sched/backfill performance improvements

Remove duplicate backfill scheduling tests. For example there is
no need to test if a job can be started if the only difference
from the previous test involves nodes in other partitions that
can not be used by the job we are trying to start.

src/plugins/sched/backfill/backfill.c

@@ -611,6 +611,7 @@ static int _attempt_backfill(void)
 	uint32_t min_nodes, max_nodes, req_nodes;
 	bitstr_t *avail_bitmap = NULL, *resv_bitmap = NULL;
 	bitstr_t *exc_core_bitmap = NULL, *non_cg_bitmap = NULL;
+	bitstr_t *previous_bitmap = NULL;
 	time_t now, sched_start, later_start, start_res, resv_end;
 	node_space_map_t *node_space;
 	struct timeval bf_time1, bf_time2;
@@ -889,6 +890,7 @@ static int _attempt_backfill(void)
 
 		/* Determine impact of any resource reservations */
 		later_start = now;
+		FREE_NULL_BITMAP(previous_bitmap);
  TRY_LATER:
 		if (slurmctld_config.shutdown_time)
 			break;
@@ -900,7 +902,7 @@ static int _attempt_backfill(void)
 			job_ptr->time_limit = orig_time_limit;
 			if (debug_flags & DEBUG_FLAG_BACKFILL) {
 				END_TIMER;
-				info("backfill: completed yielding locks 2"
+				info("backfill: completed yielding locks "
 				     "after testing %d jobs, %s",
 				     job_test_count, TIME_STR);
 			}
@@ -984,12 +986,16 @@ static int _attempt_backfill(void)
 
 		/* Test if insufficient nodes remain OR
 		 *	required nodes missing OR
-		 *	nodes lack features */
+		 *	nodes lack features OR
+		 *	no change since previously tested nodes (only changes
+		 *	in other partition nodes) */
 		if ((bit_set_count(avail_bitmap) < min_nodes) ||
 		    ((job_ptr->details->req_node_bitmap) &&
 		     (!bit_super_set(job_ptr->details->req_node_bitmap,
 				     avail_bitmap))) ||
-		    (job_req_node_filter(job_ptr, avail_bitmap))) {
+		    (job_req_node_filter(job_ptr, avail_bitmap)) ||
+		    (previous_bitmap &&
+		     bit_equal(previous_bitmap, avail_bitmap))) {
 			if (later_start) {
 				job_ptr->start_time = 0;
 				goto TRY_LATER;
@@ -1000,6 +1006,9 @@ static int _attempt_backfill(void)
 			continue;
 		}
 
+		FREE_NULL_BITMAP(previous_bitmap);
+		previous_bitmap = bit_copy(avail_bitmap);
+
 		/* Identify nodes which are definitely off limits */
 		FREE_NULL_BITMAP(resv_bitmap);
 		resv_bitmap = bit_copy(avail_bitmap);
@@ -1143,6 +1152,7 @@ static int _attempt_backfill(void)
 	FREE_NULL_BITMAP(exc_core_bitmap);
 	FREE_NULL_BITMAP(resv_bitmap);
 	FREE_NULL_BITMAP(non_cg_bitmap);
+	FREE_NULL_BITMAP(previous_bitmap);
 
 	for (i=0; ; ) {
 		FREE_NULL_BITMAP(node_space[i].avail_bitmap);