Update OpenMPI documentation

docs/software/communication/openmpi.md

@@ -4,21 +4,11 @@
 [Cray MPICH][ref-communication-cray-mpich] is the recommended MPI implementation on Alps.
 However, [OpenMPI](https://www.open-mpi.org/) can be used as an alternative in some cases, with limited support from CSCS.
 
-To use OpenMPI on Alps, it must be built against [libfabric][ref-communication-libfabric] with support for the [Slingshot 11 network][ref-alps-hsn].
+Support for the [Slingshot 11 network][ref-alps-hsn] is provided by the [libfabric][ref-communication-libfabric] library.
 
 ## Using OpenMPI
 
-!!! warning
-    Building and using OpenMPI on Alps is still [work in progress](https://eth-cscs.github.io/cray-network-stack/).
-    The instructions found on this page may be inaccurate, but are a good starting point to using OpenMPI on Alps.
-
-!!! todo
-    Deploy experimental uenv.
-
-!!! todo
-    Document OpenMPI uenv next to prgenv-gnu, prgenv-nvfortran, and linalg?
-
-OpenMPI is provided through a [uenv][ref-uenv] similar to [`prgenv-gnu`][ref-uenv-prgenv-gnu].
+OpenMPI is provided in the [`prgenv-gnu-openmpi`][ref-uenv-prgenv-gnu-openmpi] uenv.
 Once the uenv is loaded, compiling and linking with OpenMPI and libfabric is transparent.
 At runtime, some additional options must be set to correctly use the Slingshot network.
 
@@ -32,27 +22,55 @@ Additionally, the following environment variables should be set:
 ```bash
 export PMIX_MCA_psec="native" # (1)!
 export FI_PROVIDER="cxi" # (2)!
-export OMPI_MCA_pml="^ucx" # (3)!
+export OMPI_MCA_pml="cm" # (3)!
 export OMPI_MCA_mtl="ofi" # (4)!
 ```
 
 1. Ensures PMIx uses the same security domain as Slurm. Otherwise PMIx will print warnings at startup.
 2. Use the CXI (Slingshot) provider.
-3. Use anything except [UCX](https://openucx.org/documentation/) for [point-to-point communication](https://docs.open-mpi.org/en/v5.0.x/mca.html#selecting-which-open-mpi-components-are-used-at-run-time). The `^` signals that OpenMPI should exclude all listed components.
+3. Use CM for [point-to-point communication](https://docs.open-mpi.org/en/v5.0.x/mca.html#selecting-which-open-mpi-components-are-used-at-run-time).
 4. Use libfabric for the [Matching Transport Layer](https://docs.open-mpi.org/en/v5.0.x/mca.html#frameworks).
 
 !!! info "CXI provider does all communication through the network interface cards (NICs)"
     When using the libfabric CXI provider, all communication goes through NICs, including intra-node communication.
-    This means that intra-node communication can not make use of shared memory optimizations and the maximum bandwidth will not be severely limited.
+    This means that intra-node communication can not make use of shared memory optimizations and the maximum bandwidth will be severely limited.
+
+### Using the experimental LINKx provider
+
+The default configuration routes all communication through the NICs.
+While performance may sometimes be acceptable, this mode does not make full use of the much higher intra-node bandwidth available on Grace-Hopper nodes.
+In particular, GPU-GPU communication is significantly faster when using the appropriate intra-node links.
+
+The experimental [LINKx](https://ofiwg.github.io/libfabric/v2.3.1/man/fi_lnx.7.html) libfabric provider allows composing multiple libfabric providers for inter- and intra-node communication.
+The CXI provider can be used for inter-node communication while the shared memory (`shm`) provider can be used to take advantage of xpmem for CPU-CPU communication and GDRCopy for GPU-GPU communication.
+
+!!! warning "The LINKx provider is experimental and may contain bugs, in particular for intra-node communication"
+
+    A patch has been included in the [`prgenv-gnu-openmpi`][ref-uenv-prgenv-gnu-openmpi] uenv for [this LINKx issue](https://github.com/ofiwg/libfabric/issues/11231).
+    However, the patch may be incomplete and other issues may still be present.
+    Always validate your results to ensure MPI is working correctly.
+
+To use the LINKx provider, set `--mpi=pmix`, as without the LINKx provider.
+Additionally, set the following environment variables:
+
+```bash
+export PMIX_MCA_psec="native"
+export FI_PROVIDER="lnx" # (1)!
+export FI_LNX_PROV_LINKS="shm+cxi:cxi0|shm+cxi:cxi1|shm+cxi:cxi2|shm+cxi:cxi3" # (2)!
+export FI_SHM_USE_XPMEM=1 # (3)!
+export OMPI_MCA_pml="cm"
+export OMPI_MCA_mtl="ofi"
+export OMPI_MCA_mtl_ofi_av=table # (4)!
+```
 
