Run MPI Hello World Examples

This tutorial introduces examples for running a simple MPI “Hello World” Fortran code using SLURM, both as an interactive job and as a batch job.

Note

The files mentioned in this tutorial are accessible on the P-Cluster at /efs_ecco/ECCO/misc/hello/. Some of them are also provided as downloadable links within this tutorial.

MPI Hello World Fortran Code

We use an MPI Fortran code called hello.f in which each process prints out “Hello, world!”.

Create a Local Test Directory and Obtain the Relevant Files

Users first need to copy the directory /efs_ecco/ECCO/misc/hello/ to a preferred location (e.g., /home/USERNAME/mpitest/ in the example below, with USERNAME replaced by the actual username), and then change into ~/mpitest/hello/, as shown in the following example:

echo "navigate to your home directory"
cd ~

echo "show your home directory "
pwd
/home/USERNAME

echo "make a directory for the mpi hello test files"
mkdir mpitest

echo "navigate into that directory"
cd mpitest

echo "copy the hello world mpi files here"
cp -pr /efs_ecco/ECCO/misc/hello .

echo "navigate  into that directory"
cd hello

echo "show contents
ls -1

Users should see the following:

README
hello
hello.f
hello_job_1140.log
hello_salloc.log
hello_sbatch.csh

Compile

The executable, called hello, has been pre-gerenated. If needed, use the following command to re-compile the Fortran program hello.f and generate the executable:

mpiifort -O2 -o hello hello.f

Tip: The "O" in -O2 is letter "O", not Zero. `-O2` stands for Optimization level 2.

Run as an Interactive Job

To run the executable as an interactive job using 6 CPUs, use the following command to request the computing resoure:

salloc --ntasks=6 --ntasks-per-node=2 --partition=sealevel-c5xl-demand --time=00:05:00

Here, salloc requests 6 tasks with 2 tasks per node, effectively requesting 3 nodes from the partition sealevel-c5xl-demand. We only request 5 minute of wall clock time (--time=00:05:00) since the job runs quickly (the actual run completes in at most a couple of seconds). Although the job itself finishes quickly, it often takes a few minutes for the computing resources to become ready. During this period, users will see a message similar to the following:

salloc: Granted job allocation 1263
salloc: Waiting for resource configuration

When the allocation is granted, users will see the following message, and the prompt will appear:

salloc: Nodes sealevel-c5xl-demand-dy-c5xlarge-[1-3] are ready for job

Users can then use the following command to use of the node as the inactive node (replace the job ID 1263 with the actual job ID):

srun --jobid=1263 --pty /bin/bash

Uses will notice the prompt changes

USERNAME@sealevel-c5xl-demand-dy-c5xlarge-1:/SOMEWORKINGDIR/$

Then, issue the following command to launch the executable ./hello using 6 procrosses.

mpirun -np 6 ./hello

The output of the interactive job is as follows:

salloc: Granted job allocation 1139
salloc: Waiting for resource configuration
salloc: Nodes sealevel-c5xl-demand-dy-c5xlarge-[1-3] are ready for job
  Process   1 says "Hello, world!"
12 May       2025  10:08:00.038 PM      

HELLO_MPI - Master process:
  FORTRAN77/MPI version
  A simple MPI program.

  The number of processes is   6

  Process   0 says "Hello, world!"

Elapsed wall clock time =   0.109248E-04 seconds.
  Process   2 says "Hello, world!"
  Process   3 says "Hello, world!"
  Process   4 says "Hello, world!"
  Process   5 says "Hello, world!"

HELLO_MPI:
  Normal end of execution.

12 May       2025  10:08:00.062 PM      
salloc: Relinquishing job allocation 1139

There are 6 printouts of “Hello, world!”, corresponding to the 6 processors.

Issue the exit command times (see explanations below) and release the allocated resources:

exit
exit

The first exit exits the interactive shell started by the srun command, while the second exit ends the salloc session and releases the allocated resources. After the first “exit”, you will see the prompt changes back to USERNAME@ip-10-20-22-69:~$. After the second exit, you will see the following message:

scsalloc: Relinquishing job allocation 1139

If you now issue the command squeue, you will see your job ID is not longer in the job list.

Note: If you look carefully, you'll see that the lines with 'Process n says "Hello, world!" can be out of order (really scrambled) because how *nix machines flush the caches of text sent to STDOUT. It's not a big deal if the order of the 'Process n says "Hello World"' is scrambled. If you see Hello Worlds!, you're good.

Run as a Batch Job

We can also run the executable as a batch job. To do this, we create a shell script called hello_sbatch.csh to run the executable using 12 processors as a batch job. To submit it, issue the following command:

sbatch hello_sbatch.csh

The script first specifies some SLURM directives, such as #SBATCH -J hello_sbatch, which define the job submission parameters and are interpreted by SLURM when you submit the script using sbatch. It then sets the number of processors (nprocs) and use mpirun to lauch the executable.

The output will be written to a file called hello_job_NNNN.log (where NNNN is the actual job number) in the directory where the job is submitted. You should see 12 printouts of “Hello, world!” in the hello_job_NNNN.log file. An example output file from the batch job is as follows:

  Process   1 says "Hello, world!"
  Process   2 says "Hello, world!"
  Process   3 says "Hello, world!"
  Process   4 says "Hello, world!"
  Process   5 says "Hello, world!"
13 May       2025   0:09:28.738 AM

HELLO_MPI - Master process:
  FORTRAN77/MPI version
  A simple MPI program.

  The number of processes is  12

  Process   0 says "Hello, world!"

Elapsed wall clock time =   0.138135E-05 seconds.
  Process   6 says "Hello, world!"
  Process   7 says "Hello, world!"
  Process   8 says "Hello, world!"
  Process   9 says "Hello, world!"
  Process  10 says "Hello, world!"
  Process  11 says "Hello, world!"

HELLO_MPI:
  Normal end of execution.

13 May       2025   0:09:28.824 AM

See, here the Hello, world! statements are also jumbled up. No big deal, delcare success and move on.