A Glimpse of Parallel Computing - Elements of Scientific Computing - page 446

MPI_Send(void * buf, int count, MPI_Datatype datatype,
int dest, int tag, MPI_Comm comm)
The first three arguments constitute the outgoing message, by specifying the ini-
tial memory address of the array, the number of data elements, and the data type.
The dest argument gives the rank of the receiving process relative to the comm
communicator, whereas tag is an integer argument used to label the particular
message.
Correspondingly, the simplest MPI function for receiving a message is
MPI_Recv(void * buf, int count, MPI_Datatype datatype,
int source, int tag, MPI_Comm comm, MPI_Status * status)
Compared with the MPI Send function, the extra argument in the MPI Recv function
is a pointer to a so-called MPI Status object, which can be used to check some
details of a received message.
Below we will give two examples of MPI programming in the C language. Com-
pared with Examples 10.6 and 10.7 , we will see that a programmer is responsible for
more details of the parallelization. Communications have to be enforced explicitly
in MPI. Despite the extra programming effort, parallel MPI programs are usually
good with respect to data locality, because the local data structure owned by each
MPI process is small relative to the global data structure. In addition, the user
has full control over work division, which can be of great value for performance
enhancement.
For a beginner, it is important to realize that each MPI process executes the
same MPI program. The distinction between the processes, which are spawned by
some parallel runtime system, is through the unique process rank. The rank typi-
cally determines the work assignment for each process. Moreover, an if -test with
respect to a particular process rank can allow the chosen process to perform different
operations than the other processes.
Example 10.8. The following is the most important part of an MPI implementation
of the composite trapezoidal integration rule (10.6):
#include <mpi.h>
/ * code omitted for defining function f
* /
int main (int nargs, char ** args)
{
int P, my_id, n, n_p, i_start_p, i, remainder;
double a, b, h, x, s_p, sum;
MPI_Init (&nargs, &args);
MPI_Comm_size(MPI_COMM_WORLD,&P);
MPI_Comm_rank(MPI_COMM_WORLD,&my_id);
n = 1000000; a = 0.; b = 1.;
remainder = (n-1)%P;