我想在使用MPI的所有节点上找到前k个结果。为此，我想使用MPI_Reduce自带的功能。但是我的代码不起作用，因为函数的len参数与赋予MPI_Reduce的count参数不同。
我发现here实现可以执行此操作来管理计算。
我的代码与此类似：
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;
}
}
其中acctbal_pair是一个带有suppkey和acctbal字段的结构
我像这样调用MPI_Reduce。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);
但是对于稍微大一点的k值，计数会被分割成更小的块，使我的函数失败。
有没有办法告诉Reduce不要管道计算？或者你知道另一种（有效的）实现方法吗？我真的不想使用MPI_Gather并且因为通信开销很大而在根上找到第一个k结果。
我不能仅使用固定参数k创建函数（并将所有k个元素视为1 MPI_type），因为k是在运行时计算的。
我知道这不是MPI_Reduce的目的（它应该只是计算一些操作元素）但如果count没有分块，这就完美了。
p.S。：我的MPI实现是OpenMPI
答案 0 :(得分：1)
当然，你可以这样做 - 你只需要创建一个大小为k的类型（在运行时很容易做到）并进行选择。唯一的诀窍是，你没有办法将状态（例如，k）传递给选择操作，所以你需要通过一个全局变量进行通信 - 这显然不是很好，但是人们需要做的就是做。如果需要使用不同大小的k重复运行算法，则只需根据需要创建类型并重置全局变量。
（如果你以某种其他方式将k的值潜入选择操作中，你可以避免使用全局变量 - 例如，在每个数组中传递给它的数据的第一个元素是值k。）
下面是一些执行此操作的代码;它允许处理器具有小于k值的情况。每个处理器选择其k个最小值并将它们填充到本地数组中，然后选择操作执行部分排序合并操作以仅拾取k个最少元素。
#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;
}
编译并运行：
$ 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 ]
（没有全局变量的版本如下：）
#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;
}