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Subject: Re: [OMPI users] Bug MPI_Iscatter
From: Pierre Jolivet (jolivet_at_[hidden])
Date: 2013-11-22 19:18:43


George,

On Nov 22, 2013, at 5:21 AM, George Bosilca <bosilca_at_[hidden]> wrote:

> Pierre,
>
> On Nov 22, 2013, at 02:39 , Pierre Jolivet <jolivet_at_[hidden]> wrote:
>
>> 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 ?
>
> Because it is a strong requirement of the MPI standard: the typemap of a send should be matched by its corresponding receive. Otherwise, it is legal to raise an exception of type MPI_ERR_TYPE.
>
>> 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.
>
> Different algorithms implemented by different people. Some of them are more robust, while others less. In this case Scatter translate the count = 0 to a message length = 0, while the Iscatter always look for the extent of the datatype.
>
>> I don’t see any remark on that matter on the MPI 3.0 documentation.
>
> Indeed, and there is at least one example where MPI_DATATYPE_NULL is explicitly used for calls where the datatype does not matter (4.23 as an example). Horrible!!!

That’s what I don’t get, why are you saying it’s horrible ? It is clearly written in the spec. that the data type is only significant at root (for Scatter), and that Iscatter is nothing else than a nonblocking variant of Scatter (so the value should also be significant only at root).

Moreover, there is at least one thing that is wrong in the sources:
1) in ompi/mca/coll/libnbc/nbc_igather.c, line 55 should read:
  if (MPI_SUCCESS != res) { printf("MPI Error in MPI_Comm_size() (%i)\n", res); return res; }
instead of:
  if (MPI_SUCCESS != res) { printf("MPI Error in MPI_Comm_rank() (%i)\n", res); return res; }

And I still have a hard time believing that the test line 56 in ompi/mca/coll/libnbc/nbc_igather.c — if (rank == root) — is not missing in ompi/mca/coll/libnbc/nbc_iscatter.c line 58, but I guess I will have to trust you on this one.

You should probably specify somewhere that you differ from the standard for that function, other MPI implementations don’t have this limitation, c.f.http://pic.dhe.ibm.com/infocenter/zos/v1r12/index.jsp?topic=%2Fcom.ibm.zos.r12.fomp200%2Fipezps0025.htm or http://trac.mpich.org/projects/mpich/browser/src/mpi/coll/iscatter.c#L601

Pierre

> George.
>
>>
>> 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 <bosilca_at_[hidden]> 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 <jolivet_at_[hidden]> 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
>>>> users_at_[hidden]
>>>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>>>
>>> _______________________________________________
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>>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>>
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>
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