George,
I completely agree that the code I sent was a good example of what NOT to do
with collective and non-blocking communications, so I’m sending a better one.
1. I’m setting MPI_DATATYPE_NULL only on non-root processes. The root has a
real datatype. Why should both match when using MPI_IN_PLACE ?
2-3-4. Yes, all these points are valid, this is of course just a minimalist
example.
My question is, if you are indeed saying that it is not a OpenMPI bug, what is
the rationale for changing the behavior between MPI_Scatter and MPI_Iscatter
when it comes down to the send type on non-root processes.
I don’t see any remark on that matter on the MPI 3.0 documentation.
Thanks.
#include <mpi.h>
int main(int argc, char** argv) {
int taskid, ntasks;
MPI_Init(&argc, &argv);
MPI_Request rq;
MPI_Comm_rank(MPI_COMM_WORLD,&taskid);
MPI_Comm_size(MPI_COMM_WORLD,&ntasks);
double* r;
int l = 0;
// This will run fine. MPI_DOUBLE
if(taskid > 0)
MPI_Iscatter(NULL, 0, MPI_DOUBLE, r, l, MPI_DOUBLE, 0, MPI_COMM_WORLD,
&rq);
else
MPI_Iscatter(r, l, MPI_DOUBLE, MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, 0,
MPI_COMM_WORLD, &rq);
MPI_Wait(&rq, MPI_STATUS_IGNORE);
// This will run fine. MPI_DATATYPE_NULL
if(taskid > 0)
MPI_Scatter(NULL, 0, MPI_DATATYPE_NULL, r, l, MPI_DOUBLE, 0,
MPI_COMM_WORLD);
else
MPI_Scatter(r, l, MPI_DOUBLE, MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, 0,
MPI_COMM_WORLD);
// This will not run fine. MPI_DATATYPE_NULL
if(taskid > 0)
MPI_Iscatter(NULL, 0, MPI_DATATYPE_NULL, r, l, MPI_DOUBLE, 0,
MPI_COMM_WORLD, &rq);
else
MPI_Iscatter(r, l, MPI_DOUBLE, MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, 0,
MPI_COMM_WORLD, &rq);
MPI_Wait(&rq, MPI_STATUS_IGNORE);
MPI_Finalize();
}
On Nov 21, 2013, at 4:34 PM, George Bosilca <bosi...@icl.utk.edu> wrote:
> Pierre,
> There are several issues with the code you provided.
>
> 1. You can’t use an MPI_DATATYPE_NULL as the send datatype, not even when
> count is zero. At least the root must provide a real datatype. In fact the
> type signature of the send message (datatype and count) should match the type
> signature of the receiving datatype.
>
> 2. I know your count is zero, and no data will be transmitted but your code
> is difficult to read and understand.
>
> 3. MPI_Iscatter is a collective communication. As such all processes in the
> associated communicator (MPI_COMM_WORLD in your case) must participate to the
> collective. Thus, calling MPI_Iscatter only where tasked > 0 is incorrect
> (you explicitly excluded 0).
>
> 4. From the MPI standard perspective your example is not correct, as you are
> not allowed to call MPI_Finalize while there are messages pending. Now, Open
> MPI tolerate this, but it is clearly not standard behavior.
>
> #include <mpi.h>
>
> int main(int argc, char** argv)
> {
> int taskid, ntasks;
> MPI_Init(&argc, &argv);
> MPI_Request rq;
> MPI_Comm_rank(MPI_COMM_WORLD,&taskid);
> MPI_Comm_size(MPI_COMM_WORLD,&ntasks);
> double r;
> int l = 0;
>
> MPI_Iscatter(NULL, 0, MPI_DOUBLE, &r, l, MPI_DOUBLE, 0, MPI_COMM_WORLD,
> &rq);
> MPI_Wait(&rq, MPI_STATUS_IGNORE);
>
> MPI_Finalize();
> }
>
> George.
>
>
> On Nov 21, 2013, at 23:19 , Pierre Jolivet <joli...@ann.jussieu.fr> wrote:
>
>> Hello,
>> The following code doesn’t execute properly :
>> #include <mpi.h>
>>
>> int main(int argc, char** argv) {
>> int taskid, ntasks;
>> MPI_Init(&argc, &argv);
>> MPI_Request rq;
>>
>> MPI_Comm_rank(MPI_COMM_WORLD,&taskid);
>> MPI_Comm_size(MPI_COMM_WORLD,&ntasks);
>> double* r;
>> int l = 0;
>> if(taskid > 0)
>> MPI_Iscatter(NULL, 0, MPI_DATATYPE_NULL, r, l, MPI_DOUBLE, 0,
>> MPI_COMM_WORLD, &rq);
>> MPI_Finalize();
>> }
>>
>> Outcome:
>> *** An error occurred in MPI_Type_extent
>> *** MPI_ERR_TYPE: invalid datatype
>>
>> Hotfix: change MPI_DATATYPE_NULL to something else.
>>
>> Thanks for a quick fix.
>> _______________________________________________
>> users mailing list
>> us...@open-mpi.org
>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>
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