gnu: openmpi: Raise the priority of the PSM2 component.

This allows us to get better performance by default on machines with
OmniPath hardware.

* gnu/packages/patches/openmpi-psm2-priority.patch: New file.
* gnu/packages/mpi.scm (openmpi)[source]: Use it.
* gnu/local.mk (dist_patch_DATA): Add it.

gnu/packages/mpi.scm

@@ -174,7 +174,8 @@ bind processes, and much more.")
                           (version-major+minor version)
                           "/downloads/openmpi-" version ".tar.bz2"))
       (sha256
-       (base32 "0ms0zvyxyy3pnx9qwib6zaljyp2b3ixny64xvq3czv3jpr8zf2wh"))))
+       (base32 "0ms0zvyxyy3pnx9qwib6zaljyp2b3ixny64xvq3czv3jpr8zf2wh"))
+      (patches (search-patches "openmpi-psm2-priority.patch"))))
     (build-system gnu-build-system)
     (inputs
      `(("hwloc" ,hwloc-2 "lib")

gnu/packages/patches/openmpi-psm2-priority.patch

@@ -0,0 +1,21 @@
+PSM2 is the only valid implementation for OmniPath (OPA).  UCX also
+supports OmniPath, but it does so via Verbs, thus getting much lower
+performance (typically 3 GiB/s instead of 10 GiB/s).
+
+To work around that, give the mtl_psm2 component a higher priority
+than the pml_ucx component.
+
+See <https://www.mail-archive.com/users@lists.open-mpi.org/msg33581.html>.
+
+--- openmpi-4.0.2/ompi/mca/mtl/psm2/mtl_psm2_component.c	2019-11-15 17:06:15.142076840 +0100
++++ openmpi-4.0.2/ompi/mca/mtl/psm2/mtl_psm2_component.c	2019-11-15 17:06:22.242009379 +0100
+@@ -126,7 +126,7 @@ ompi_mtl_psm2_component_register(void)
+         setenv("PSM2_DEVICES", "self,shm", 0);
+     }
+ 
+-    param_priority = 40;
++    param_priority = 55;
+     (void) mca_base_component_var_register (&mca_mtl_psm2_component.super.mtl_version,
+                                             "priority", "Priority of the PSM2 MTL component",
+                                             MCA_BASE_VAR_TYPE_INT, NULL, 0, 0,
+