Add variant to mvapich2 to set size of rank bits (#3886)

var/spack/repos/builtin/packages/mvapich2/package.py

@@ -58,6 +58,11 @@ class Mvapich2(Package):
     variant('threads', default='multiple',
             description='Control the level of thread support')
 
+    # 32 is needed when job size exceeds 32768 cores
+    variant('ch3_rank_bits', default=32,
+            description='Number of bits allocated to the rank field (16 or 32)'
+            )
+
     ##########
     # TODO : Process managers should be grouped into the same variant,
     # as soon as variant capabilities will be extended See
@@ -262,6 +267,8 @@ def install(self, spec, prefix):
             "--enable-romio",
             "--disable-silent-rules",
             "--enable-threads={0}".format(spec.variants['threads'].value),
+            "--with-ch3-rank-bits={0}".format(
+                spec.variants['ch3_rank_bits'].value),
         ]
 
         if self.compiler.f77 and self.compiler.fc: