MPI的数据类型适用于花车，而不是双打。它是一个对齐问题？ [英] MPI derived datatype works for floats, but not for doubles. Is it an alignment issue?

问题描述

我必须与正在沟通与MPI的数据类型的帮助下，C-结构，一个奇怪的问题。以下作品中的例子。它简单的发送由一个整数加4 浮动值的消息。

I have a weird issue related to a C-structure that is communicated with the help of an MPI derived datatype. The example below works; it simply sends a message consisting of one integer plus 4 float values.

Minmum的例子：

#include <mpi.h>
#include <stdio.h>

int main(int argc, char *argv[]) {
    MPI_Init(&argc, &argv);

    int i, rank, tag = 1;
    MPI_Status status;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    // Array of doubles plus element count
    typedef struct {
        int row;
        float elements[4];
    } My_array;

    // Derived datatype for an array of doubles plus element count
    MPI_Datatype MY_ARRAY_TYPE;
    const int nr_blocks = 2;
    int blocklengths[2] = {1, 4};
    MPI_Datatype oldtypes[2] = {MPI_INT, MPI_FLOAT};
    MPI_Aint extent, lb;
    MPI_Type_get_extent(MPI_INT, &lb, &extent);
    MPI_Aint displacements[2] = {0, extent};
    MPI_Type_create_struct(nr_blocks, blocklengths, displacements,
                       oldtypes, &MY_ARRAY_TYPE);
    MPI_Type_commit(&MY_ARRAY_TYPE);

   if(rank == 0) {
        My_array array1  = {3, 3.1, 3.2, 3.3, 3.4};
        MPI_Send(&array1, 1, MY_ARRAY_TYPE, 1, tag, MPI_COMM_WORLD);
   }
   if(rank == 1) {
        My_array array2;
        MPI_Recv(&array2, 1, MY_ARRAY_TYPE, 0, tag, MPI_COMM_WORLD, &status);
        printf("Rank %d received elements of row %d:\n", rank, array2.row);
        for(i = 0; i < 4; i++)
            printf("\t%.1f\n", array2.elements[i]);
    }
    MPI_Type_free(&MY_ARRAY_TYPE);
    MPI_Finalize();
}

如果你有机会到MPI安装，例如可以通过 mpicc -o例如example.c 编译和的mpirun -np运行例如2 。
输出应该是

If you have access to an MPI installation, the example can be compiled by mpicc -o example example.c and run by mpirun -np 2 example. The output should be

Rank 1 received elements of row 3:
    3.1
    3.2
    3.3
    3.4

问题：
现在，当浮动的阵s被改为数组双击 S，并据此 MPI_FLOAT 到 MPI_DOUBLE ，我得到一个错误的结果。

The problem: Now when the array of floats is changed to an array of doubles, and accordingly MPI_FLOATto MPI_DOUBLE, I get a wrong result.

这code：

#include <mpi.h>
#include <stdio.h>

int main(int argc, char *argv[]) {
    MPI_Init(&argc, &argv);

    int i, rank, tag = 1;
    MPI_Status status;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    // Array of doubles plus element count
    typedef struct {
        int row;
        double elements[4];
    } My_array;

    // Derived datatype for an array of doubles plus element count
    MPI_Datatype MY_ARRAY_TYPE;
    const int nr_blocks = 2;
    int blocklengths[2] = {1, 4};
    MPI_Datatype oldtypes[2] = {MPI_INT, MPI_DOUBLE};
    MPI_Aint extent, lb;
    MPI_Type_get_extent(MPI_INT, &lb, &extent);
    MPI_Aint displacements[2] = {0, extent};
    MPI_Type_create_struct(nr_blocks, blocklengths, displacements,
                       oldtypes, &MY_ARRAY_TYPE);
    MPI_Type_commit(&MY_ARRAY_TYPE);

   if(rank == 0) {
        My_array array1  = {3, 3.1, 3.2, 3.3, 3.4};
        MPI_Send(&array1, 1, MY_ARRAY_TYPE, 1, tag, MPI_COMM_WORLD);
   }
   if(rank == 1) {
        My_array array2;
        MPI_Recv(&array2, 1, MY_ARRAY_TYPE, 0, tag, MPI_COMM_WORLD, &status);
        printf("Rank %d received elements of row %d:\n", rank, array2.row);
        for(i = 0; i < 4; i++)
            printf("\t%.1f\n", array2.elements[i]);
    }
    MPI_Type_free(&MY_ARRAY_TYPE);
    MPI_Finalize();
}

生产：

Rank 1 received elements of row 3:
    3.1
    3.2
    3.3
    0.0

我尝试了一下周围，在结构和派生数据类型使用其它数据（例如，整数而不是一个数组， INT / MPI_INT 而不是浮动 / MPI_FLOAT 等），并看到问题使用时，双打只出现。这让我怀疑这可能是各种各样的对齐问题 - 但我卡在那里。 MPI应自动照顾比对。

I tried around a bit, using other data in the struct and the derived datatype (e.g., an array of integers instead of just one, int/MPI_INT instead of float/MPI_FLOAT, etc.) and saw that the problem arises only when doubles are used. Which makes me suspect that this might be an alignment issue of sorts - but I am stuck there. MPI should take care of alignments automatically.

问：为什么用浮动 / MPI_FLOAT 上面的例子中的工作，但与双击 / MPI_DOUBLE ，我该如何解决这个问题？

Question: Why does the above example work with float/MPI_FLOAT, but not with double/MPI_DOUBLE and how can I fix it?

有些机细节，可能是相关的：

Some machine specifics that might be relevant:

• CPU：AMD皓龙6134


• 地址尺寸：48位


• 对齐：64


• 编译器：GCC 4.4.7


• MPI库：（可惜）厂商特定

编辑：在弗拉基米尔·F中的意见建议，我增加了code不起作用

as suggested in the comments by Vladimir F, I added the code that does not work.

推荐答案

我只是发现了问题所在：它确实是对齐。第二code上市产生正确的第3双打也不过是一个奇怪的巧合...通过使用 MPI_INT 为以下值的偏移量的扩大，我认为不会有什么填充。这是更好地计算这样的偏移量：

I just found out what the problem is: it is indeed alignment. That the second code listing produces the first 3 doubles correctly is nothing but a weird coincidence...By using the extend of MPI_INT as the offset of the following value, I assumed that there would be no padding. It is better to compute the offsets like this:

#include <stddef.c> 
...
MPI_Datatype MY_ARRAY_TYPE;
const int nr_blocks = 2;
int blocklengths[2] = {1, 4};
MPI_Datatype oldtypes[2] = {MPI_INT, MPI_DOUBLE};
MPI_Aint displacements[2];
displacements[0] = offsetof(My_array, row);
displacements[1] = offsetof(My_array, elements);
MPI_Type_create_struct(nr_blocks, blocklengths, displacements,
                   oldtypes, &MY_ARRAY_TYPE);
MPI_Type_commit(&MY_ARRAY_TYPE);
...

我真的有兴趣看看它是如何制定出这样......为什么我们拿到3正确的价值观和一个0.0？由于取向是关闭的4个字节和双打重新由8个字节我的平台，为什么我没有得到一些随机数上psented $ P $？如何前3已经去codeD正确，如果他们每人拿了一张双人床再加上接下来的双重的上4下4个字节？

I'd really be interested to see how it can work out this way...why do we get 3 correct values and one 0.0? Since the alignment was off by 4 bytes and doubles are represented by 8 bytes on my platform, why didn't I get some random numbers? How can the first 3 have been decoded correctly if they each took the lower 4 bytes of one double plus the upper 4 of the next double?

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