Minor tweak to scheduler cycle timing

Decrease maximimum scheduler main loop run time from 10 secs to
4 secs for improved performance.
If running with sched/backfill, do not run through all jobs on
periodic scheduling loop, but only the default depth. The
backfill scheduler can go through more jobs anyway due to its
ability to relinquish and recover locks.
See bug 616

src/slurmctld/job_scheduler.c

@@ -737,8 +737,8 @@ extern int schedule(uint32_t job_limit)
 #ifdef HAVE_BG
 	char *ionodes = NULL;
 	char tmp_char[256];
-	static bool backfill_sched = false;
 #endif
+	static bool backfill_sched = false;
 	static time_t sched_update = 0;
 	static bool wiki_sched = false;
 	static bool fifo_sched = false;
@@ -754,11 +754,11 @@ extern int schedule(uint32_t job_limit)
 		char *sched_params, *tmp_ptr;
 		char *sched_type = slurm_get_sched_type();
 		char *prio_type = slurm_get_priority_type();
-#ifdef HAVE_BG
+
 		/* On BlueGene, do FIFO only with sched/backfill */
 		if (strcmp(sched_type, "sched/backfill") == 0)
 			backfill_sched = true;
-#endif
+
 		if ((strcmp(sched_type, "sched/builtin") == 0) &&
 		    (strcmp(prio_type, "priority/basic") == 0) &&
 		    _all_partition_priorities_same())
@@ -800,9 +800,14 @@ extern int schedule(uint32_t job_limit)
 
 		sched_timeout = slurm_get_msg_timeout() / 2;
 		sched_timeout = MAX(sched_timeout, 1);
-		sched_timeout = MIN(sched_timeout, 10);
+		sched_timeout = MIN(sched_timeout, 4);
 		sched_update = slurmctld_conf.last_update;
 	}
+	/* Rather than periodicallly going to bottom of queue, let the
+	 * backfill scheduler do so since it can periodically relinquish
+	 * locks rather than blocking all RPCs. */
+	if ((job_limit == INFINITE) && backfill_sched)
+		job_limit = def_job_limit;
 	if (job_limit == 0)
 		job_limit = def_job_limit;