9.12. Troubleshooting

Launching applications can be a complex process that involves many moving parts. This section attempts to provide solutions to some of the most common problems users encounter.

9.12.1. Errors about missing libraries

When building PRRTE with the compilers that have libraries in non-default search path locations, you may see errors about those compiler’s support libraries when trying to launch applications if their corresponding environments were not setup properly.

For example, you may see warnings similar to the following:

# With the Intel compiler suite
shell$ prterun -n 1 --host node1.example.com hello
prted: error while loading shared libraries: libimf.so: cannot open shared object file: No such file or directory
--------------------------------------------------------------------------
A daemon (pid 11893) died unexpectedly with status 127 while
attempting to launch so we are aborting.
...more error messages...

# With the PGI compiler suite
shell$ prterun -n 1 --host node1.example.com hello
prted: error while loading shared libraries: libpgcc.so: cannot open shared object file: No such file or directory
...more error messages...

# With the PathScale compiler suite
shell$ prterun -n 1 --host node1.example.com hello
prted: error while loading shared libraries: libmv.so: cannot open shared object file: No such file or directory
...more error messages...

Specifically, PRRTE first attempts to launch a “helper” daemon prted on node1.example.com, but it failed because one of prted’s dependent libraries was not able to be found. The libraries shown above (libimf.so, libpgcc.so, and libmv.so) are specific to their compiler suites (Intel, PGI, and PathScale, respectively). As such, it is likely that the user did not setup the compiler library in their environment properly on this node.

Double check that you have setup the appropriate user environment on the target node, for both interactive and non-interactive logins.

Note

It is a common error to ensure that the user environment is setup properly for interactive logins, but not for non-interactive logins.

Here’s an example of a user-compiled MPI application working fine locally, but failing when invoked non-interactively on a remote node:

# Compile a trivial MPI application
head_node$ cd $HOME
head_node$ mpicc mpi_hello.c -o mpi_hello

# Run it locally; it works fine
head_node$ ./mpi_hello
Hello world, I am 0 of 1.

# Run it remotely interactively; it works fine
head_node$ ssh node2.example.com

Welcome to node2.
node2$ ./mpi_hello
Hello world, I am 0 of 1.
node2$ exit

# Run it remotely *NON*-interactively; it fails
head_node$ ssh node2.example.com $HOME/mpi_hello
mpi_hello: error while loading shared libraries: libimf.so: cannot open shared object file: No such file or directory

In cases like this, check your shell script startup files and verify that the appropriate compiler environment is setup properly for non-interactive logins.

9.12.2. Problems when running across multiple hosts

When you are able to run jobs on a single host, but fail to run them across multiple hosts, try the following:

Ensure that your launcher is able to launch across multiple hosts. For example, if you are using ssh, try to ssh to each remote host and ensure that you are not prompted for a password. For example:
```
shell$ ssh remotehost hostname
remotehost
```
If you are unable to launch across multiple hosts, check that your SSH keys are setup properly. Or, if you are running in a managed environment, such as in a Slurm, Torque, or other job launcher, check that you have reserved enough hosts, are running in an allocated job, etc.
Ensure that your PATH and LD_LIBRARY_PATH are set correctly on each remote host on which you are trying to run. For example, with ssh:
```
shell$ ssh remotehost env | grep -i path
PATH=...path on the remote host...
LD_LIBRARY_PATH=...LD library path on the remote host...
```
If your PATH or LD_LIBRARY_PATH are not set properly, see this section for the correct values. Keep in mind that it is fine to have multiple PRRTE installations installed on a machine; the first PRRTE installation found by PATH and LD_LIBARY_PATH is the one that matters.
Run a simple operating system job across multiple hosts. This verifies that the PRRTE run-time system is functioning properly across multiple hosts. For example, try running the hostname command:
```
shell$ prterun --host remotehost hostname
remotehost
shell$ prterun --host remotehost,otherhost hostname
remotehost
otherhost
```
If you are unable to run operating system jobs across multiple hosts, check for common problems such as:
1. Check your non-interactive shell setup on each remote host to ensure that it is setting up the PATH and LD_LIBRARY_PATH properly.
2. Check that PRRTE is finding and launching the correct version of PRRTE on the remote hosts.
3. Ensure that you have firewalling disabled between hosts (PRRTE opens random TCP and sometimes random UDP ports between hosts in a single MPI job).
4. Try running with the plm_base_verbose MCA parameter at level 10, which will enable extra debugging output to see how PRRTE launches on remote hosts. For example:
```
prterun --prtemca plm_base_verbose 10 --host remotehost hostname``
```
Now run a simple PMIx-based job across multiple hosts that does not involve inter-process communications. The hello_c program in the examples directory in the PRRTE distribution is a good choice. This verifies that the PMIx subsystem is able to initialize and terminate properly. For example:
```
shell$ prterun --host remotehost,otherhost hello_c
Hello, world, I am 0 of 1, (PRRTE VERSION, package: PRRTE jsquyres@example.com Distribution, ident: VERSION, DATE)
Hello, world, I am 1 of 1, (PRRTE VERSION, package: PRRTE jsquyres@example.com Distribution, ident: VERSION, DATE)
```
If you are unable to run simple, non-communication jobs, this can indicate that your PRRTE installation is unable to initialize properly on remote hosts. Double check your non-interactive login setup on remote hosts.