balance_semaphores() returns the number of unsemaphored jobs, and the second...

balance_semaphores() returns the number of unsemaphored jobs, and the second sync() is only done if a nonzero number of semaphored jobs were unsemaphored

modules/Bio/EnsEMBL/Hive/DBSQL/AnalysisJobAdaptor.pm

@@ -608,6 +608,8 @@ sub balance_semaphores {
     my $find_sth    = $self->prepare($find_sql);
     my $update_sth  = $self->prepare($update_sql);
 
+    my $rebalanced_jobs_counter = 0;
+
     $find_sth->execute();
     while(my ($job_id, $was, $should) = $find_sth->fetchrow_array()) {
         my $msg;
@@ -618,11 +620,14 @@ sub balance_semaphores {
             $update_sth->execute($should-$was, $job_id);
             $msg = "Semaphore count needed rebalancing now, so performing: $was -> $should";
             $self->db->get_LogMessageAdaptor->store_job_message( $job_id, $msg, 1 );
+            $rebalanced_jobs_counter++;
         }
         warn "[Job $job_id] $msg\n";    # TODO: integrate the STDERR diagnostic output with LogMessageAdaptor calls in general
     }
     $find_sth->finish;
     $update_sth->finish;
+
+    return $rebalanced_jobs_counter;
 }
 
 

modules/Bio/EnsEMBL/Hive/Scheduler.pm

@@ -67,16 +67,19 @@ sub schedule_workers_resync_if_necessary {
             $queen->check_for_dead_workers($valley, 1);
         }
 
+        print "Scheduler: re-synchronizing the Hive...\n";
+        $queen->synchronize_hive($filter_analysis);
+
         if($queen->db->hive_auto_rebalance_semaphores) {    # make sure rebalancing only ever happens for the pipelines that asked for it
             print "Scheduler: re-balancing of semaphore_counts...\n";
-            $queen->db->get_AnalysisJobAdaptor->balance_semaphores($filter_analysis && $filter_analysis->dbID);
+            if( my $rebalanced_jobs_counter = $queen->db->get_AnalysisJobAdaptor->balance_semaphores($filter_analysis && $filter_analysis->dbID) ) {
+                print "Scheduler: re-balanced $rebalanced_jobs_counter jobs, going through another re-synchronization of the Hive...\n";
+                $queen->synchronize_hive($filter_analysis);
+            }
         } else {
             print "Scheduler: automatic re-balancing of semaphore_counts is off by default. If you think your pipeline might benefit from it, set hive_auto_rebalance_semaphores => 1 in the PipeConfig's hive_meta_table.\n";
         }
 
-        print "Scheduler: re-synchronizing the Hive...\n";
-        $queen->synchronize_hive($filter_analysis);
-
         ($workers_to_submit_by_meadow_type_rc_name, $total_extra_workers_required, $log_buffer)
             = schedule_workers($queen, $submit_capacity, $default_meadow_type, undef, undef, $filter_analysis, $meadow_capacity_limiter_hashed_by_type, $analysis_id2rc_name);
         print $log_buffer;