I implemented some `MPI_Scatterv` and `MPI_Gatherv` routines for a parallel matrix matrix multiplication. Everything works fine for small matrix sizes up to N = 180, if I exceed this size, e.g. N = 184 MPI throws some errors while using `MPI_Scatterv`.
For the 2D Scatter I used some constructions with MPI_Type_create_subarray and MPI_TYPE_create_resized. Explanations of these constructions can be found in this question http://stackoverflow.com/questions/9269399/sending-blocks-of-2d-array-in-c-using-mpi.
The minimal example code I wrote filles a matrix A with some values scatters it to the local processes and write the rank number of each process in the local copy of the scattered A. After that the local copies will be gathered to the master process.
#include "mpi.h"
#define N 184 // grid size
#define procN 2 // size of process grid
int main(int argc, char **argv) {
double* gA = nullptr; // pointer to array
int rank, size; // rank of current process and no. of processes
// mpi initialization
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
// force to use correct number of processes
if (size != procN * procN) {
if (rank == 0) fprintf(stderr,"%s: Only works with np = %d.\n", argv[0], procN * procN);
MPI_Abort(MPI_COMM_WORLD,1);
}
// allocate and print global A at master process
if (rank == 0) {
gA = new double[N * N];
for (int i = 0; i < N; i++) {
for (int j = 0; j < N; j++) {
gA[j * N + i] = j * N + i;
}
}
printf("A is:\n");
for (int i = 0; i < N; i++) {
for (int j = 0; j < N; j++) {
printf("%f ", gA[j * N + i]);
}
printf("\n");
}
}
// create local A on every process which we'll process
double* lA = new double[N / procN * N / procN];
// create a datatype to describe the subarrays of the gA array
int sizes[2] = {N, N}; // gA size
int subsizes[2] = {N / procN, N / procN}; // lA size
int starts[2] = {0,0}; // where this one starts
MPI_Datatype type, subarrtype;
MPI_Type_create_subarray(2, sizes, subsizes, starts, MPI_ORDER_C, MPI_DOUBLE, &type);
MPI_Type_create_resized(type, 0, N / procN * sizeof(double), &subarrtype);
MPI_Type_commit(&subarrtype);
// compute number of send blocks
// compute distance between the send blocks
int sendcounts[procN * procN];
int displs[procN * procN];
if (rank == 0) {
for (int i = 0; i < procN * procN; i++) {
sendcounts[i] = 1;
}
int disp = 0;
for (int i = 0; i < procN; i++) {
for (int j = 0; j < procN; j++) {
displs[i * procN + j] = disp;
disp += 1;
}
disp += ((N / procN) - 1) * procN;
}
}
// scatter global A to all processes
MPI_Scatterv(gA, sendcounts, displs, subarrtype, lA,
N*N/(procN*procN), MPI_DOUBLE,
0, MPI_COMM_WORLD);
// print local A's on every process
for (int p = 0; p < size; p++) {
if (rank == p) {
printf("la on rank %d:\n", rank);
for (int i = 0; i < N / procN; i++) {
for (int j = 0; j < N / procN; j++) {
printf("%f ", lA[j * N / procN + i]);
}
printf("\n");
}
}
MPI_Barrier(MPI_COMM_WORLD);
}
MPI_Barrier(MPI_COMM_WORLD);
// write new values in local A's
for (int i = 0; i < N / procN; i++) {
for (int j = 0; j < N / procN; j++) {
lA[j * N / procN + i] = rank;
}
}
// gather all back to master process
MPI_Gatherv(lA, N*N/(procN*procN), MPI_DOUBLE,
gA, sendcounts, displs, subarrtype,
0, MPI_COMM_WORLD);
// print processed global A of process 0
if (rank == 0) {
printf("Processed gA is:\n");
for (int i = 0; i < N; i++) {
for (int j = 0; j < N; j++) {
printf("%f ", gA[j * N + i]);
}
printf("\n");
}
}
