What I have
I have a C-program using MPI, and it uses 4 processes: 1 vehicle(taskid=0) and 3 passengers. Vehicle can accommodate 2 passengers at a time. 3 customers keep coming back to get a ride.
For Vehicle, I have:
int passengers[C] = {0};
while(1)
MPI_Recv(&pid, 1, MPI_INT, MPI_ANY_SOURCE, 1, MPI_COMM_WORLD, &status);
//put pid in passengers[totalNumberArrived]
if(totalNumberArrived == 2)
printf("vehicle left...");
sleep(5);
printf("vehicle came back...");
for (i=0; i<2; i++)
MPI_Send(&passengers[i], 1, MPI_INT, passengers[i], 1, MPI_COMM_WORLD);
totalNumberArrived = 0;
if(done)//omitting the details here
break;
And, for each passengers, I have:
for (i to NumOfRound)
sleep(X);
printf("%d is sending a msg", tasked)
MPI_Send(&taskid, 1, MPI_INT, 0, 1, MPI_COMM_WORLD);
MPI_Recv(&pid, 1, MPI_INT, 0, 1, MPI_COMM_WORLD, &pstatus);
printf("%d received from %d\n", tasked, pid, pstatus.MPI_SOURCE);
issue
If the vehicle left for a ride with taskid 1 and 3, I expect to see this kind of output:
vehicle left...
vehicle came back...
1 is sending a msg
3 is sending a msg (this could be before 1's msg though)
but I sometimes get
vehicle left...
1 is sending a msg
3 is sending a msg
vehicle came back...
which looks like the passenger is not blocked until the vehicle comes back.
I thought that MPI_Recv blocks the task until it gets a msg from the vehicle, so I researched and read that MPI_Recv does block but this kind of issues occur because the printf is not necessarily printing in order. I also read that some recommends to use flush but in some cases flush doesn't work.
I'm not sure what I should do in my case. Is it really just the matter of printf order?
I've also read this: Ordering Output in MPI
and wonder if I should add a master thread and let it be the central controller for vehicle and passengers??
