The following text is taken directly from the Open MPI README file in the development branch.
Open MPI Version Numbers and Binary Compatibility
Open MPI has two sets of version numbers that are likely of interest
to end users / system administrator:
* Software version number
* Shared library version numbers
Both are predicated on Open MPI's definition of "backwards
NOTE: The version numbering conventions were changed with the release
of v1.10.0. Most notably, Open MPI no longer uses an "odd/even"
release schedule to indicate feature development vs. stable
releases. See the README in releases prior to v1.10.0 for more
Open MPI version Y is backwards compatible with Open MPI version X
(where Y>X) if users can:
* Compile an MPI/OpenSHMEM application with version X, mpirun/oshrun
it with version Y, and get the same user-observable behavior.
* Invoke ompi_info with the same CLI options in versions X and Y and
get the same user-observable behavior.
Note that this definition encompasses several things:
* Application Binary Interface (ABI)
* MPI / OpenSHMEM run time system
* mpirun / oshrun command line options
* MCA parameter names / values / meanings
However, this definition only applies when the same version of Open
MPI is used with all instances of the runtime and MPI / OpenSHMEM
processes in a single MPI job. If the versions are not exactly the
same everywhere, Open MPI is not guaranteed to work properly in any
Backwards compatibility tends to work best when user applications are
dynamically linked to one version of the Open MPI / OSHMEM libraries,
and can be updated at run time to link to a new version of the Open
MPI / OSHMEM libraries.
For example, if an MPI / OSHMEM application links statically against
the libraries from Open MPI vX, then attempting to launch that
application with mpirun / oshrun from Open MPI vY is not guaranteed to
work (because it is mixing vX and vY of Open MPI in a single job).
Similarly, if using a container technology that internally bundles all
the libraries from Open MPI vX, attempting to launch that container
with mpirun / oshrun from Open MPI vY is not guaranteed to work.
Software Version Number
Official Open MPI releases use the common "A.B.C" version identifier
format. Each of the three numbers has a specific meaning:
* Major: The major number is the first integer in the version string
Changes in the major number typically indicate a significant
change in the code base and/or end-user functionality, and also
indicate a break from backwards compatibility. Specifically: Open
MPI releases with different major version numbers are not
backwards compatibale with each other.
CAVEAT: This rule does not extend to versions prior to v1.10.0.
Specifically: v1.10.x is not guaranteed to be backwards
compatible with other v1.x releases.
* Minor: The minor number is the second integer in the version
string. Changes in the minor number indicate a user-observable
change in the code base and/or end-user functionality. Backwards
compatibility will still be preserved with prior releases that
have the same major version number (e.g., v2.5.3 is backwards
compatible with v2.3.1).
* Release: The release number is the third integer in the version
string. Changes in the release number typically indicate a bug
fix in the code base and/or end-user functionality. For example,
if there is a release that only contains bug fixes and no other
user-observable changes or new features, only the third integer
will be increased (e.g., from v4.3.0 to v4.3.1).
The "A.B.C" version number may optionally be followed by a Quantifier:
* Quantifier: Open MPI version numbers sometimes have an arbitrary
string affixed to the end of the version number. Common strings
o aX: Indicates an alpha release. X is an integer indicating the
number of the alpha release (e.g., v1.10.3a5 indicates the 5th
alpha release of version 1.10.3).
o bX: Indicates a beta release. X is an integer indicating the
number of the beta release (e.g., v1.10.3b3 indicates the 3rd
beta release of version 1.10.3).
o rcX: Indicates a release candidate. X is an integer indicating
the number of the release candidate (e.g., v1.10.3rc4 indicates
the 4th release candidate of version 1.10.3).
Nightly development snapshot tarballs use a different version number
scheme; they contain three distinct values:
* The git branch name from which the tarball was created.
* The date/timestamp, in YYYYMMDDHHMM format.
* The hash of the git commit from which the tarball was created.
For example, a snapshot tarball filename of
"openmpi-v2.x-201703070235-e4798fb.tar.gz" indicates that this tarball
was created from the v2.x branch, on March 7, 2017, at 2:35am GMT,
from git hash e4798fb.
Shared Library Version Number
The GNU Libtool official documentation details how the versioning
scheme works. The quick version is that the shared library versions
are a triple of integers: (current,revision,age), or "c:r:a". This
triple is not related to the Open MPI software version number. There
are six simple rules for updating the values (taken almost verbatim
from the Libtool docs):
1. Start with version information of "0:0:0" for each shared library.
2. Update the version information only immediately before a public
release of your software. More frequent updates are unnecessary,
and only guarantee that the current interface number gets larger
3. If the library source code has changed at all since the last
update, then increment revision ("c:r:a" becomes "c:r+1:a").
4. If any interfaces have been added, removed, or changed since the
last update, increment current, and set revision to 0.
5. If any interfaces have been added since the last public release,
then increment age.
6. If any interfaces have been removed since the last public release,
then set age to 0.
Here's how we apply those rules specifically to Open MPI:
1. The above rules do not apply to MCA components (a.k.a. "plugins");
MCA component .so versions stay unspecified.
2. The above rules apply exactly as written to the following
libraries starting with Open MPI version v1.5 (prior to v1.5,
libopen-pal and libopen-rte were still at 0:0:0 for reasons
discussed in bug ticket #2092
3. The following libraries use a slightly modified version of the
above rules: rules 4, 5, and 6 only apply to the official MPI and
OpenSHMEM interfaces (functions, global variables). The rationale
for this decision is that the vast majority of our users only care
about the official/public MPI/OpenSHMEM interfaces; we therefore
want the .so version number to reflect only changes to the
official MPI/OpenSHMEM APIs. Put simply: non-MPI/OpenSHMEM API /
internal changes to the MPI-application-facing libraries are
irrelevant to pure MPI/OpenSHMEM applications.