Fix time tick lag too much (#23265)

Signed-off-by: Nlongjiquan <jiquan.long@zilliz.com>

internal/rootcoord/timeticksync.go

@@ -128,7 +128,9 @@ func newTimeTickSync(ctx context.Context, sourceID int64, factory msgstream.Fact
 
 		lock:           sync.Mutex{},
 		sess2ChanTsMap: make(map[typeutil.UniqueID]*chanTsMsg),
-		sendChan:       make(chan map[typeutil.UniqueID]*chanTsMsg, 16),
+
+		// 1 is the most reasonable capacity. In fact, Milvus can only focus on the latest time tick.
+		sendChan: make(chan map[typeutil.UniqueID]*chanTsMsg, 1),
 
 		syncedTtHistogram: newTtHistogram(),
 	}
@@ -169,7 +171,17 @@ func (t *timetickSync) sendToChannel() bool {
 		ptt[k] = v
 		t.sess2ChanTsMap[k] = nil
 	}
-	t.sendChan <- ptt
+
+	select {
+	case t.sendChan <- ptt:
+	default:
+		// The consumer of `sendChan` haven't completed its operation. If we send the `ptt` here, the consumer will
+		// always get an older time tick. The older time tick in `sendChan` will block newer time tick in next window.
+		// However, in fact the consumer can only focus on the newest.
+
+		// TODO: maybe a metric should be here.
+	}
+
 	return true
 }