coll/libnbc: add recursive doubling algorithm for MPI_Iexscan

Implements recursive doubling algorithm for MPI_Iexscan.
The algorithm preserves order of operations so it can be used both
by commutative and non-commutative operations.

The MCA parameter 'coll_libnbc_iexscan_algorithm' was added for dynamic
algorithm selection.

Signed-off-by: Mikhail Kurnosov <mkurnosov@gmail.com>

Author: mkurnosov

ompi/mca/coll/libnbc/coll_libnbc.h

@@ -70,6 +70,7 @@ BEGIN_C_DECLS
 #define NBC_NUM_COLL 17
 
 extern bool libnbc_ibcast_skip_dt_decision;
+extern int libnbc_iexscan_algorithm;
 
 struct ompi_coll_libnbc_component_t {
     mca_coll_base_component_2_0_0_t super;

ompi/mca/coll/libnbc/coll_libnbc_component.c

@@ -46,6 +46,13 @@ static int libnbc_priority = 10;
 static bool libnbc_in_progress = false;     /* protect from recursive calls */
 bool libnbc_ibcast_skip_dt_decision = true;
 
+int libnbc_iexscan_algorithm = 0;             /* iexscan user forced algorithm */
+static mca_base_var_enum_value_t iexscan_algorithms[] = {
+    {0, "ignore"},
+    {1, "linear"},
+    {2, "recursive_doubling"},
+    {0, NULL}
+};
 
 static int libnbc_open(void);
 static int libnbc_close(void);
@@ -128,6 +135,8 @@ libnbc_close(void)
 static int
 libnbc_register(void)
 {
+    mca_base_var_enum_t *new_enum = NULL;
+
     /* Use a low priority, but allow other components to be lower */
     libnbc_priority = 10;
     (void) mca_base_component_var_register(&mca_coll_libnbc_component.super.collm_version,
@@ -158,6 +167,16 @@ libnbc_register(void)
                                            MCA_BASE_VAR_SCOPE_READONLY,
                                            &libnbc_ibcast_skip_dt_decision);
 
+    libnbc_iexscan_algorithm = 0;
+    (void) mca_base_var_enum_create("coll_libnbc_iexscan_algorithms", iexscan_algorithms, &new_enum);
+    mca_base_component_var_register(&mca_coll_libnbc_component.super.collm_version,
+                                    "iexscan_algorithm",
+                                    "Which iexscan algorithm is used: 0 ignore, 1 linear, 2 recursive_doubling",
+                                    MCA_BASE_VAR_TYPE_INT, new_enum, 0, MCA_BASE_VAR_FLAG_SETTABLE,
+                                    OPAL_INFO_LVL_5, MCA_BASE_VAR_SCOPE_ALL,
+                                    &libnbc_iexscan_algorithm);
+    OBJ_RELEASE(new_enum);
+
     return OMPI_SUCCESS;
 }