C++

The tutorial assumes you have already worked through the MPI Example Tutorial. Therefore, the instructions here are abbreviated but will follow the same format so you may easily consult the extended tutorial.

Table of Contents

• Step 1: Access Your Allocation

• Step 2: Create a SLURM Script

  • Example SLURM Script

  • SLURM Procedure

• Step 3: Compile the C++ Program from Source

  • MPI Hello World Source Code

  • C++ Procedure

• Step 4: Run the Job

Note: Do not execute jobs on the login nodes; only use the login nodes to access your compute nodes. Processor-intensive, memory-intensive, or otherwise disruptive processes running on login nodes will be killed without warning.

Step 1: Access Your Allocation

If you need to request an allocation, see instructions here.

1. Open a Bash terminal (or PuTTY for Windows users).

2. Execute ssh [e-mail].

3. When prompted, enter your or password.

Step 2: Create a SLURM Script

Example SLURM Script

Here is an example SLURM script for running a batch job on the cluster.

#!/bin/bash

#SBATCH -p stdmemq           # Submit to 'stdmemq' Partitiion or queue
#SBATCH -J MPItest           # Name the job as 'MPItest'
#SBATCH -o MPItest-%j.out    # Write the standard output to file named 'jMPItest-<job_number>.out'
#SBATCH -e MPItest-%j.err    # Write the standard error to file named 'jMPItest-<job_number>.err'
#SBATCH -t 0-12:00:00        # Run for a maximum time of 0 days, 12 hours, 00 mins, 00 secs
#SBATCH --nodes=1            # Request N nodes
#SBATCH --ntasks-per-node=20 # Request n cores or task per node
#SBATCH --mem-per-cpu=4000   # Request 4000MB (4GB) RAM per core
#SBATCH --mail-type=ALL      # Send email notification at the start and end of the job
#SBATCH --mail-user=<user>@cofc.edu  # Send email notification to this address

module list                 # will list modules loaded by default.
pwd                         # prints current working directory
date                        # prints the date and time

mpirun hello_world_cpp        # run the MPI job

Step 3: Compile the C++ Program from Source

MPI Hello World Source Code

#include<iostream>
#include "mpi.h"

using namespace std;
#include "mpi.h"

int main(int argc, char *argv[])
{
    int id, p, name_len;
    char processor_name[MPI_MAX_PROCESSOR_NAME];     
    MPI::Init(argc, argv);
    p = MPI::COMM_WORLD.Get_size();
    id = MPI::COMM_WORLD.Get_rank();
    MPI_Get_processor_name(processor_name, &name_len);
    cout<<" Processor " << processor_name<<" ID="<<id<< " Hello world\n";
    MPI::Finalize();
return 0;
}

C++ Procedure

1. Compile the C++ source into a binary executable file.

   $user@host[~] mpic++ -o hello_world_cpp hello_world.cpp

2. Use ls -al to verify the presence of the hello_world_cpp binary in your working directory.

Step 4: Run the Job

1. Use sbatch to schedule your batch job in the queue.

   $user@host[~] sbatch hello_world_cpp.slurm

   This command will automatically queue your job using SLURM and produce a job ID number (shown below). You can check the status of your job at any time with the squeue -j <JOB_ID> command.

   $user@host[~] squeue -j 12345

   You can also stop your job at any time with the scancel command.

   $user@host[~] scancel 12345

2. View your results. You can view the contents of these files using the more command followed by the file name.

  $user@host[~]  more mpi_hello_world_cpp.o12345

Your output should look something like this (the output is truncated.):

   Processor compute001 ID=5 Hello world
   Processor compute001 ID=1 Hello world
   Processor compute001 ID=4 Hello world
   Processor compute001 ID=8 Hello world
   .
   .
   .

1. Download your results (using the scp command or an SFTP client) or move them to persistent storage.

Additional Examples

• Working with C

• Working with Fortran

• Working with Python

• Working with Makefiles