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Subject: Re: [OMPI devel] Problem with MPI_Type_indexed and hole (defined with MPI_Type_create_resized )
From: George Bosilca (bosilca_at_[hidden])
Date: 2010-03-19 11:52:00


Pascal,

I went inside the code, and I have to say it's a long tricky story. Let me try to sort it out:

- you create two types:
  - the indexed one containing just one element. This type is contiguous as there are no holes around the data, i.e. the size and the extent of this datatype are equal.
  - the resized one. This type resize the previous one by adding a hole in the beginning, thus it is not a contiguous type, even if the memory layout is in a single piece.

Now, let's go one step up in the ROMIO code attached to your previous email. You get the content of the main type, in this example RESIZED, and them the content of the internal type which is TYPE indexed. When you look if the internal type is contiguous, Open MPI answer yes as the indexed type has its extent equal to its size. While this is true, the fact that this type is resized make it non-contiguous, as by resizing it you explicitly alter the lower bound.

The fix you proposed in your last email (i.e. modify the ADIO is contig function) is a workaround. Let me think a little bit more about this. I'll be in right here, please read below...

If I read the MPI 2-2 standard in the Chapter about the Datatypes (page 87), at the section about the MPI_Type_indexed. I have the original typemap, i.e. the one for the MPI_CHAR type (char,0). When I create the indexed type I get the typemap (char, 1). Based on the definition of lower and upper bounds on the page 100, lb is equal to 1 and ub is equal to 2, which make the extent of the indexed type equal to 1. So far so good. Now let's look what the MPI standard says about having multiple of such datatype in an array, aka MPI_Type_contiguous based on your MPI_Type_indexed. As a reminder you indexed type has the typemap (char, 1) and the extent 1. Based on the definition of MPI_Type_contiguous on page 84, the typemap of the MPI_Type_contiguous( 4, your_indexed_type) is: (char,1), (char, 2), (char, 3), (char, 4) which as far as I can say it is __contiguous__. So the Open MPI ompi_datatype_is_contigous_memory correctly returns the fact that the resulting datatype even with a count greater than 1 is contiguous.
Welcome to the fancy world of MPI datatypes.

Therefore, I think the Open MPI functions __really__ do the correct thing, and the problem is in the COMBINER_RESIZED code. As the datatype is explicitly resized by the user, you should not look if the previous type (types[0]) is contiguous or not, it doesn't matter as it was clearly resized. I wonder what the ROMIO developers had in mind for the ADIOI_Datatype_iscontig function, but it doesn't look like they just want to know if the content is contiguous. I guess this function return true if the content is contiguous AND the content start at the pointer position (displacement zero).

  george.

On Mar 19, 2010, at 06:14 , Pascal Deveze wrote:

