Optimize allocations in compactor: use buffer for priorities

db.go

@@ -1617,7 +1617,7 @@ func (db *DB) Flatten(workers int) error {
 			}
 		}
 		if len(levels) <= 1 {
-			prios := db.lc.pickCompactLevels()
+			prios := db.lc.pickCompactLevels(nil)
 			if len(prios) == 0 || prios[0].score <= 1.0 {
 				db.opt.Infof("All tables consolidated into one level. Flattening done.\n")
 				return nil

levels.go

@@ -473,8 +473,13 @@ func (s *levelsController) runCompactor(id int, lc *z.Closer) {
 		}
 		return false
 	}
+
+	var priosBuffer []compactionPriority
 	runOnce := func() bool {
-		prios := s.pickCompactLevels()
+		prios := s.pickCompactLevels(priosBuffer)
+		defer func() {
+			priosBuffer = prios
+		}()
 		if id == 0 {
 			// Worker ID zero prefers to compact L0 always.
 			prios = moveL0toFront(prios)
@@ -536,7 +541,9 @@ func (s *levelsController) lastLevel() *levelHandler {
 
 // pickCompactLevel determines which level to compact.
 // Based on: https://github.com/facebook/rocksdb/wiki/Leveled-Compaction
-func (s *levelsController) pickCompactLevels() (prios []compactionPriority) {
+// It tries to reuse priosBuffer to reduce memory allocation,
+// passing nil is acceptable, then new memory will be allocated.
+func (s *levelsController) pickCompactLevels(priosBuffer []compactionPriority) (prios []compactionPriority) {
 	t := s.levelTargets()
 	addPriority := func(level int, score float64) {
 		pri := compactionPriority{
@@ -548,6 +555,12 @@ func (s *levelsController) pickCompactLevels() (prios []compactionPriority) {
 		prios = append(prios, pri)
 	}
 
+	// Grow buffer to fit all levels.
+	if cap(priosBuffer) < len(s.levels) {
+		priosBuffer = make([]compactionPriority, 0, len(s.levels))
+	}
+	prios = priosBuffer[:0]
+
 	// Add L0 priority based on the number of tables.
 	addPriority(0, float64(s.levels[0].numTables())/float64(s.kv.opt.NumLevelZeroTables))
 
@@ -1707,7 +1720,7 @@ type LevelInfo struct {
 
 func (s *levelsController) getLevelInfo() []LevelInfo {
 	t := s.levelTargets()
-	prios := s.pickCompactLevels()
+	prios := s.pickCompactLevels(nil)
 	result := make([]LevelInfo, len(s.levels))
 	for i, l := range s.levels {
 		l.RLock()