Merge branch 'mpi-hints' into 'master'

Documenting MPI settings

See merge request !1074

README-mpi-hints.md

+Various hints on settings needed to get various MPIs running with SWIFT.
+
+Last update 5th May 2020.
+
+
+## Intel MPI
+
+_Intel MPI 2018_ usually runs without any needs for special settings.
+
+_Intel MPI 2019 and 2020_ can run for small tests, but without flags will
+generally deadlock in the MPI exchanges of the engine, or worse. In that case
+try the following settings.
+
+```
+  FI_OFI_RXM_RX_SIZE=4096
+  FI_OFI_RXM_TX_SIZE=4096
+  FI_UNIVERSE_SIZE=2048
+```
+
+If you want use the `release_mt` library, then you also need to use:
+
+```
+  source $I_MPI_ROOT/intel64/bin/mpivars.sh release_mt
+```
+
+when initializing the library environment. Some success has also been seen
+using the asynchronous progression settings:
+
+```
+  I_MPI_ASYNC_PROGRESS=1
+  I_MPI_ASYNC_PROGRESS_THREADS=1
+
+```
+(note these are tested with `2019 update-4` and `2020 update-1` on Mellanox).
+
+## OpenMPI
+
+_Open MPI_ comes in many flavours with many combinations of underlying
+transport libraries and running on many different fabrics. A complete
+description of all combinations is beyond the scope of this guide.
+
+On Mellanox hardware, we have had success running version 4.0 with the
+UCX layer version 1.6 and using the following settings:
+
+```
+-mca coll_hcoll_enable 0
+UCX_TLS=ud_x,shm,self
+UCX_RC_MLX5_TM_ENABLE=n
+UCX_DC_MLX5_TM_ENABLE=n
+```