Merge branch 'issue-497' into 'preview'

Issue 497 - OpenMPI 4.1x. Performance Loss w.r.t. MPI-IO

Closes #497

See merge request !885

doc.zih.tu-dresden.de/docs/index.md

@@ -31,6 +31,7 @@ Please also find out the other ways you could contribute in our
 
 ## News
 
+* **2023-11-16** [OpenMPI 4.1.x  - Workaround for MPI-IO Performance Loss](jobs_and_resources/mpi_issues/#openmpi-v41x-performance-loss-with-mpi-io-module-ompio)
 * **2023-10-04** [User tests on Barnard](jobs_and_resources/barnard_test.md)
 * **2023-06-01** [New hardware and complete re-design](jobs_and_resources/architecture_2023.md)
 * **2023-01-04** [New hardware: NVIDIA Arm HPC Developer Kit](jobs_and_resources/arm_hpc_devkit.md)

doc.zih.tu-dresden.de/docs/jobs_and_resources/mpi_issues.md

@@ -2,6 +2,32 @@
 
 This pages holds known issues observed with MPI and concrete MPI implementations.
 
+## OpenMPI v4.1.x - Performance Loss with MPI-IO-Module OMPIO
+
+OpenMPI v4.1.x introduced a couple of major enhancements, e.g., the `OMPIO` module is now the
+default module for MPI-IO on **all** filesystems incl. Lustre (cf.
+[NEWS file in OpenMPI source code](https://raw.githubusercontent.com/open-mpi/ompi/v4.1.x/NEWS)).
+Prior to this, `ROMIO` was the default MPI-IO module for Lustre.
+
+Colleagues of ZIH have found that some MPI-IO access patterns suffer a significant performance loss
+using `OMPIO` as MPI-IO module with OpenMPI/4.1.x modules on ZIH systems. At the moment, the root
+cause is unclear and needs further investigation.
+
+**A workaround** for this performance loss is to use "old", i.e., `ROMIO` MPI-IO-module. This
+is achieved by setting the environment variable `OMPI_MCA_io` before executing the application as
+follows
+
+```console
+export OMPI_MCA_io=^ompio
+srun ...
+```
+
+or setting the option as argument, in case you invoke `mpirun` directly
+
+```console
+mpirun --mca io ^ompio ...
+```
+
 ## Mpirun on partition `alpha`and `ml`
 
 Using `mpirun` on partitions `alpha` and `ml` leads to wrong resource distribution when more than