Collective exercises

collectives/README.md

+## Collective operations
+
+In this exercise we test different routines for collective
+communication. Write a program for four MPI processes, such that each
+process has a data vector with the following data:
+
+![](img/sendbuffer.png)
+
+In addition, each task has a receive buffer for eight elements and the
+values in the buffer are initialized to -1.
+
+Implement communication that sends and receives values from these data
+vectors to the receive buffers using a single collective routine in
+each case, so that the receive buffers will have the following values:
+
+a) 
+
+![](img/bcast.png)
+
+b) 
+
+![](img/scatter.png)
+
+c) 
+
+![](img/gatherv.png)
+
+d) 
+
+![](img/alltoall.png)
+
+
+You can start from scratch or use the skeleton code found in
+[c/collective.c](c/collective.c),
+[fortran/collective.F90](fortran/collective.F90) or
+[python/collective.py](python/collective.py)
+
+

collectives/c/collective.c

+#include <stdio.h>
+#include <stdlib.h>
+#include <mpi.h>
+
+#define NTASKS 4
+
+void print_buffers(int *printbuffer, int *sendbuffer, int buffersize);
+void init_buffers(int *sendbuffer, int *recvbuffer, int buffersize);
+
+
+int main(int argc, char *argv[])
+{
+    int ntasks, rank, color;
+    int sendbuf[2 * NTASKS], recvbuf[2 * NTASKS];
+    int printbuf[2 * NTASKS * NTASKS];
+
+    MPI_Comm sub_comm;
+
+    MPI_Init(&argc, &argv);
+    MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+
+    if (ntasks != NTASKS) {
+        if (rank == 0) {
+            fprintf(stderr, "Run this program with %i tasks.\n", NTASKS);
+        }
+        MPI_Abort(MPI_COMM_WORLD, -1);
+    }
+
+    /* Initialize message buffers */
+    init_buffers(sendbuf, recvbuf, 2 * NTASKS);
+
+    /* Print data that will be sent */
+    print_buffers(printbuf, sendbuf, 2 * NTASKS);
+
+    /* TODO: use a single collective communication call (and maybe prepare
+     *       some parameters for the call) */
+
+    /* Print data that was received */
+    /* TODO: add correct buffer */
+    print_buffers(printbuf, ..., 2 * NTASKS);
+
+    MPI_Finalize();
+    return 0;
+}
+
+
+void init_buffers(int *sendbuffer, int *recvbuffer, int buffersize)
+{
+    int rank, i;
+
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    for (i = 0; i < buffersize; i++) {
+        recvbuffer[i] = -1;
+        sendbuffer[i] = i + buffersize * rank;
+    }
+}
+
+
+void print_buffers(int *printbuffer, int *sendbuffer, int buffersize)
+{
+    int i, j, rank, ntasks;
+
+    MPI_Gather(sendbuffer, buffersize, MPI_INT,
+               printbuffer, buffersize, MPI_INT, 0, MPI_COMM_WORLD);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
+
+    if (rank == 0) {
+        for (j = 0; j < ntasks; j++) {
+            printf("Task %i:", j);
+            for (i = 0; i < buffersize; i++) {
+                printf(" %2i", printbuffer[i + buffersize * j]);
+            }
+            printf("\n");
+        }
+        printf("\n");
+    }
+}

collectives/c/solution/alltoall.c

+#include <stdio.h>
+#include <stdlib.h>
+#include <mpi.h>
+
+#define NTASKS 4
+
+void print_buffers(int *printbuffer, int *sendbuffer, int buffersize);
+void init_buffers(int *sendbuffer, int *recvbuffer, int buffersize);
+
+
+int main(int argc, char *argv[])
+{
+    int ntasks, rank, color;
+    int sendbuf[2 * NTASKS], recvbuf[2 * NTASKS];
+    int printbuf[2 * NTASKS * NTASKS];
+
+    MPI_Comm sub_comm;
+
+    MPI_Init(&argc, &argv);
+    MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+
+    if (ntasks != NTASKS) {
+        if (rank == 0) {
+            fprintf(stderr, "Run this program with %i tasks.\n", NTASKS);
+        }
+        MPI_Abort(MPI_COMM_WORLD, -1);
+    }
+
+    /* Initialize message buffers */
+    init_buffers(sendbuf, recvbuf, 2 * NTASKS);
+
+    /* Print data that will be sent */
+    print_buffers(printbuf, sendbuf, 2 * NTASKS);
+
+    /* Perform the all-to-all communication pattern */
+    MPI_Alltoall(sendbuf, 2, MPI_INT, recvbuf, 2, MPI_INT, MPI_COMM_WORLD);
+
+    /* Print data that was received */
+    print_buffers(printbuf, recvbuf, 2 * NTASKS);
+
+    MPI_Finalize();
+    return 0;
+}
+
+
+void init_buffers(int *sendbuffer, int *recvbuffer, int buffersize)
+{
+    int rank, i;
+
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    for (i = 0; i < buffersize; i++) {
+        recvbuffer[i] = -1;
+        sendbuffer[i] = i + buffersize * rank;
+    }
+}
+
+
+void print_buffers(int *printbuffer, int *sendbuffer, int buffersize)
+{
+    int i, j, rank, ntasks;
+
+    MPI_Gather(sendbuffer, buffersize, MPI_INT,
+               printbuffer, buffersize, MPI_INT, 0, MPI_COMM_WORLD);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
+
+    if (rank == 0) {
+        for (j = 0; j < ntasks; j++) {
+            printf("Task %i:", j);
+            for (i = 0; i < buffersize; i++) {
+                printf(" %2i", printbuffer[i + buffersize * j]);
+            }
+            printf("\n");
+        }
+        printf("\n");
+    }
+}

