MPI_Scatter issues (C) - c

Question

My assignment is to parallelize some provided sequential code that takes an array of numbers and merges it with another array in order to come up with a sorted list. The first step is to initialize the array on each processor, then fill it with values on the root process. Next, I'm supposed to scatter the array out to the other processors so each processor has a chunk of the data for sorting, then sort these small lists locally and then gather them back up with the root process. My problem here is that no matter what I try the scatter won't actually send any data to the other processors. The array in these processors is always full of 0's, whereas the root processor does contain a list of numbers. Anybody care to take a look at my code and tell me what I'm missing?
The original sequential code
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/**
* Prints a vector without decimal places...
*/
void print_vector(int n, double vector[]) {
int i;
printf("[%.0f", vector[0]);
for (i = 1; i < n; i++) {
printf(", %.0f", vector[i]);
}
printf("]\n");
}
/**
* Just checks sequentially if everything is in ascending order.
*
* Note: we don't care about stability in this sort since we
* have no data attached to the double value, see
* http://en.wikipedia.org/wiki/Sorting_algorithm#Stability
* for more detail.
*/
void test_correctness(int n, double v[]) {
int i;
for (i = 1; i < n; i++) {
if (v[i] < v[i-1]) {
printf("Correctness test found error at %d: %.4f is not < %.4f but appears before it\n", i, v[i-1], v[i]);
}
}
}
/**
* Initialize random vector.
*
* You may not parallelize this (even though it could be done).
*/
void init_random_vector(int n, double v[]) {
int i, j;
for (i = 0; i < n; i++) {
v[i] = rand() % n;
}
}
double *R;
/**
* Merges two arrays, left and right, and leaves result in R
*/
void merge(double *left_array, double *right_array, int leftCount, int rightCount) {
int i,j,k;
// i - to mark the index of left aubarray (left_array)
// j - to mark the index of right sub-raay (right_array)
// k - to mark the index of merged subarray (R)
i = 0; j = 0; k =0;
while (i < leftCount && j < rightCount) {
if(left_array[i] < right_array[j])
R[k++] = left_array[i++];
else
R[k++] = right_array[j++];
}
while (i < leftCount)
R[k++] = left_array[i++];
while (j < rightCount)
R[k++] = right_array[j++];
}
/**
* Recursively merges an array of n values using group sizes of s.
* For example, given an array of 128 values and starting s value of 16
* will result in 8 groups of 16 merging into 4 groups of 32, then recursively
* calling merge_all which merges them into 2 groups of 64, then once more
* recursively into 1 group of 128.
*/
void merge_all(double *v, int n, int s) {
if (s < n) {
int i;
for (i = 0; i < n; i += 2*s) {
merge(v+i, v+i+s, s, s);
// result is in R starting at index 0
memcpy(v+i, R, 2*s*sizeof(double));
}
merge_all(v, n, 2*s);
}
}
void merge_sort(int n, double *v) {
merge_all(v, n, 1);
}
int main(int argc, char *argv[]) {
if (argc < 2) {
// () means optional
printf("usage: %s n (seed)\n", argv[0]);
return 0;
}
int n = atoi(argv[1]);
int seed = 0;
if (argc > 2) {
seed = atoi(argv[2]);
}
srand(seed);
int mpi_p, mpi_rank;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &mpi_p);
MPI_Comm_rank(MPI_COMM_WORLD, &mpi_rank);
// init the temporary array used later for merge
R = malloc(sizeof(double)*n);
// all MPI processes will allocate a vector of length n, but only
// process 0 will initialize the values. You must distribute the
// values to processes. You will also likely need to allocate
// additional memory in your processes, make sure to clean it up
// by adding a correct free at the end of each process.
double *v = malloc(sizeof(double)*n);
if (mpi_rank == 0) {
init_random_vector(n, v);
}
double start = MPI_Wtime();
// do all the work ourselves! (you should make a better algorithm here!)
if (mpi_rank == 0) {
merge_sort(n, v);
}
double end = MPI_Wtime();
if (mpi_rank == 0) {
printf("Total time to solve with %d MPI Processes was %.6f\n", mpi_p, (end-start));
test_correctness(n, v);
}
MPI_Finalize();
free(v);
free(R);
return 0;
}
The parts I edited
double start = MPI_Wtime();
// do all the work ourselves! (you should make a better algorithm here!)
if (mpi_rank == 0)
{
//Scatter(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype, root, comm)
MPI_Scatter(&v, sizeOf, MPI_DOUBLE, &v, sizeOf, MPI_DOUBLE, 0, MPI_COMM_WORLD);
}
if (mpi_rank > 0)
{
//merge_sort(sizeOf, R);
printf("%f :v[0]", v[0]); printf("%s", "\n");
printf("%f :v[1]", v[1]); printf("%s", "\n");
printf("%f :v[2]", v[0]); printf("%s", "\n");
}
if (mpi_rank == 0)
{
//Gather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype, root, comm)
//MPI_Gather(&v, sizeOf, MPI_DOUBLE, test, sizeOf, MPI_DOUBLE, 0, MPI_COMM_WORLD);
//merge(v, test, n, sizeOf);
merge_sort(n, v);
}
double end = MPI_Wtime();
The code should output three values from each processor that isn't the root, but it just gives me 0's. I tried many different parameters inside the scatter function call, to no avail. Almost everything I tried produces the same output found below. Sample output:
0.000000 :v[0]
0.000000 :v[1]
0.000000 :v[2]
Total time to solve with 2 MPI Processes was 0.000052
EDIT: Calling scatter from every process is also something I tried, as it sounds like this is the way you're supposed to call it instead of just calling it from the root. However, then I get a bunch of errors instead of any output at all. Errors look like this:
*** Process received signal ***
Signal: Segmentation fault (11)
Signal code: (128)
Failing at address: (nil)
*** Process received signal ***
Signal: Segmentation fault (11)
Signal code: (128)
Failing at address: (nil)
Something is clearly wrong here. Not sure what I'm doing wrong though.