> Hi George,
>
> I went further on my investigations, and I found a solution.
>
> ADIOI_Datatype_iscontig is defined in the file ompi/mca/io/romio/src/io_romio_module.c as:
>
> void ADIOI_Datatype_iscontig(MPI_Datatype datatype, int *flag)
> {
> /*
> * Open MPI contiguous check return true for datatype with
> * gaps in the beginning and at the end. We have to provide
> * a count of 2 in order to get these gaps taken into acount.
> */
> *flag = ompi_datatype_is_contiguous_memory_layout(datatype, 2);
> }
>
> It is clearly written here that the gaps should be taken into account with a count of 2. But that's not everytime the case.
>
> Your proposition is to modify ROMIO code.
> So, I propose to fix ADIOI_Datatype_iscontig and add the following code after the call
> to ompi_datatype_is_contiguous_memory_layout():
>
> if (*flag) {
> MPI_Aint true_extent, true_lb;
>
> ompi_datatype_get_true_extent(datatype, &true_lb, &true_extent);
>
> if (true_lb > 0)
> *flag = 0;
> }
>
> Regards,
>
> Pascal
>
> On Mar 18, 2010, at 13:24, George Bosilca wrote:
>> We will disagree on that, but your datatype is contiguous. It doesn't matter that there are gaps in the beginning and at the end, as long as you only send one such datatype the real data that has to go over the network _is_ contiguous. And this is what the Open MPI datatype engine is reporting back.
>>
>> Apparently, ROMIO expect a contiguous datatype to start from the position 0 relative to the beginning of the user buffer. I don't see why they have such a restrictive view, but I guess the original MPICH datatype engine was not able to distinguish between gaps in the middle and gaps at the beginning and the end of the datatype.
>>
>> I don't see how to fix that in ROMIO code. But in case you plan to fix it, the correct solution is to retrieve the true lower bound of the datatype in the contiguous case and add it to st_offset.
>>
>> george.
>>
>> On Mar 18, 2010, at 12:27 , Pascal Deveze wrote:
>>
>>> Hi all,
>>>
>>> Sorry, I missed my porting from MPICH2 to OpenMPI concerning the file
>> romio/adio/comm/flatten.c
>>> (flatten.c in OpenMPI does not support MPI_COMBINER_RESIZED).
>>>
>>> Here is the diff:
>>>
>>> diff -u flatten.c flatten.c.old
>>> --- flatten.c 2010-03-18 17:07:43.000000000 +0100
>>> +++ flatten.c.old 2010-03-18 17:14:04.000000000 +0100
>>> @@ -525,44 +525,6 @@
>>> }
>>> break;
>>> - case MPI_COMBINER_RESIZED:
>>> - /* This is done similar to a type_struct with an lb, datatype, ub */
>>> -
>>> - /* handle the Lb */
>>> - j = *curr_index;
>>> - flat->indices[j] = st_offset + adds[0];
>>> - flat->blocklens[j] = 0;
>>> -
>>> - (*curr_index)++;
>>> -
>>> - /* handle the datatype */
>>> -
>>> - MPI_Type_get_envelope(types[0], &old_nints, &old_nadds,
>>> - &old_ntypes, &old_combiner);
>>> - ADIOI_Datatype_iscontig(types[0], &old_is_contig);
>>> -
>>> - if ((old_combiner != MPI_COMBINER_NAMED) && (!old_is_contig)) {
>>> - ADIOI_Flatten(types[0], flat, st_offset+adds[0], curr_index);
>>> - }
>>> - else {
>>> - /* current type is basic or contiguous */
>>> - j = *curr_index;
>>> - flat->indices[j] = st_offset;
>>> - MPI_Type_size(types[0], (int*)&old_size);
>>> - flat->blocklens[j] = old_size;
>>> -
>>> - (*curr_index)++;
>>> - }
>>> -
>>> - /* take care of the extent as a UB */
>>> - j = *curr_index;
>>> - flat->indices[j] = st_offset + adds[0] + adds[1];
>>> - flat->blocklens[j] = 0;
>>> -
>>> - (*curr_index)++;
>>> -
>>> - break;
>>> -
>>> default:
>>> /* TODO: FIXME (requires changing prototypes to return errors...) */
>>> FPRINTF(stderr, "Error: Unsupported datatype passed to ADIOI_Flatten\n");
>>> @@ -827,29 +789,6 @@
>>> }
>>> }
>>> break;
>>> -
>>> - case MPI_COMBINER_RESIZED:
>>> - /* treat it as a struct with lb, type, ub */
>>> -
>>> - /* add 2 for lb and ub */
>>> - (*curr_index) += 2;
>>> - count += 2;
>>> -
>>> - /* add for datatype */
>>> - MPI_Type_get_envelope(types[0], &old_nints, &old_nadds,
>>> - &old_ntypes, &old_combiner);
>>> - ADIOI_Datatype_iscontig(types[0], &old_is_contig);
