong) because MPI_Irecv are trying to do pointer arithmetic with void
values. In fact, if i write:
double*buff = &recvbuff[0];
for( nprocs..){
MPI_Irecv(&buff[(i-1)], recvcount, recvtype, i, 0, comm,
&array_request[i-1]);
}
it works well. Do you have an idea to use a portalbe way to do this?(
if is it possible)
2009/3/30 Massimo Cafaro <massimo.caf...@unisalento.it>:
Dear Gabriele,
to the best of my knowledge the MPI standard does not provide such a
function.
The reason is that when calling MPI_Gather, the standard requires
matching
type signatures (i.e., the sendcount and sendtype argument on each
of the
non root processes must match the recvcount and recvtype arguments
at the
root process). This still allows having disting type maps (type and
displacement pairs) at a sender process and at the root process,
but it is a
feature seldom used in practice, at least in my experience.
Therefore, you must know in advance the datatype you are receiving
even in
the case this datatype is a derived datatype.
If not, the likely outcome is that the receive buffer at the root
process
gets overwritten, which causes MPI_Gather to return an error.
Due to the signature of the MPI_Gather function, the only
possibility I see
to achieve what you are trying to do is to use the MPI_BYTE
datatype, and
use the communicator argument to distinguish between a collective
gather in
which you receive MPI_INT, MPI_DOUBLE etc. Of course I would not
create nor
recommend to create new communicators for this purpose only.
Kind regards,
Massimo
On 30/mar/09, at 17:43, Gabriele Fatigati wrote:
Dear OpenMPI developers,
i'm writing an MPI_Gather wrapper to collect void elements. My
queation is: is there a portable way to know the type of received
elements, like MPI_INT or MPI_DOUBLE? I've noted that i can retrieve
this information by ompi_datatype_t-> name field, but i think isn't
portable. Is there aMPI function that does this check?
--
Ing. Gabriele Fatigati
Parallel programmer
CINECA Systems & Tecnologies Department
Supercomputing Group
Via Magnanelli 6/3, Casalecchio di Reno (BO) Italy
www.cineca.it Tel: +39 051 6171722
g.fatigati [AT] cineca.it
_______________________________________________
users mailing list
us...@open-mpi.org
http://www.open-mpi.org/mailman/listinfo.cgi/users
--
*******************************************************************************************************
Massimo Cafaro, Ph.D. Additional
affiliations:
Assistant Professor National
Nanotechnology Laboratory (NNL/CNR-INFM)
Dept. of Engineering for Innovation Euro-Mediterranean
Centre for
Climate Change
University of Salento, Lecce, Italy SPACI Consortium
Via per Monteroni E-mail
massimo.caf...@unisalento.it
73100 Lecce, Italy
caf...@ieee.org
Voice +39 0832 297371
caf...@acm.org
Fax +39 0832 298173
Web http://sara.unisalento.it/~cafaro
*******************************************************************************************************
_______________________________________________
users mailing list
us...@open-mpi.org
http://www.open-mpi.org/mailman/listinfo.cgi/users
--
Ing. Gabriele Fatigati
Parallel programmer
CINECA Systems & Tecnologies Department
Supercomputing Group
Via Magnanelli 6/3, Casalecchio di Reno (BO) Italy
www.cineca.it Tel: +39 051 6171722
g.fatigati [AT] cineca.it
_______________________________________________
users mailing list
us...@open-mpi.org
http://www.open-mpi.org/mailman/listinfo.cgi/users
--
*******************************************************************************************************
Massimo Cafaro, Ph.D. Additional
affiliations:
Assistant Professor National
Nanotechnology Laboratory (NNL/CNR-INFM)
Dept. of Engineering for Innovation Euro-Mediterranean
Centre for Climate Change
University of Salento, Lecce, Italy SPACI Consortium
Via per Monteroni E-mail
massimo.caf...@unisalento.it
73100 Lecce, Italy
caf...@ieee.org
Voice +39 0832 297371
caf...@acm.org
Fax +39 0832 298173
Web http://sara.unisalento.it/~cafaro
*******************************************************************************************************
