I am having trouble in writing a parallel MPI I/O program that will write in a particular pattern. I was able to have process 0 write integers 0-9, process 1 write integers 10-19, process 2 write integers 20-29, etc.
proc 0: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
proc 1: [10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
proc 2: [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]
proc 3: [30, 31, 32, 33, 34, 35, 36, 37, 38, 39]
result: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20
Here is the code that accomplishes that:
int main(int argc, char *argv[]) {
// MPI_Finalize();
int i, rank, size, offset;
MPI_File fhw;
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
int N = size * 10;
int buf[N];
for ( i = 0; i < N; ++i ) {
buf[i] = rank * 10 + i;
}
offset = rank * (N/size) * sizeof(int);
MPI_File_open(MPI_COMM_WORLD, "datafile", MPI_MODE_CREATE|MPI_MODE_WRONLY,
MPI_INFO_NULL, &fhw);
printf("(%d) Writing to file...\n", rank);
printf("\nRank: (%d), Offset: %d\n", rank, offset);
MPI_File_write_at(fhw, offset, buf, (N/size), MPI_INT, &status);
MPI_File_close(&fhw);
MPI_Finalize();
return 0;
}
However, I am confused about how I should produce the following result:
// starting out:
proc 0: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
proc 1: [10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
proc 2: [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]
proc 3: [30, 31, 32, 33, 34, 35, 36, 37, 38, 39]
// proc 0 writes first 2 slots, then proc 1 writes next 2, etc.
result: [0, 1, 10, 11, 20, 21, 30, 31, 2, 3, 12, 13, 22, 23, ..., 8, 9, 18, 19, 28, 29, 38, 29]
I have tried using MPI_File_set_view while looking up examples and documentation online for the past few hours but cannot get it to work. Can someone guide me in the right direction?
As you had understood you need to setup a view...
Then some small mistake in your code: 1) Do you really need a buf of more that 10 number for each processes? 2) The offset in MPI_File_wite_at is note in byte, but in number of element (in respect of the element size of your view)
So to set up the view you only need 1 line:
#include "mpi.h"
#include <cstdio>
int main(int argc, char *argv[]) {
// MPI_Finalize();
int i, rank, size, offset;
MPI_File fhw;
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
int N = 10;
int buf[N];
for ( i = 0; i < N; ++i ) {
buf[i] = rank * 10 + i;
}
offset = 10*rank;
MPI_File_open(MPI_COMM_WORLD,"datafile",MPI_MODE_CREATE|MPI_MODE_WRONLY,
MPI_INFO_NULL, &fhw);
printf("(%d) Writing to file...\n", rank);
printf("\nRank: (%d), Offset: %d\n", rank, offset);
MPI_File_set_view( fhw,0,MPI_INT, MPI_INT, "native", MPI_INFO_NULL ) ;
MPI_File_write_at(fhw, offset, buf, N, MPI_INT, &status);
MPI_File_close(&fhw);
MPI_Finalize();
return 0;
}
Then you can do exactly the same using MPI_File_write :-), and setting the view differently for each processes, just replace the view and write:
MPI_File_set_view(fhw,offset*sizeof(int),MPI_INT,MPI_INT,
"native",MPI_INFO_NULL ) ;
MPI_File_write_at(fhw, 0, buf, N, MPI_INT, &status);
or simply:
MPI_File_set_view(fhw,offset*sizeof(int),MPI_INT,MPI_INT,
"native",MPI_INFO_NULL ) ;
MPI_File_write(fhw, buf, N, MPI_INT, &status);
NOTE: in view the offset is in byte, while in write it is in size of elements of the view.... Might be a bit confusing :-)
Then the magics:
You need to write blocks of 2 ints with a stride of 2*size, and you have N/2 of those blocks, so you create a type:
MPI_Type_vector(N/2, 2 , size*2, MPI_INT, &ftype);
MPI_Type_commit(&ftype);
and set the view:
MPI_File_set_view( fhw, rank*2*sizeof(int), MPI_INT, ftype, "native", MPI_INFO_NULL ) ;
then you have to think that in memory you have a continuous storage of your data, to fit your view, it has to be something as N/2 blocks, so you create a datatype:
MPI_Type_contiguous(2, MPI_INT, &mtype);
MPI_Type_commit(&mtype);
Then you are ready for the write:
MPI_File_write(fhw, buf, N/2, mtype, &status);
MPI_File_close(&fhw);
And so the entire code will become:
#include "mpi.h"
#include <cstdio>
int main(int argc, char *argv[]) {
int i, rank, size, offset;
MPI_File fhw;
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
int N = 10; //need to be even!!!
int buf[N];
for ( i = 0; i < N; ++i ) {
buf[i] = rank * N + i;
}
offset = 10*rank;
MPI_File_open(MPI_COMM_WORLD, "datafile", MPI_MODE_CREATE|MPI_MODE_WRONLY,
MPI_INFO_NULL, &fhw);
printf("(%d) Writing to file...\n", rank);
printf("\nRank: (%d), Offset: %d\n", rank, offset);
MPI_Datatype ftype,mtype;
MPI_Type_vector(N/2, 2 , size*2, MPI_INT, &ftype);
MPI_Type_commit(&ftype);
MPI_File_set_view( fhw, rank*2*sizeof(int), MPI_INT, ftype,
"native",MPI_INFO_NULL ) ;
MPI_Type_contiguous(2, MPI_INT, &mtype);
MPI_Type_commit(&mtype);
MPI_File_write(fhw, buf, N/2, mtype, &status);
MPI_File_close(&fhw);
MPI_Finalize();
return 0;
}