>>> -
>>> - if ((old_combiner != MPI_COMBINER_NAMED) && (!old_is_contig)) {
>>> - count += ADIOI_Count_contiguous_blocks(types[0], curr_index);
>>> - }
>>> - else {
>>> - /* basic or contiguous type */
>>> - count++;
>>> - (*curr_index)++;
>>> - }
>>> - break;
>>> -
>>> default:
>>> /* TODO: FIXME */
>>> FPRINTF(stderr, "Error: Unsupported datatype passed to
>> ADIOI_Count_contiguous_blocks, combiner = %d\n", combiner);
>>>
>>>
>>> Regards,
>>>
>>> Pascal
>>>
>>> Pascal Deveze a écrit :
>>> > Hi all,
>>> >
>>> > I use a very simple datatype defined as follow:
>>> > lng[0]= 1;
>>> > dsp[0]= 1;
>>> > err=MPI_Type_indexed(1, lng, dsp, MPI_CHAR, &offtype);
>>> > err=MPI_Type_create_resized(offtype, 0, 2, &filetype);
>>> > MPI_Type_commit(&filetype);
>>> >
>>> > This datatype consists of a hole (of length 1 char) followed by a char.
>>> >
>>> > The datatype with hole at the beginning is not correctly handled by
>> ROMIO integrated in OpenMPI (I tried with MPICH2 and it worked fine).
>>> > You will see bellow a program to reproduce the problem.
>>> >
>>> > After investigations, I see that the difference between OpenMPI and
>> MPICH appears at line 542 in the file romio/adio/comm/flatten.c:
>>> >
>>> > case MPI_COMBINER_RESIZED:
>>> > /* This is done similar to a type_struct with an lb, datatype, ub */
>>> >
>>> > /* handle the Lb */
>>> > j = *curr_index;
>>> > flat->indices[j] = st_offset + adds[0];
>>> > flat->blocklens[j] = 0;
>>> >
>>> > (*curr_index)++;
>>> >
>>> > /* handle the datatype */
>>> >
>>> > MPI_Type_get_envelope(types[0], &old_nints, &old_nadds,
>>> > &old_ntypes, &old_combiner);
>>> > ADIOI_Datatype_iscontig(types[0], &old_is_contig); <========== ligne 542
>>> >
>>> > For MPICH2, the datatype is not contiguous, but it is for OpenMPI.
>> The routine ADIOI_Datatype_iscontig is
>>> > quite different in OpenMPI because the datatypes are handled very
>> differently. If I reset old_is_contig just after
>>> > line 542, the problem disappears (Of course, this is not a solution).
>>> >
>>> > I am not able to propose a right solution. Can somebody help ?
>>> >
>>> > Pascal
>>> >
>>> > ============ Program to reproduce the problem ========
>>> > #include <stdio.h>
>>> > #include "mpi.h"
>>> >
>>> > char filename[256]="VIEW_TEST";
>>> > char buffer[100];
>>> > int err, i, myid, dsp[3], lng[3];
>>> > MPI_Status status;
>>> > MPI_File fh;
>>> > MPI_Datatype filetype, offtype;
>>> > MPI_Aint lb, extent;
>>> >
>>> > int main(int argc, char **argv) {
>>> >
>>> > MPI_Init(&argc, &argv);
>>> > MPI_Comm_rank(MPI_COMM_WORLD, &myid);
>>> > for (i=0; i<sizeof(buffer); i++) buffer[i] = i;
>>> >
>>> > if (!myid) {
>>> > MPI_File_open(MPI_COMM_SELF, filename, MPI_MODE_CREATE |
>> MPI_MODE_RDWR , MPI_INFO_NULL, &fh);
>>> > MPI_File_write(fh, buffer, sizeof(buffer), MPI_CHAR, &status);
>>> > MPI_File_close(&fh);
>>> >
>>> > lng[0]= 1;
>>> > dsp[0]= 1;
>>> > MPI_Type_indexed(1, lng, dsp, MPI_CHAR, &offtype);
>>> > MPI_Type_create_resized(offtype, 0, 2, &filetype);
>>> > MPI_Type_commit(&filetype);
>>> >
>>> > MPI_File_open(MPI_COMM_SELF, filename, MPI_MODE_RDONLY ,
>> MPI_INFO_NULL, &fh);
>>> > MPI_File_set_view(fh, 0, MPI_CHAR, filetype,"native", MPI_INFO_NULL);
>>> > MPI_File_read(fh, buffer, 5, MPI_CHAR, &status);
>>> >
>>> > printf("Data: ");
>>> > for (i=0 ; i<5 ; i++) printf(" %x ", buffer[i]);
>>> > if (buffer[1] != 3) printf("\n =======> test KO : buffer[1]=%d
>> instead of %d \n", buffer[1], 4);
>>> > else printf("\n =======> test OK\n");
>>> > MPI_Type_free(&filetype);
>>> > MPI_File_close(&fh);
>>> > }
>>> > MPI_Barrier(MPI_COMM_WORLD);
>>> > MPI_Finalize();
>>> > }
>>> > ============ The result of the program with MPICH2 ========
>>> > Data: 1 3 5 7 9
>>> > =======> test OK
>>> >
>>> > ============ The result of the program with OpenMPI ========
>>> > Data: 0 2 4 6 8
>>> > =======> test KO : buffer[1]=2 instead of 4
>>> >
>>> > Comment: Only the first hole is ommited.
>>> >
>>> >
>>> >
>>> >
>>>
>>
>>
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