12.8. Internal frameworks

The Modular Component Architecture (MCA) is the backbone of Open MPI – most services and functionality are implemented through MCA components.

12.8.1. MPI layer frameworks

Here is a list of all the component frameworks in the MPI layer of Open MPI:

• bml: BTL management layer

• coll: MPI collective algorithms

• fbtl: point to point file byte transfer layer: abstraction for individual read: collective read and write operations for MPI I/O

• fcoll: collective file system functions for MPI I/O

• fs: file system functions for MPI I/O

• hook: Generic hooks into Open MPI

• io: MPI I/O

• mtl: Matching transport layer, used for MPI point-to-point messages on some types of networks

• op: Back end computations for intrinsic MPI_Op operators

• osc: MPI one-sided communications

• pml: MPI point-to-point management layer

• sharedfp: shared file pointer operations for MPI I/O

• topo: MPI topology routines

• vprotocol: Protocols for the “v” PML

12.8.2. OpenSHMEM component frameworks

• atomic: OpenSHMEM atomic operations

• memheap: OpenSHMEM memory allocators that support the PGAS memory model

• scoll: OpenSHMEM collective operations

• spml: OpenSHMEM “pml-like” layer: supports one-sided, point-to-point operations

• sshmem: OpenSHMEM shared memory backing facility

12.8.3. Miscellaneous frameworks

• allocator: Memory allocator

• backtrace: Debugging call stack backtrace support

• btl: Point-to-point Byte Transfer Layer

• dl: Dynamic loading library interface

• hwloc: Hardware locality (hwloc) versioning support

• if: OS IP interface support

• installdirs: Installation directory relocation services

• memchecker: Run-time memory checking

• memcpy: Memory copy support

• memory: Memory management hooks

• mpool: Memory pooling

• patcher: Symbol patcher hooks

• pmix: Process management interface (exascale)

• rcache: Memory registration cache

• reachable: Network reachability determination

• shmem: Shared memory support (NOT related to OpenSHMEM)

• smsc: Shared memory single-copy support

• threads: OS and userspace thread support

• timer: High-resolution timers

12.8.4. Framework notes

Each framework typically has one or more components that are used at run-time. For example, the btl framework is used by the MPI layer to send bytes across different types underlying networks. The tcp btl, for example, sends messages across TCP-based networks; the ucx pml sends messages across InfiniBand-based networks.

12.8.5. MCA parameter notes

Each component typically has some tunable parameters that can be changed at run-time. Use the ompi_info(1) command to check a component to see what its tunable parameters are. For example:

shell$ ompi_info --param btl tcp

shows some of the parameters (and default values) for the tcp btl component (use --all or --level 9 to show all the parameters).

Note that ompi_info (without --all or a specified level) only shows a small number a component’s MCA parameters by default. Each MCA parameter has a “level” value from 1 to 9, corresponding to the MPI-3 MPI_T tool interface levels. See the LEVELS section in the ompi_info(1) man page for an explanation of the levels and how they correspond to Open MPI’s code.

Here’s rules of thumb to keep in mind when using Open MPI’s levels:

• Levels 1-3:

  • These levels should contain only a few MCA parameters.

  • Generally, only put MCA parameters in these levels that matter to users who just need to run Open MPI applications (and don’t know/care anything about MPI). Examples (these are not comprehensive):

    • Selection of which network interfaces to use.

    • Selection of which MCA components to use.

    • Selective disabling of warning messages (e.g., show warning message XYZ unless a specific MCA parameter is set, which disables showing that warning message).

    • Enabling additional stderr logging verbosity. This allows a user to run with this logging enabled, and then use that output to get technical assistance.

• Levels 4-6:

  • These levels should contain any other MCA parameters that are useful to expose to end users.

  • There is an expectation that “power users” will utilize these MCA parameters — e.g., those who are trying to tune the system and extract more performance.

  • Here’s some examples of MCA parameters suitable for these levels (these are not comprehensive):

    • When you could have hard-coded a constant size of a resource (e.g., a resource pool size or buffer length), make it an MCA parameter instead.

    • When there are multiple different algorithms available for a particular operation, code them all up and provide an MCA parameter to let the user select between them.

• Levels 7-9:

  • Put any other MCA parameters here.

  • It’s ok for these MCA parameters to be esoteric and only relevant to deep magic / the internals of Open MPI.

  • There is little expectation of users using these MCA parameters.

See this section for details on how to set MCA parameters at run time.