MPI_Type_free(&subarrtype);
if (rank == 0) {
delete gA;
}
delete lA;
MPI_Finalize();
return 0;
}It can be compiled and run using
mpicxx -std=c++11 -o test test.cpp mpirun -np 4 ./test
For small N=4,...,180 everything goes fine
A is:
0.000000 6.000000 12.000000 18.000000 24.000000 30.000000
1.000000 7.000000 13.000000 19.000000 25.000000 31.000000
2.000000 8.000000 14.000000 20.000000 26.000000 32.000000
3.000000 9.000000 15.000000 21.000000 27.000000 33.000000
4.000000 10.000000 16.000000 22.000000 28.000000 34.000000
5.000000 11.000000 17.000000 23.000000 29.000000 35.000000
la on rank 0:
0.000000 6.000000 12.000000
1.000000 7.000000 13.000000
2.000000 8.000000 14.000000
la on rank 1:
3.000000 9.000000 15.000000
4.000000 10.000000 16.000000
5.000000 11.000000 17.000000
la on rank 2:
18.000000 24.000000 30.000000
19.000000 25.000000 31.000000
20.000000 26.000000 32.000000
la on rank 3:
21.000000 27.000000 33.000000
22.000000 28.000000 34.000000
23.000000 29.000000 35.000000
Processed gA is:
0.000000 0.000000 0.000000 2.000000 2.000000 2.000000
0.000000 0.000000 0.000000 2.000000 2.000000 2.000000
0.000000 0.000000 0.000000 2.000000 2.000000 2.000000
1.000000 1.000000 1.000000 3.000000 3.000000 3.000000
1.000000 1.000000 1.000000 3.000000 3.000000 3.000000
1.000000 1.000000 1.000000 3.000000 3.000000 3.000000 Here you see the errors when I use N = 184:
Fatal error in PMPI_Scatterv: Other MPI error, error stack:
PMPI_Scatterv(655)..............: MPI_Scatterv(sbuf=(nil), scnts=0x7ffee066bad0, displs=0x7ffee066bae0, dtype=USER<resized>, rbuf=0xe9e590, rcount=8464, MPI_DOUBLE, root=0, MPI_COMM_WORLD) failed
MPIR_Scatterv_impl(205).........: fail failed
I_MPIR_Scatterv_intra(265)......: Failure during collective
I_MPIR_Scatterv_intra(259)......: fail failed
MPIR_Scatterv(141)..............: fail failed
MPIC_Recv(418)..................: fail failed
MPIC_Wait(269)..................: fail failed
PMPIDI_CH3I_Progress(623).......: fail failed
pkt_RTS_handler(317)............: fail failed
do_cts(662).....................: fail failed
MPID_nem_lmt_dcp_start_recv(288): fail failed
dcp_recv(154)...................: Internal MPI error! cannot read from remote process
Fatal error in PMPI_Scatterv: Other MPI error, error stack:
PMPI_Scatterv(655)..............: MPI_Scatterv(sbuf=(nil), scnts=0x7ffef0de9b50, displs=0x7ffef0de9b60, dtype=USER<resized>, rbuf=0x21a7610, rcount=8464, MPI_DOUBLE, root=0, MPI_COMM_WORLD) failed
MPIR_Scatterv_impl(205).........: fail failed
I_MPIR_Scatterv_intra(265)......: Failure during collective
I_MPIR_Scatterv_intra(259)......: fail failed
MPIR_Scatterv(141)..............: fail failed
MPIC_Recv(418)..................: fail failed
MPIC_Wait(269)..................: fail failed
PMPIDI_CH3I_Progress(623).......: fail failed
pkt_RTS_handler(317)............: fail failed
do_cts(662).....................: fail failed
MPID_nem_lmt_dcp_start_recv(288): fail failed
dcp_recv(154)...................: Internal MPI error! cannot read from remote processI found some information abut an issue with MPI_Bcast hang on large user defined types, see (https://software.intel.com/en-us/articles/intel-mpi-library-2017-known-issue-mpi-bcast-hang-on-large-user-defined-datatypes) but I'm not sure if its the same for Scatterv and Gatherv. I'm using Intel MPI Library 2017 Update 2 for Linux.
I hope someone knows a sollution for this problem.
Thread Topic:
Question