MPI_Reduce选择前k个结果(MPI_Reduce select first k result

我想找到了使用MPI的所有节点的前k个结果。为此，我想用MPI_Reduce与自身的功能。但是我的代码不能正常工作，因为功能的LEN参数是不一样的给予MPI_Reduce计数参数。
我发现这里是实现可以做到这一点，以流水线的计算。

我的代码是类似于此：

inline void MPI_user_select_top_k(int *invec, acctbal_pair *inoutvec, int *len, MPI_Datatype *dtpr) {
    std::vector<acctbal_pair> temp;
    for(int i = 0; i < *len; ++i) {
        acctbal_pair p1 = {invec[i].suppkey, invec[i].acctbal};
        acctbal_pair p2 = {inoutvec[i].suppkey, inoutvec[i].acctbal};
        temp.push_back(p1);
        temp.push_back(p2);
    }
    std::sort(temp.begin(), temp.end(), [&](acctbal_pair a, acctbal_pair b) { return a.acctbal > b.acctbal;});
    for(int i = 0; i < *len; ++i) {
        inoutvec[i].suppkey = temp[i].suppkey;
        inoutvec[i].acctbal = temp[i].acctbal;
    }
}

凡acctbal_pair与领域suppkey和acctbal一个struct
我叫MPI_Reduce这样。其中localResults和globalResults是大小为k的向量。

MPI_Reduce(localResults.data(), globalResults.data(), k, mpi_suppkey_acctbal, select_top_k, ROOT, MPI_COMM_WORLD);

然而，对于稍大k的值，则计数被分裂成更小的块，让我的功能失效。

有没有办法告诉没有降低到管道的计算？或者你知道另一个（高效）的方式来实现这一点？我真的不希望使用MPI_Gather并找到根的前k个结果，因为大的通信开销。

我不能只创建具有固定参数k的函数（和治疗所有k个元素作为1个MPI_type）为k在运行时计算。

我知道，这是不是MPI_Reduce的目的（这应该只是计算一些操作元素方面），但这个工程完美，如果计数不会分块。

PS：我的MPI实现是的openmpi

当然，你可以做到这一点 - 你只需要创建一个类型的大小k（这是很容易在运行时做），做选择。唯一的技巧是，你没有办法通过状态（例如，K）的选择操作，所以你需要通过一个全局变量进行沟通 - 这显然不是很大，但一做什么人需要做的。如果您需要以不同的大小k的重复运行的算法，一个只可根据需要创建类型和复位全局变量。

（您可避免全局变量，如果你潜入k的值转换为选择操作一些其他的方式-比如，传递给它的每个阵列中的数据的第一个元素是值k ）。

下面是一些代码，做这一点; 它允许其中处理器具有小于k个值的情况。每个处理器选择其ķ最小值和充塞他们本地阵列中，然后选择操作做了局部排序合并操作摘掉正好k个最低元件。

#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
#include <limits.h>
#include <math.h>

const int invalid_data = -1;
static int globalselectk;        /* yuk */

int min2(int a, int b) {
    if (b < a)
        return b;
    return a;
}

MPI_Datatype createtype(int selectk) {

    MPI_Datatype selectktype;
    MPI_Type_contiguous(selectk, MPI_INT, &selectktype);
    MPI_Type_commit(&selectktype);

    return selectktype;
}

void initselectk(int *d, size_t selectk) {
    for (int i=0; i<selectk; i++)
        d[i] = invalid_data;
}

void printselectk(int *d, size_t selectk) {
    printf("[");
    for (int i=0; i<selectk; i++)
        printf("%3d ",d[i]);
    printf("] ");
}

int countselectk(int *d, size_t selectk) {
    int count = 0;

    while ( (d[count] != invalid_data) && (count < selectk) )
        count++;

    return count;
}
int mergeselect(int *d1, int *d2, int *dout, size_t selectk) {
    int count1 = countselectk(d1, selectk);
    int count2 = countselectk(d2, selectk);

    int count = 0;
    int total = count1+count2;
    if (total >= selectk) total = selectk;

    int idx1=0, idx2=0;

    while (count < total) {
        int minloc = -1;
        int minval = INT_MAX;
        if (idx1 < count1) {
            minloc = 1;
            minval = d1[idx1];
        }
        if ( (idx2 < count2) && (d2[idx2] < minval ) ) {
            minloc = 2;
            minval = d2[idx2];
        }
        dout[count++] = minval;
        if (minloc == 1)
            idx1++;
        else
            idx2++;
    }
    return count;
}

void selectop(void *in, void *inout, int *len, MPI_Datatype *type) {
    int *invals = (int *)in;
    int *inoutvals = (int *)inout;

    int out[globalselectk];

    for (int i=0; i<*len; i++) {
        initselectk(out, globalselectk);
        int count = mergeselect(invals, inoutvals, out, globalselectk);

        for (int j=0; j<count; j++)
            inoutvals[j] = out[j];

        invals += globalselectk;
        inoutvals += globalselectk;
    }

    return;
}

int intcompar(const void *v1, const void *v2) {
    int *i1 = (int *)v1;
    int *i2 = (int *)v2;

    return (*i1 - *i2);
}

int main(int argc, char **argv) {

    int rank, size;
    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    if (argc < 3) {
        fprintf(stderr,"Usage: %s localn k-to-select\n", argv[0]);
        MPI_Abort(MPI_COMM_WORLD,1);
    }

    int locn    = atoi(argv[1]);
    int selectk = atoi(argv[2]);
    globalselectk = selectk;     /* yuk */

    int localdata[locn];
    int local[selectk], global[selectk];

    /* create our new data type */
    MPI_Datatype mpi_datatype = createtype(selectk);

    MPI_Op mpi_selectop;
    MPI_Op_create(selectop, 1, &mpi_selectop);

    srand(rank*37);
    for (int i=0; i<locn; i++)
        localdata[i] = floor(500.*rand()/RAND_MAX);
    /* get our local k selected */
    /* could use quickselect for this, but to focus on the MPI, let's just sort */

    initselectk(local, selectk);
    qsort(localdata, locn, sizeof(int), intcompar);
    for (int i=0; i<min2(selectk,locn); i++)
        local[i] = localdata[i];

    for (int proc=0; proc<size; proc++) {
        if (rank == proc) {
            printf("Rank %2d has values: ",rank);
            for (int i=0; i<locn; i++)
                printf("%3d ", localdata[i]);
            printf("\n");
        }
        MPI_Barrier(MPI_COMM_WORLD);
    }

    MPI_Reduce(local, global, 1, mpi_datatype, mpi_selectop, 0, MPI_COMM_WORLD);

    if (rank == 0) {
        printf("Result is: \n");
        printselectk(global,selectk);
        printf("\n");
    }

    MPI_Op_free(&mpi_selectop);
    MPI_Type_free(&mpi_datatype);
    MPI_Finalize();

    return 0;
}

编译和运行提供了：

$ mpicc kselect.c -o kselect -Wall -std=c99 
$ mpirun -np 10 kselect 12 5
Rank  0 has values:  98 138 167 197 238 276 314 384 391 399 420 455 
Rank  1 has values:  16  87 119 134 156 164 225 299 321 380 409 441 
Rank  2 has values:  22  81 155 219 285 295 330 342 364 399 435 499 
Rank  3 has values:   3   7  75 164 181 271 285 358 379 438 466 491 
Rank  4 has values:   7  63  74 132 173 178 197 244 304 337 352 457 
Rank  5 has values:  21  62 104 138 240 346 377 382 411 446 455 482 
Rank  6 has values:  19  90 142 231 246 269 281 307 331 380 413 451 
Rank  7 has values:  43 191 193 232 236 331 399 429 439 445 446 457 
Rank  8 has values:  10 111 128 165 277 277 371 394 413 438 443 470 
Rank  9 has values:   2   2  34  57  97 105 128 187 265 329 344 409 
Result is: 
[  2   2   3   7   7 ]

（版本没有全局变量如下:)

