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Developer-level technical information on the internals of Open MPI

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This FAQ is for Open MPI v4.x and earlier.
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Table of contents:

  1. Open MPI terminology
  2. How do I get a copy of the most recent source code?
  3. Ok, I got a Git checkout. Now how do I build it?
  4. What is the main tree layout of the Open MPI source tree? Are there directory name conventions?
  5. Is there more information available?

1. Open MPI terminology

Open MPI is a large project containing many different sub-systems and a relatively large code base. Let's first cover some fundamental terminology in order to make the rest of the discussion easier.

Open MPI has three sections of code:

  • OMPI: The MPI API and supporting logic
  • ORTE: The Open Run-Time Environment (support for different back-end run-time systems)
  • OPAL: The Open Portable Access Layer (utility and "glue" code used by OMPI and ORTE)

There are strict abstraction barriers in the code between these sections. That is, they are compiled into three separate libraries: libmpi, liborte, and libopal with a strict dependency order: OMPI depends on ORTE and OPAL, and ORTE depends on OPAL. More specifically, OMPI executables are linked with:

shell$ mpicc myapp.c -o myapp
# This actually turns into:
shell$ cc myapp.c -o myapp -lmpi -lopen-rte -lopen-pal ...

More system-level libraries may listed after -lopal, but you get the idea.

Strictly speaking, these are not "layers" in the classic software engineering sense (even though it is convenient to refer to them as such). They are listed above in dependency order, but that does not mean that, for example, the OMPI code must go through the ORTE and OPAL code in order to reach the operating system or a network interface.

As such, this code organization more reflects abstractions and software engineering, not a strict hierarchy of functions that must be traversed in order to reach a lower layer. For example, OMPI can call OPAL functions directly — it does not have to go through ORTE. Indeed, OPAL has a different set of purposes than ORTE, so it wouldn't even make sense to channel all OPAL access through ORTE. OMPI can also directly call the operating system as necessary. For example, many top-level MPI API functions are quite performance sensitive; it would not make sense to force them to traverse an arbitrarily deep call stack just to move some bytes across a network.

Here's a list of terms that are frequently used in discussions about the Open MPI code base:

  • MCA: The Modular Component Architecture (MCA) is the foundation upon which the entire Open MPI project is built. It provides all the component architecture services that the rest of the system uses. Although it is the fundamental heart of the system, its implementation is actually quite small and lightweight — it is nothing like CORBA, COM, JINI, or many other well-known component architectures. It was designed for HPC — meaning that it is small, fast, and reasonably efficient — and therefore offers few services other than finding, loading, and unloading components.

  • Framework: An MCA framework is a construct that is created for a single, targeted purpose. It provides a public interface that is used by external code, but it also has its own internal services. A list of Open MPI frameworks is available here. An MCA framework uses the MCA's services to find and load components at run-time — implementations of the framework's interface. An easy example framework to discuss is the MPI framework named "btl", or the Byte Transfer Layer. It is used to send and receive data on different kinds of networks. Hence, Open MPI has btl components for shared memory, TCP, Infiniband, Myrinet, etc.

  • Component: An MCA component is an implementation of a framework's interface. Another common word for component is "plugin". It is a standalone collection of code that can be bundled into a plugin that can be inserted into the Open MPI code base, either at run-time and/or compile-time.

  • Module: An MCA module is an instance of a component (in the C++ sense of the word "instance"; an MCA component is analogous to a C++ class). For example, if a node running an Open MPI application has multiple ethernet NICs, the Open MPI application will contain one TCP btl component, but two TCP btl modules. This difference between components and modules is important because modules have private state; components do not.

Frameworks, components, and modules can be dynamic or static. That is, they can be available as plugins or they may be compiled statically into libraries (e.g., libmpi).

2. How do I get a copy of the most recent source code?

See the instructions here.

3. Ok, I got a Git checkout. Now how do I build it?

See the instructions here.

4. What is the main tree layout of the Open MPI source tree? Are there directory name conventions?

There are a few notable top-level directories in the source tree:

  • config/: M4 scripts supporting the top-level configure script mpi.h)
  • etc/: Some miscellaneous text files
  • include/: Top-level include files that will be installed
  • ompi/: The Open MPI code base
  • orte/: The Open RTE code base
  • opal/: The OPAL code base

Each of the three main source directories (ompi/, orte/, and opal/) generate a top-level library named libmpi, liborte, and libopal, respectively. They can be built as either static or shared libraries. Executables are also produced in subdirectories of some of the trees.

Each of the sub-project source directories have similar (but not identical) directory structures under them:

  • class/: C++-like "classes" (using the OPAL class system) specific to this project
  • include/: Top-level include files specific to this project
  • mca/: MCA frameworks and components specific to this project
  • runtime/: Startup and shutdown of this project at runtime
  • tools/: Executables specific to this project (currently none in OPAL)
  • util/: Random utility code

There are other top-level directories in each of the three sub-projects, each having to do with specific logic and code for that project. For example, the MPI API implementations can be found under ompi/mpi/LANGUAGE, where LANGUAGE is c, cxx, f77, and f90.

The layout of the mca/ trees are strictly defined. They are of the form:

<project>/mca/<framework name>/<component name>/

To be explicit: it is forbidden to have a directory under the mca trees that does not meet this template (with the exception of base directories, explained below). Hence, only framework and component code can be in the mca/ trees.

That is, framework and component names must be valid directory names (and C variables; more on that later). For example, the TCP BTL component is located in the following directory:

# In v1.6.x and earlier:

# In v1.7.x and later: opal/mca/btl/tcp/

The name base is reserved; there cannot be a framework or component named "base." Directories named base are reserved for the implementation of the MCA and frameworks. Here are a few examples (as of the v1.8 source tree):

# Main implementation of the MCA

# Implementation of the btl framework opal/mca/btl/base

# Implementation of the rml framework orte/mca/rml/base

# Implementation of the pml framework ompi/mca/pml/base

Under these mandated directories, frameworks and/or components may have arbitrary directory structures, however.

5. Is there more information available?

Yes. In early 2006, Cisco hosted an Open MPI workshop where the Open MPI Team provided several days of intensive dive-into-the-code tutorials. The slides from these tutorials are available here.

Additionally, Greenplum videoed several Open MPI developers discussing Open MPI internals in 2012. The videos are available here.