# 10.6. Launching with SSH

When launching Open MPI jobs in a non-scheduled environment, `ssh` is typically used to launch commands on remote nodes. As listed in the quick start section, successfully launching MPI applications with `ssh` requires the following:

1. You must be able to non-interactively login — without entering a password or passphrase — to all remote nodes from all remotes nodes.

2. Open MPI’s executables must be findable (e.g., in your `PATH`).

3. Open MPI’s libraries must be findable (e.g., in your `LD_LIBRARY_PATH`).

## 10.6.1. Specifying the hosts for an MPI job

There are three mechanisms for specifying the hosts that an MPI job will run on:

1. The `--hostfile` option to mpirun(1).

   Use this option to specify a list of hosts on which to run. Note that for compatibility with other MPI implementations, `--machinefile` is a synonym for `--hostfile`. See this section for more information about the `--hostfile` option.

2. The `--host` option to mpirun(1).

   This option can be used to specify a list of hosts on which to run on the command line. See this section for more information about the `--host` option.

3. Running in a scheduled environment.

   If you are running in a scheduled environment (e.g., in a Slurm, Torque, or LSF job), Open MPI will automatically get the lists of hosts from the scheduler. See the next subsections for details about launching MPI jobs in supported scheduled environements.

Important

The specification of hosts using any of the above methods has nothing to do with the network interfaces that are used for MPI traffic. The list of hosts is only used for specifying which hosts on which to launch MPI processes.

## 10.6.2. Non-interactive `ssh` logins

SSH keys must be setup such that the following can be executed without being prompted for password or passphrase:

    shell$ ssh othernode echo hello
    hello
    shell$

Consult instructions and tutorials from around the internet to learn how to setup SSH keys. Try Google search terms like “passwordless SSH” or “SSH key authentication”.

For simplicity, it may be desirable to configure your SSH keys without passphrases. This adds some risk, however (e.g., if your SSH keys are compromised). But it simplifies your SSH setup because you will not need to use `ssh-agent`. Evaluate the risk level you are comfortable with.

Important

Open MPI uses a tree-based pattern to launch processes on remote nodes. This means that Open MPI must be able to non-interactively login — without being prompted for password or passphrase — to any node in the host list from any node in the host list.

It may not be sufficient to only setup an SSH key from the node where you are invoking mpirun(1) to all other nodes.

If you have a shared `$HOME` filesystem between your nodes, you can setup a single SSH key that is used to login to all nodes.

## 10.6.3. Finding Open MPI executables and libraries

Once Open MPI is able to use `ssh` to invoke executables on a remote node, it must be able to find its helper executables and shared libraries on that remote node.

If Open MPI is installed in a system-level folder (e.g., in `/usr/bin`), Open MPI will likely be able to find its executables and libraries on the remote node with no additional assistance.

If, however, Open MPI is installed into a path that is not searched by default, you will need to provide assistance so that Open MPI can find its executables and libraries.

Important

For simplicity, it is strongly recommended that you install Open MPI in the same location on all nodes in your job. See the Installation location section for more details.

You can do this in one of two ways.

### 10.6.3.1. Use “prefix” behavior

Note

“Prefix” behavior is only available with mpirun(1); it is not available via resource manager direct launch mechanisms. However, this section is about using `ssh` to launch MPI jobs, which means that there is no resource manager, and therefore there is no direct launch mechanism available.

When “prefix” behavior is enabled, Open MPI will automatically set the `$PATH` and `$LD_LIBRARY_PATH` on remote nodes before executing remote commands.

Important

Open MPI assumes that the installation `prefix`, `bindir`, and `libdir` are the same on the remote node as they are on the local node. If they are not, then you should not use the “prefix” behavior.

You can enable “prefix” behavior in one of three ways:

1. Use an absolute path name to invoke mpirun(1).

       shell$ $HOME/my-openmpi/bin/mpirun --hostfile my-hostfile.txt mpi-hello-world

   Simply using the absolute path name to mpirun(1) tells Open MPI to enable “prefix” mode.

2. Use the `--prefix` option to mpirun(1).

>       shell$ $HOME/my-openmpi/bin/mpirun --hostfile my-hostfile.txt \
>           --prefix $HOME/my-openmpi \
>           mpi-hello-world

>     The ``-prefix`` option takes a single argument: the prefix path to
>     use for the bindir and libdir on the remote node.

1. Configure Open MPI with `--enable-mpirun-prefix-by-default`.

   If Open MPI is built this way, mpirun(1) will always enable “prefix” behavior.

### 10.6.3.2. Set the `PATH` and `LD_LIBRARY_PATH` in your shell startup files

Consider the case where Open MPI was configured with:

    shell$ ./configure --prefix=$HOME/my-openmpi ...

In this cause, Open MPI will be installed into `$HOME/my-openmpi`. This path is almost certainly not in any system-default search paths, so it must be added to the `$PATH` and `$LD_LIBRARY_PATH` environment variables.

Specifically: the goal is that the following non-interactive commands must be able to execute without error:

    # First, ensure that this command returns the correct ompi_info
    # instance (i.e., $HOME/my-openmpi/bin/ompi_info).
    shell$ ssh remotenode which ompi_info
    /home/myusername/my-openmpi/bin/ompi_info

    # Next, ensure that you can run that ompi_info command without
    # error
    shell$ ssh remotenode ompi_info

    # ... lots of output ...

Ensure that you do not see any errors about libraries that cannot be found.

All shells have some kind of script file that is executed at login time perform environmental setup tasks. This startup file is the one that needs to be edited to:

1. Add Open MPI’s executable path (which is likely `$prefix/bin`, or `$HOME/my-openmpi/bin` in this example) to the `$PATH` environment variable.

2. Add Open MPI’s library path (which is likely `$prefix/lib`, or `$HOME/my-openmpi/lib` in this example) to the `$LD_LIBRARY_PATH` environment variable.

You probably want to add Open MPI’s libraries to the front of `$PATH` and `$LD_LIBRARY_PATH` to ensure that this Open MPI installation’s files are found first.

Consult the manual page for your shell for specific details (some shells are picky about the permissions of the startup file, for example). The list below contains some common shells and the startup files that they read/execute upon login:

Shell | Non-interactive login | Interactive login
--- | --- | ---
`bash` or `zsh` | `$HOME/.bashrc` if it exists. | `$HOME/.bash_profile` if it exists, or `$HOME/.bash_login` if it exists, or `$HOME/.profile` if it exists (in that order). Note that some Linux distributions automatically come with `$HOME/.bash_profile` scripts for users that automatically execute `$HOME/.bashrc` as well. Consult the bash man page for more information.
`sh` | This shell does not execute any file automatically, so Open MPI will execute the `$HOME/.profile` script before invoking Open MPI executables on remote nodes | `$HOME/.profile`
`csh` | `$HOME/.cshrc` | `$HOME/.cshrc` followed by `$HOME/.login`
`tcsh` | `$HOME/.tcshrc` if it exists, or `$HOME/.cshrc` if it does not | `$HOME/.tcshrc` if it exists, or `$HOME/.cshrc` if it does not Afterwards, execute `$HOME/.login`