collectives/c/solution/broadcast.c

+#include <stdio.h>
+#include <stdlib.h>
+#include <mpi.h>
+
+#define NTASKS 4
+
+void print_buffers(int *printbuffer, int *sendbuffer, int buffersize);
+void init_buffers(int *sendbuffer, int *recvbuffer, int buffersize);
+
+
+int main(int argc, char *argv[])
+{
+    int ntasks, rank, color;
+    int sendbuf[2 * NTASKS], recvbuf[2 * NTASKS];
+    int printbuf[2 * NTASKS * NTASKS];
+
+    MPI_Comm sub_comm;
+
+    MPI_Init(&argc, &argv);
+    MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+
+    if (ntasks != NTASKS) {
+        if (rank == 0) {
+            fprintf(stderr, "Run this program with %i tasks.\n", NTASKS);
+        }
+        MPI_Abort(MPI_COMM_WORLD, -1);
+    }
+
+    /* Initialize message buffers */
+    init_buffers(sendbuf, recvbuf, 2 * NTASKS);
+
+    /* Print data that will be sent */
+    print_buffers(printbuf, sendbuf, 2 * NTASKS);
+
+    /* Send (0,1,2,...,7) everywhere */
+    MPI_Bcast(sendbuf, 2 * NTASKS, MPI_INT, 0, MPI_COMM_WORLD);
+
+    /* Print data that was received */
+    print_buffers(printbuf, sendbuf, 2 * NTASKS);
+
+    MPI_Finalize();
+    return 0;
+}
+
+
+void init_buffers(int *sendbuffer, int *recvbuffer, int buffersize)
+{
+    int rank, i;
+
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    for (i = 0; i < buffersize; i++) {
+        recvbuffer[i] = -1;
+        sendbuffer[i] = i + buffersize * rank;
+    }
+}
+
+
+void print_buffers(int *printbuffer, int *sendbuffer, int buffersize)
+{
+    int i, j, rank, ntasks;
+
+    MPI_Gather(sendbuffer, buffersize, MPI_INT,
+               printbuffer, buffersize, MPI_INT, 0, MPI_COMM_WORLD);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
+
+    if (rank == 0) {
+        for (j = 0; j < ntasks; j++) {
+            printf("Task %i:", j);
+            for (i = 0; i < buffersize; i++) {
+                printf(" %2i", printbuffer[i + buffersize * j]);
+            }
+            printf("\n");
+        }
+        printf("\n");
+    }
+}

collectives/c/solution/gatherv.c

+#include <stdio.h>
+#include <stdlib.h>
+#include <mpi.h>
+
+#define NTASKS 4
+
+void print_buffers(int *printbuffer, int *sendbuffer, int buffersize);
+void init_buffers(int *sendbuffer, int *recvbuffer, int buffersize);
+
+
+int main(int argc, char *argv[])
+{
+    int ntasks, rank, color;
+    int sendbuf[2 * NTASKS], recvbuf[2 * NTASKS];
+    int printbuf[2 * NTASKS * NTASKS];
+
+    MPI_Comm sub_comm;
+
+    MPI_Init(&argc, &argv);
+    MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+
+    if (ntasks != NTASKS) {
+        if (rank == 0) {
+            fprintf(stderr, "Run this program with %i tasks.\n", NTASKS);
+        }
+        MPI_Abort(MPI_COMM_WORLD, -1);
+    }
+
+    /* Initialize message buffers */
+    init_buffers(sendbuf, recvbuf, 2 * NTASKS);
+
+    /* Print data that will be sent */
+    print_buffers(printbuf, sendbuf, 2 * NTASKS);
+
+    /* Gather varying size data to task 1 */
+    int offsets[NTASKS] = { 0, 1, 2, 4 };
+    int counts[NTASKS] = { 1, 1, 2, 4 };
+    MPI_Gatherv(sendbuf, counts[rank], MPI_INT, recvbuf, counts,
+                offsets, MPI_INT, 1, MPI_COMM_WORLD);
+
+    /* Print data that was received */
+    print_buffers(printbuf, recvbuf, 2 * NTASKS);
+
+    MPI_Finalize();
+    return 0;
+}
+
+
+void init_buffers(int *sendbuffer, int *recvbuffer, int buffersize)
+{
+    int rank, i;
+
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    for (i = 0; i < buffersize; i++) {
+        recvbuffer[i] = -1;
+        sendbuffer[i] = i + buffersize * rank;
+    }
+}
+
+
+void print_buffers(int *printbuffer, int *sendbuffer, int buffersize)
+{
+    int i, j, rank, ntasks;
+
+    MPI_Gather(sendbuffer, buffersize, MPI_INT,
+               printbuffer, buffersize, MPI_INT, 0, MPI_COMM_WORLD);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
+
+    if (rank == 0) {
+        for (j = 0; j < ntasks; j++) {
+            printf("Task %i:", j);
+            for (i = 0; i < buffersize; i++) {
+                printf(" %2i", printbuffer[i + buffersize * j]);
+            }
+            printf("\n");
+        }
+        printf("\n");
+    }
+}

collectives/c/solution/scatter.c

+#include <stdio.h>
+#include <stdlib.h>
+#include <mpi.h>
+
+#define NTASKS 4
+
+void print_buffers(int *printbuffer, int *sendbuffer, int buffersize);
+void init_buffers(int *sendbuffer, int *recvbuffer, int buffersize);
+
+
+int main(int argc, char *argv[])
+{
+    int ntasks, rank, color;
+    int sendbuf[2 * NTASKS], recvbuf[2 * NTASKS];
+    int printbuf[2 * NTASKS * NTASKS];
+
+    MPI_Comm sub_comm;
+
+    MPI_Init(&argc, &argv);
+    MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+
+    if (ntasks != NTASKS) {
+        if (rank == 0) {
+            fprintf(stderr, "Run this program with %i tasks.\n", NTASKS);
+        }
+        MPI_Abort(MPI_COMM_WORLD, -1);
+    }
+
+    /* Initialize message buffers */
+    init_buffers(sendbuf, recvbuf, 2 * NTASKS);
+
+    /* Print data that will be sent */
+    print_buffers(printbuf, sendbuf, 2 * NTASKS);
+
+    /* Scatter the elements from task 0 */
+    MPI_Scatter(sendbuf, 2, MPI_INT, recvbuf, 2, MPI_INT, 0,
+                MPI_COMM_WORLD);
+
+    /* Print data that was received */
+    print_buffers(printbuf, recvbuf, 2 * NTASKS);
+
+    MPI_Finalize();
+    return 0;
+}
+
+
+void init_buffers(int *sendbuffer, int *recvbuffer, int buffersize)
+{
+    int rank, i;
+
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    for (i = 0; i < buffersize; i++) {
+        recvbuffer[i] = -1;
+        sendbuffer[i] = i + buffersize * rank;
+    }
+}
+
+
+void print_buffers(int *printbuffer, int *sendbuffer, int buffersize)
+{
+    int i, j, rank, ntasks;
+
+    MPI_Gather(sendbuffer, buffersize, MPI_INT,
+               printbuffer, buffersize, MPI_INT, 0, MPI_COMM_WORLD);
+    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+    MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
+
+    if (rank == 0) {
+        for (j = 0; j < ntasks; j++) {
+            printf("Task %i:", j);
+            for (i = 0; i < buffersize; i++) {
+                printf(" %2i", printbuffer[i + buffersize * j]);
+            }
+            printf("\n");
+        }
+        printf("\n");
+    }
+}

