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Exercise 2

Overview:

Login to the LC workshop cluster, if you are not already logged in
Using your “Hello World” MPI program from Exercise 1, add MPI blocking point-to-point routines to send and receive messages
Successfully compile your program
Successfully run your program - several different ways
Try the same thing with nonblocking send/receive routines

Approx. 20 minutes

1. Still logged into the workshop cluster?

If so, then continue to the next step. If not, then login as you did previously for Exercise 1.

2. Create, compile and run an MPI blocking send/receive program

Assuming you were able to create a successful “hello world” MPI program in Exercise 1, copy your source file to a new file and call it something like helloBsend.c or helloBsend.f. If you were not successful, you can use the provided mpi_hello.c or mpi_hello.f. as your starting point.

Edit your new helloBsend source file and modify it to do the following - after the master task has printed the number of tasks, but before MPI_Finalize: Have each task determine a unique partner task to send/receive with. One easy way to do this:

C:

if (taskid < numtasks/2) then partner = numtasks/2 + taskid
else if (taskid >= numtasks/2) then partner = taskid - numtasks/2

Fortran:

if (taskid .lt. numtasks/2) then partner = numtasks/2 + taskid
else if (taskid .ge. numtasks/2) then partner = taskid - numtasks/2

Each task sends its partner a single integer message: its taskid.

Each task receives from its partner a single integer message: the partner’s taskid.

For confirmation, after the send/receive, each task prints something like “Task ## is partner with ##” where ## is the taskid of the task and its partner.

If you need help, see the provided example files mpi_helloBsend.c or mpi_helloBsend.f

3. Compile your `helloBsend` program

Using your choice of compiler as in Exercise 1, compile your helloBsend program until you get a clean compile.

4. Run your executable

Using the srun command as in Exercise 1 to run your executable. For example:

srun -N2 -n8 -ppReserved helloBsend

Did your job run successfully? Based on the output, did it behave as expected? If not, figure out any problems and fix them before proceeding.

5. Vary numbers of nodes and total tasks

Run your program a few more times, but vary the number of nodes and total tasks. Observe the task output statements to confirm.

6. Convert to non-blocking

If time permits, copy your helloBsend source file to a new helloNBsend source file. Then convert the blocking routines to non-blocking routines. See the provided or source files if you need assistance.