Update the documentation for the LCI parcelport.

docs/sphinx/manual/using_the_lci_parcelport.rst

@@ -97,13 +97,29 @@ If you are using ``mpirun`` or ``srun``, you can just pass
 ``--hpx:ini=hpx.parcel.lci.priority=1000``, ``--hpx:ini=hpx.parcel.lci.enable=1``, and
 ``--hpx:ini=hpx.parcel.bootstrap=lci`` to the |hpx| applications.
 
+If you are running on a Cray machine, you need to pass `--mpi=pmix` or `--mpi=pmi2` to srun
+to enable the PMIx or PMI2 support of SLURM since LCI does not support the default Cray PMI.
+For example,
+
+.. code-block:: shell-session
+
+   $ srun --mpi=pmix [hpx application]
+
 .. _tune_lci_pp:
 
 Performance tuning of the LCI parcelport
 ========================================
 
-We encourage users to increase the zero-copy serialization threshold of LCI to ``8192`` (bytes) by
-either the cmake variable ``-DHPX_WITH_ZERO_COPY_SERIALIZATION_THRESHOLD=8192``
-(this will affect all the parcelports) or the command line option
-``--hpx:ini=hpx.parcel.lci.zero_copy_serialization_threshold=8192``
-(this will only affect the LCI parcelport).
+We encourage users to set the following environmental variables
+when using the LCI parcelport to get better performance.
+
+.. code-block:: shell-session
+
+   $ export LCI_SERVER_MAX_SENDS=1024
+   $ export LCI_SERVER_MAX_RECVS=4096
+   $ export LCI_SERVER_NUM_PKTS=65536
+   $ export LCI_SERVER_MAX_CQES=65536
+   $ export LCI_PACKET_SIZE=12288
+
+This setting needs roughly 800MB memory per process. The memory consumption mainly
+comes from the packets, which can be calculated using `LCI_SERVER_NUM_PKTS x LCI_PACKET_SIZE`.