-    Libfabric has a new [LINKx](https://ofiwg.github.io/libfabric/v2.1.0/man/fi_lnx.7.html) provider, which allows using different libfabric providers for inter- and intra-node communication.
-    This provider is not as well tested, but can in theory perform better for intra-node communication, because it can use shared memory.
-    To use the LINKx provider, set the following, instead of `FI_PROVIDER=cxi`:
+1. Use the libfabric LINKx provider, to allow using different libfabric providers for inter- and intra-node communication.
+2. Specify which providers LINKx should use.
+   Use the shared memory provider for intra-node communication and the CXI (Slingshot) provider for inter-node communication.
+   Choose one of the four available NICs on a node in a round-robin fashion.
+3. Explicitly use xpmem for CPU-CPU communication.
+   The default is to use CMA.
+4. The LINKx provider requires this option to be set. TODO: Better explanation?
 
-    ```bash
-    export FI_PROVIDER="lnx" # (1)!
-    export FI_LNX_PROV_LINKS="shm+cxi" # (2)!
-    ```
+## Known issues
 
-    1. Use the libfabric LINKx provider, to allow using different libfabric providers for inter- and intra-node communication.
-    2. Use the shared memory provider for intra-node communication and the CXI (Slingshot) provider for inter-node communication.
+TODO: async collectives don't work.

docs/software/prgenv/index.md

@@ -10,6 +10,10 @@ CSCS provides "programming environments" on Alps vClusters that provide compiler
     Provides compilers, MPI, tools and libraries built around the GNU compiler toolchain.
     It is the go to programming environment on all systems and target node types, that is it is the first that you should try out when starting to compile an application or create a python virtual environment.
 
+-   :fontawesome-solid-layer-group: [__prgenv-gnu-openmpi__][ref-uenv-prgenv-gnu-openmpi] uenv
+
+    Same as [`prgenv-gnu`][ref-uenv-prgenv-gnu] but with OpenMPI instead of Cray MPICH.
+
 -   :fontawesome-solid-layer-group: [__prgenv-nvfortran__][ref-uenv-prgenv-nvfortran] uenv
 
     Provides a set of tools and libraries for building applications that need the NVIDIA Fortran compiler, commonly required for OpenACC and CUDA-Fortran applications.

docs/software/prgenv/prgenv-gnu-openmpi.md

@@ -0,0 +1,18 @@
+[](){#ref-uenv-prgenv-gnu-openmpi}
+# prgenv-gnu-openmpi
+
+The `prgenv-gnu-openmpi` uenv is otherwise similar to [`prgenv-gnu`][ref-uenv-prgenv-gnu] except it provides OpenMPI instead of Cray MPICH.
+
+!!! warning "OpenMPI is not officially supported on CSCS systems"
+    Cray MPICH is the preferred, and officially supported, MPI implementation on CSCS systems.
+    OpenMPI is provided on a best effort basis.
+    While most applications should work correctly with OpenMPI, there may be missing features, broken functionality, or bad performance compared to Cray MPICH.
+    Issues are best reported upstream, but CSCS is happy to help facilitate and coordinate issue reporting if you [get in touch][ref-get-in-touch].
+
+Use of the uenv is similar to [`prgenv-gnu`][ref-uenv-prgenv-gnu].
+See the [OpenMPI documentation][ref-communication-openmpi] for important information on configuring OpenMPI to take advantage of the Slingshot network.
+
+### Versions
+
+=== "25.11"
+    TODO

docs/software/prgenv/prgenv-gnu.md

@@ -10,6 +10,8 @@ It is the go to programming environment on all systems and target node types, th
 
     The [`linalg`][ref-uenv-linalg] environment is similar to prgenv-gnu, with additional linear algebra and mesh partitioning algorithms.
 
+    The [`prgenv-gnu-openmpi`][ref-uenv-prgenv-gnu-openmpi] is otherwise similar to `prgenv-gnu`, but provides OpenMPI instead of Cray MPICH.
+
 [](){#ref-uenv-prgenv-gnu-versioning}
 ## Versioning