Add LINKx info

docs/merlin7/05-Software-Support/openmpi.md

@@ -31,7 +31,7 @@ Administrators recommend caution when relying on `unstable` versions for critica
     At present, we recommend using OpenMPI **5.0.8 or newer** on Merlin7, compiled with
     **libfabric/2.2.0-oss** or superior.
 
-### Using srun in Merlin7
+### Running OpenMPI on Merlin7
 
 In OpenMPI versions prior to 5.0.x, using `srun` for direct task launches was faster than `mpirun`.
 Although this is no longer the case, `srun` remains the recommended method due to its simplicity and ease of use.
@@ -40,19 +40,43 @@ Key benefits of `srun`:
 * Automatically handles task binding to cores.
 * In general, requires less configuration compared to `mpirun`.
 * Best suited for most users, while `mpirun` is recommended only for advanced MPI configurations.
+* However, `mpirun` has a much faster MPI initialization, which might be suitable for short runs.
 
-Guidelines:
-* Always adapt your scripts to use srun before seeking support.
-* For any module-related issues, please contact the Merlin7 administrators.
+For any module-related issues, please contact the Merlin7 administrators.
 
 Example Usage:
 ```bash
 srun ./app
+mpirun ./app
 ```
 
 !!! tip
     Always run OpenMPI applications with `srun` for a seamless experience.
 
+#### CXI vs LinkX provider
+
+Open MPI on Merlin7 is built with **libfabric** support. By default, libfabric uses the **CXI** (Cassini) provider
+on Slingshot-based networks, which is the standard behavior on Merlin7. However, the default CXI provider does not
+always deliver the best performance for applications with significant inter-node communication.
+
+To address this, a new provider plugin specifically designed for Open MPI, **LINKx**, has been built on Merlin7 using
+libfabric `-oss` releases. This provider is still considered **preview** and is therefore **not enabled by default**,
+but is **strongly recommended**.
+
+In addition, some environment variables must be configured explicitly, in particular `FI_LNX_PROV_LINKS`, whose value
+depends on the node type and on the resources assigned to the job. For this reason, if you want to use the LINKx
+provider, you should set:
+
+```bash
+# CPU nodes
+export FI_PROVIDER="lnx"
+export FI_LNX_PROV_LINKS="shm+cxi:cxi0"
+
+# GPU nodes
+export FI_PROVIDER="lnx"
+export FI_LNX_PROV_LINKS="shm+cxi:cxi0|shm+cxi:cxi1|shm+cxi:cxi2|shm+cxi:cxi3"
+```
+
 ### PMIx Support in Merlin7
 
 Merlin7's SLURM installation includes support for multiple PMI types, including pmix. To view the available options, use the following command: