# 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:

1. 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.

2. 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.

3. 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``

4. 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.

5. 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