fastcache.go: re-create the bucket.m map inside cleanLocked() in order to reduce map fragmentation

Hopefully, this may help reducing memory usage and GC pressure for long-running processes, which use fastcache.

See VictoriaMetrics/VictoriaMetrics#5379

Author: valyala

fastcache.go

@@ -272,13 +272,27 @@ func (b *bucket) cleanLocked() {
 	bGen := b.gen & ((1 << genSizeBits) - 1)
 	bIdx := b.idx
 	bm := b.m
-	for k, v := range bm {
+	newItems := 0
+	for _, v := range bm {
 		gen := v >> bucketSizeBits
 		idx := v & ((1 << bucketSizeBits) - 1)
 		if (gen+1 == bGen || gen == maxGen && bGen == 1) && idx >= bIdx || gen == bGen && idx < bIdx {
-			continue
+			newItems++
 		}
-		delete(bm, k)
+	}
+	if newItems < len(bm) {
+		// Re-create b.m with valid items, which weren't expired yet instead of deleting expired items from b.m.
+		// This should reduce memory fragmentation and the number Go objects behind b.m.
+		// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5379
+		bmNew := make(map[uint64]uint64, newItems)
+		for k, v := range bm {
+			gen := v >> bucketSizeBits
+			idx := v & ((1 << bucketSizeBits) - 1)
+			if (gen+1 == bGen || gen == maxGen && bGen == 1) && idx >= bIdx || gen == bGen && idx < bIdx {
+				bmNew[k] = v
+			}
+		}
+		b.m = bmNew
 	}
 }