10.12. Troubleshooting

Launching MPI jobs 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.

10.12.1. Messages about missing symbols

Open MPI loads a lot of plugins (sometimes called “components” or “modules”) at run time. Sometimes a plugin can fail to load because it can’t resolve all the symbols that it needs. There are a few reasons why this can happen.

The plugin is for a different version of Open MPI. See this section for an explanation of how Open MPI might try to open the “wrong” plugins.
An application is trying to manually dynamically open libmpi in a private symbol space. For example, if an application is not linked against libmpi, but rather calls something like this:
```
/* This is a Linux example |mdash| the issue is similar/the same on other
   operating systems */
handle = dlopen("libmpi.so", RTLD_NOW | RTLD_LOCAL);
```
This is due to some deep run-time linker voodoo — it is discussed towards the end of this post to the Open MPI developer’s list. Briefly, the issue is this:
1. The dynamic library libmpi is opened in a “local” symbol space.
2. MPI_INIT is invoked, which tries to open Open MPI’s plugins.
3. Open MPI’s plugins rely on symbols in libmpi (and other Open MPI support libraries); these symbols must be resolved when the plugin is loaded.
4. However, since libmpi was opened in a “local” symbol space, its symbols are not available to the plugins that it opens.
5. Hence, the plugin fails to load because it can’t resolve all of its symbols, and displays a warning message to that effect.
The ultimate fix for this issue is a bit bigger than Open MPI, unfortunately — it’s a POSIX issue (as briefly described in the devel mailing list posting, above).

However, there are several common workarounds:
- Dynamically open libmpi in a public / global symbol scope — not a private / local scope. This will enable libmpi’s symbols to be available for resolution when Open MPI dynamically opens its plugins.
- If libmpi is opened as part of some underlying framework where it is not possible to change the private / local scope to a public / global scope, then dynamically open libmpi in a public / global scope before invoking the underlying framework. This sounds a little gross (and it is), but at least the run-time linker is smart enough to not load libmpi twice — but it does keeps libmpi in a public scope.
- Use the --disable-dlopen or --disable-mca-dso options to Open MPI’s configure script (see this TODO NONEXISTENT FAQ entry for more details on these options). These options slurp all of Open MPI’s plugins up in to libmpi — meaning that the plugins physically reside in libmpi and will not be dynamically opened at run time.
- Build Open MPI as a static library by configuring Open MPI with --disable-shared and --enable-static. This has the same effect as --disable-dlopen, but it also makes libmpi.a (as opposed to a shared library).

10.12.2. Errors about missing libraries

When building Open MPI 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 MPI 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$ mpirun -n 1 --host node1.example.com mpi_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$ mpirun -n 1 --host node1.example.com mpi_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$ mpirun -n 1 --host node1.example.com mpi_hello
prted: error while loading shared libraries: libmv.so: cannot open shared object file: No such file or directory
...more error messages...

Specifically, Open MPI 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 compiler environment on the target node, for both interactive and non-interactive logins.

Note

It is a common error to ensure that the compiler 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.

10.12.3. Problems when running across multiple hosts

When you are able to run MPI 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 Open MPI installations installed on a machine; the first Open MPI installation found by PATH and LD_LIBARY_PATH is the one that matters.
Run a simple, non-MPI job across multiple hosts. This verifies that the Open MPI run-time system is functioning properly across multiple hosts. For example, try running the hostname command:
```
shell$ mpirun --host remotehost hostname
remotehost
shell$ mpirun --host remotehost,otherhost hostname
remotehost
otherhost
```
If you are unable to run non-MPI 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 Open MPI is finding and launching the correct version of Open MPI on the remote hosts.
3. Ensure that you have firewalling disabled between hosts (Open MPI 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 Open MPI launches on remote hosts. For example:
```
mpirun --mca plm_base_verbose 10 --host remotehost hostname``
```
Now run a simple MPI job across multiple hosts that does not involve MPI communications. The hello_c program in the examples directory in the Open MPI distribution is a good choice. This verifies that the MPI subsystem is able to initialize and terminate properly. For example:
```
shell$ mpirun --host remotehost,otherhost hello_c
Hello, world, I am 0 of 1, (Open MPI VERSION, package: Open MPI jsquyres@example.com Distribution, ident: VERSION, DATE)
Hello, world, I am 1 of 1, (Open MPI VERSION, package: Open MPI jsquyres@example.com Distribution, ident: VERSION, DATE)
```
If you are unable to run simple, non-communication MPI jobs, this can indicate that your Open MPI installation is unable to initialize properly on remote hosts. Double check your non-interactive login setup on remote hosts.
Now run a simple MPI job across multiple hosts that does does some simple MPI communications. The ring_c program in the examples directory in the Open MPI distribution is a good choice. This verifies that the MPI subsystem is able to pass MPI traffic across your network. For example:
```
shell$ mpirun --host remotehost,otherhost ring_c
Process 0 sending 10 to 0, tag 201 (1 processes in ring)
Process 0 sent to 0
Process 0 decremented value: 9
Process 0 decremented value: 8
Process 0 decremented value: 7
Process 0 decremented value: 6
Process 0 decremented value: 5
Process 0 decremented value: 4
Process 0 decremented value: 3
Process 0 decremented value: 2
Process 0 decremented value: 1
Process 0 decremented value: 0
Process 0 exiting
```
If you are unable to run simple MPI jobs across multiple hosts, this may indicate a problem with the network(s) that Open MPI is trying to use for MPI communications. Try limiting the networks that it uses, and/or exploring levels 1 through 3 MCA parameters for the communications module that you are using. For example, if you’re using the TCP BTL, see the output of:
```
ompi_info --level 3 --param btl tcp
```