Search/wait for any transmission in MS-MPI - c

Question

It's a master - slave situation. How can I make the master process search in a non-blocking way for a message transmitted to him. If at the point of searching there is no message transmited to master, it will continue with iterations. However, if there is a message transmitted to him, it will process the message and than continue with iterations. See the comment inside /* */
int main(int argc, char *argv[])
{
int numprocs,
rank;
MPI_Request request;
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
if(rank == 0) // the searching process
{
for (int i=0; i < 4000000; i++)
{
// do some stuff here; does not matter what
/* see if any message has been transmitted to me at this point
without blocking the process; if at this time it happens to be
transmitted, do something and than continue with for iternations;
or just continue with for iterations and maybe next time will
have a message which sends me to do something */
}
}
else
{
int flag = 1;
while(flag)
{
// something done that at some point changes flag
}
// send a message to process with rank 0 and don't get stuck here
MPI_Isend(12, 1, MPI_INT, 0, 100, MPI_COMM_WORLD, &request);
// some other stuff done
// wait for message to be transmitted
MPI_Wait(&request, &status);
}
MPI_Finalize();
return 0;
}

Answer 1

One solution is to use MPI_IProbe() to test if a message is waiting.
On this line, use a pointer instead of "12"
MPI_Isend(12, 1, MPI_INT, 0, 100, MPI_COMM_WORLD, &request);
Add flag=0 here :
while(flag!=0)
{
// something done that at some point changes flag
}
Here goes the code :
#include "mpi.h"
#include "stdio.h"
int main(int argc, char *argv[])
{
int numprocs,
rank;
MPI_Request request;
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
if(rank == 0) // the searching process
{
int i;
for (i=0; i < 4000000; i++)
{
// do some stuff here; does not matter what
//printf("I am still running!\n");
int flag;
MPI_Iprobe(MPI_ANY_SOURCE,100,MPI_COMM_WORLD,&flag,&status);
if(flag!=0){
int value;
MPI_Recv(&value, 1, MPI_INT, status.MPI_SOURCE, status.MPI_TAG, MPI_COMM_WORLD, &status);
printf("I (0) received %d \n",value);
}
/* see if any message has been transmitted to me at this point
without blocking the process; if at this time it happens to be
transmitted, do something and than continue with for iternations;
or just continue with for iterations and maybe next time will
have a message which sends me to do something */
}
}
else
{
int i;
for(i=0;i<42;i++){
int flag = 1;
while(flag!=0)
{
// something done that at some point changes flag
flag=0;
}
int bla=1000*rank+i;
// send a message to process with rank 0 and don't get stuck here
MPI_Isend(&bla, 1, MPI_INT, 0, 100, MPI_COMM_WORLD, &request);
// some other stuff done
printf("I (%d) do something\n",rank);
// wait for message to be transmitted
MPI_Wait(&request, &status);
}
}
MPI_Finalize();
return 0;
}
Bye,
Francis

Answer 2

The non-blocking test for available messages is done using the MPI_Iprobe call. In your case it would look like:
int available;
MPI_Status status;
if(rank == 0) // the searching process
{
for (int i=0; i < 4000000; i++)
{
// do some stuff here; does not matter what
/* see if any message has been transmitted to me at this point
without blocking the process; if at this time it happens to be
transmitted, do something and than continue with for iternations;
or just continue with for iterations and maybe next time will
have a message which sends me to do something */
// Tag value 100 matches the value used in the send operation
MPI_Iprobe(MPI_ANY_SOURCE, 100, MPI_COMM_WORLD, &available, &status);
if (available)
{
// Message source rank is now available in status.MPI_SOURCE
// Receive the message
MPI_Recv(..., status.MPI_SOURCE, status.MPI_TAG, MPI_COMM_WORLD, &status);
}
}
}
MPI_ANY_SOURCE is used as wildcard rank, i.e. it instruct MPI_Irecv to check for messages from any source. If a corresponding send was posted, then available will be set to true, otherwise it will be set to false. The actual source of the message is also written to the MPI_SOURCE field of the status object. If the available flags indicates the availability of a matching message, one should then post a receive operation in order to receive it. It is important that the rank and the tag are explicitly specified in the receive operation, otherwise a different message could get received instead.
You could also use persistent connections. These behave very much like non-blocking operations with the important difference that they could be restarted multiple times. The same code with persistent connections would look like:
if(rank == 0) // the searching process
{
MPI_Request req;
MPI_Status status;
int completed;
// Prepare the persistent connection request
MPI_Recv_init(buffer, buf_size, buf_type,
MPI_ANY_SOURCE, 100, MPI_COMM_WORLD, &req);
// Make the request active
MPI_Start(&req);
for (int i=0; i < 4000000; i++)
{
// do some stuff here; does not matter what
/* see if any message has been transmitted to me at this point
without blocking the process; if at this time it happens to be
transmitted, do something and than continue with for iternations;
or just continue with for iterations and maybe next time will
have a message which sends me to do something */
// Non-blocking Test for request completion
MPI_Test(&req, &completed, &status);
if (completed)
{
// Message is now in buffer
// Process the message
// ...
// Activate the request again
MPI_Start(&req);
}
}
// Cancel and free the request
MPI_Cancel(&req);
MPI_Request_free(&req);
}
Persistent operations have a slight performance edge over the non-persistent ones, shown in the previous code sample. It is important that buffer is not accessed while the request is active, i.e. after the call to MPI_Start and before MPI_Test signals completion. Persistent send/receive operations also match non-persistent receive/send operations, therefore it is not necessary to change the code of the workers and they can still use MPI_Isend.