collectives/fortran/collective.F90

+program coll_exer
+  use mpi
+  implicit none
+
+  integer, parameter :: n_mpi_tasks = 4
+
+  integer :: ntasks, rank, ierr, i, color, sub_comm
+  integer, dimension(2*n_mpi_tasks) :: sendbuf, recvbuf
+  integer, dimension(2*n_mpi_tasks**2) :: printbuf
+
+  call mpi_init(ierr)
+  call mpi_comm_size(MPI_COMM_WORLD, ntasks, ierr)
+  call mpi_comm_rank(MPI_COMM_WORLD, rank, ierr)
+
+  if (ntasks /= n_mpi_tasks) then
+     if (rank == 0) then
+        print *, "Run this program with ", n_mpi_tasks, " tasks."
+     end if
+     call mpi_abort(MPI_COMM_WORLD, -1, ierr)
+  end if
+
+  ! Initialize message buffers
+  call init_buffers
+
+  ! Print data that will be sent
+  call print_buffers(sendbuf)
+
+  ! TODO: use a single collective communication call (and maybe prepare
+  !       some parameters for the call)
+
+  ! Print data that was received
+  ! TODO: add correct buffer
+  call print_buffers(...)
+
+  call mpi_finalize(ierr)
+
+contains
+
+  subroutine init_buffers
+    implicit none
+    integer :: i
+
+    do i = 1, 2*n_mpi_tasks
+       recvbuf(i) = -1
+       sendbuf(i) = i + 2*n_mpi_tasks * rank - 1
+    end do
+  end subroutine init_buffers
+
+
+  subroutine print_buffers(buffer)
+    implicit none
+    integer, dimension(:), intent(in) :: buffer
+    integer, parameter :: bufsize = 2*n_mpi_tasks
+    integer :: i
+    character(len=40) :: pformat
+
+    write(pformat,'(A,I3,A)') '(A4,I2,":",', bufsize, 'I3)'
+
+    call mpi_gather(buffer, bufsize, MPI_INTEGER, &
+         & printbuf, bufsize, MPI_INTEGER, &
+         & 0, MPI_COMM_WORLD, ierr)
+
+    if (rank == 0) then
+       do i = 1, ntasks
+          write(*,pformat) 'Task', i - 1, printbuf((i-1)*bufsize+1:i*bufsize)
+       end do
+       print *
+    end if
+  end subroutine print_buffers
+
+end program coll_exer

collectives/fortran/solution/alltoall.F90

+program coll_exer
+  use mpi
+  implicit none
+
+  integer, parameter :: n_mpi_tasks = 4
+
+  integer :: ntasks, rank, ierr, i, color, sub_comm
+  integer, dimension(2*n_mpi_tasks) :: sendbuf, recvbuf
+  integer, dimension(2*n_mpi_tasks**2) :: printbuf
+
+  call mpi_init(ierr)
+  call mpi_comm_size(MPI_COMM_WORLD, ntasks, ierr)
+  call mpi_comm_rank(MPI_COMM_WORLD, rank, ierr)
+
+  if (ntasks /= n_mpi_tasks) then
+     if (rank == 0) then
+        print *, "Run this program with ", n_mpi_tasks, " tasks."
+     end if
+     call mpi_abort(MPI_COMM_WORLD, -1, ierr)
+  end if
+
+  ! Initialize message buffers
+  call init_buffers
+
+  ! Print data that will be sent
+  call print_buffers(sendbuf)
+
+  ! Carry out the all-to-all pattern
+  call mpi_alltoall(sendbuf, 2, MPI_INTEGER, recvbuf, 2, MPI_INTEGER, &
+       & MPI_COMM_WORLD, ierr)
+
+  ! Print data that was received
+  call print_buffers(recvbuf)
+
+  call mpi_finalize(ierr)
+
+contains
+
+  subroutine init_buffers
+    implicit none
+    integer :: i
+
+    do i = 1, 2*n_mpi_tasks
+       recvbuf(i) = -1
+       sendbuf(i) = i + 2*n_mpi_tasks * rank - 1
+    end do
+  end subroutine init_buffers
+
+
+  subroutine print_buffers(buffer)
+    implicit none
+    integer, dimension(:), intent(in) :: buffer
+    integer, parameter :: bufsize = 2*n_mpi_tasks
+    integer :: i
+    character(len=40) :: pformat
+
+    write(pformat,'(A,I3,A)') '(A4,I2,":",', bufsize, 'I3)'
+
+    call mpi_gather(buffer, bufsize, MPI_INTEGER, &
+         & printbuf, bufsize, MPI_INTEGER, &
+         & 0, MPI_COMM_WORLD, ierr)
+
+    if (rank == 0) then
+       do i = 1, ntasks
+          write(*,pformat) 'Task', i - 1, printbuf((i-1)*bufsize+1:i*bufsize)
+       end do
+       print *
+    end if
+  end subroutine print_buffers
+
+end program coll_exer

collectives/fortran/solution/broadcast.F90

+program coll_exer
+  use mpi
+  implicit none
+
+  integer, parameter :: n_mpi_tasks = 4
+
+  integer :: ntasks, rank, ierr, i, color, sub_comm
+  integer, dimension(2*n_mpi_tasks) :: sendbuf, recvbuf
+  integer, dimension(2*n_mpi_tasks**2) :: printbuf
+
+  call mpi_init(ierr)
+  call mpi_comm_size(MPI_COMM_WORLD, ntasks, ierr)
+  call mpi_comm_rank(MPI_COMM_WORLD, rank, ierr)
+