#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
#include <limits.h>
#include <math.h>
#include <assert.h>

const int invalid_data = -1;

int min2(int a, int b) {
    if (b < a)
        return b;
    return a;
}

MPI_Datatype createtype(int selectk) {
    MPI_Datatype selectktype;
    MPI_Type_contiguous(selectk, MPI_INT, &selectktype);
    MPI_Type_commit(&selectktype);

    return selectktype;
}

void initselectk(int *d, int selectk) {
    d[0] = selectk;
    for (int i=1; i<selectk+1; i++)
        d[i] = invalid_data;
}

void printselectk(int *d) {
    int selectk = d[0];
    printf("[");
    for (int i=1; i<selectk+1; i++) 
        printf("%3d ",d[i]);
    printf("] ");
}

int countselectk(int *d) {
    int selectk = d[0];
    int count = 0;
    d++;

    while ( (d[count] != invalid_data) && (count < selectk) )
        count++;

    return count;
}

int mergeselect(int *d1, int *d2, int *dout) {
    int selectk = d1[0];
    assert(selectk == d2[0]);
    dout[0] = selectk;
    dout++;

    int count1 = countselectk(d1);
    int count2 = countselectk(d2);
    int total = count1 + count2;
    if (total >= selectk) total = selectk;

    int count = 0;
    int idx1=1, idx2=1;

    while (count < total) {
        int minloc = -1;
        int minval = INT_MAX;
        if (idx1 <= count1) {
            minloc = 1;
            minval = d1[idx1];
        }
        if ( (idx2 <= count2) && (d2[idx2] < minval ) ) {
            minloc = 2;
            minval = d2[idx2];
        } 
        dout[count++] = minval;
        if (minloc == 1)
            idx1++;
        else
            idx2++; 
    }
    return count;
}

void selectop(void *in, void *inout, int *len, MPI_Datatype *type) {
    int *invals = (int *)in;
    int *inoutvals = (int *)inout;


    for (int i=0; i<*len; i++) {
        int selectk = invals[0];
        assert(selectk == inoutvals[0]);

        int out[selectk+1];
        initselectk(out, selectk);
        int count = mergeselect(invals, inoutvals, out);
        for (int j=1; j<=count; j++) 
            inoutvals[j] = out[j];

        invals += selectk+1;
        inoutvals += selectk+1;
    }

    return;
}

int intcompar(const void *v1, const void *v2) {
    int *i1 = (int *)v1;
    int *i2 = (int *)v2;

    return (*i1 - *i2);
}

int main(int argc, char **argv) {

    int rank, size;
    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    if (argc < 3) {
        fprintf(stderr,"Usage: %s localn k-to-select\n", argv[0]);
        MPI_Abort(MPI_COMM_WORLD,1);
    }

    int locn    = atoi(argv[1]);
    int selectk = atoi(argv[2]);

    int localdata[locn];
    int local[selectk+1], global[selectk+1];

    /* create our new data type */
    MPI_Datatype mpi_datatype = createtype(selectk+1);

    MPI_Op mpi_selectop;
    MPI_Op_create(selectop, 1, &mpi_selectop);

    srand(rank*37);
    for (int i=0; i<locn; i++) 
        localdata[i] = floor(500.*rand()/RAND_MAX);

    /* get our local k selected */
    /* could use quickselect for this, but to focus on the MPI, let's just sort */

    initselectk(local, selectk);
    qsort(localdata, locn, sizeof(int), intcompar);
    for (int i=0; i<min2(selectk,locn); i++) 
        local[i+1] = localdata[i];

    for (int proc=0; proc<size; proc++) {
        if (rank == proc) {
            printf("Rank %2d has values: ",rank);
            for (int i=0; i<locn; i++)
                printf("%3d ", localdata[i]);
            printf("\n");
        }
        MPI_Barrier(MPI_COMM_WORLD);
    }

    MPI_Reduce(local, global, 1, mpi_datatype, mpi_selectop, 0, MPI_COMM_WORLD);

    if (rank == 0) {
        printf("Result is: \n");
        printselectk(global);
        printf("\n");
    }

    MPI_Op_free(&mpi_selectop);
    MPI_Type_free(&mpi_datatype);
    MPI_Finalize();

    return 0;
}