lib/mergeset: move storageBlock from inmemoryPart to a sync.Pool

The lifetime of storageBlock is much shorter comparing to the lifetime of inmemoryPart,
so sync.Pool usage should reduce overall memory usage and improve performance
because of better locality of reference when marshaling inmemoryBlock to inmemoryPart.

https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2247

lib/mergeset/encoding.go

@@ -148,6 +148,21 @@ func (sb *storageBlock) Reset() {
 	sb.lensData = sb.lensData[:0]
 }
 
+func getStorageBlock() *storageBlock {
+	v := storageBlockPool.Get()
+	if v == nil {
+		return &storageBlock{}
+	}
+	return v.(*storageBlock)
+}
+
+func putStorageBlock(sb *storageBlock) {
+	sb.Reset()
+	storageBlockPool.Put(sb)
+}
+
+var storageBlockPool sync.Pool
+
 type marshalType uint8
 
 const (

lib/mergeset/inmemory_part.go

@@ -10,7 +10,6 @@ import (
 
 type inmemoryPart struct {
 	ph partHeader
-	sb storageBlock
 	bh blockHeader
 	mr metaindexRow
 
@@ -28,7 +27,6 @@ type inmemoryPart struct {
 
 func (mp *inmemoryPart) Reset() {
 	mp.ph.Reset()
-	mp.sb.Reset()
 	mp.bh.Reset()
 	mp.mr.Reset()
 
@@ -47,25 +45,27 @@ func (mp *inmemoryPart) Reset() {
 // Init initializes mp from ib.
 func (mp *inmemoryPart) Init(ib *inmemoryBlock) {
 	mp.Reset()
+	sb := getStorageBlock()
+	defer putStorageBlock(sb)
 
 	// Use the minimum possible compressLevel for compressing inmemoryPart,
 	// since it will be merged into file part soon.
 	// See https://github.com/facebook/zstd/releases/tag/v1.3.4 for details about negative compression level
 	compressLevel := -5
-	mp.bh.firstItem, mp.bh.commonPrefix, mp.bh.itemsCount, mp.bh.marshalType = ib.MarshalUnsortedData(&mp.sb, mp.bh.firstItem[:0], mp.bh.commonPrefix[:0], compressLevel)
+	mp.bh.firstItem, mp.bh.commonPrefix, mp.bh.itemsCount, mp.bh.marshalType = ib.MarshalUnsortedData(sb, mp.bh.firstItem[:0], mp.bh.commonPrefix[:0], compressLevel)
 
 	mp.ph.itemsCount = uint64(len(ib.items))
 	mp.ph.blocksCount = 1
 	mp.ph.firstItem = append(mp.ph.firstItem[:0], ib.items[0].String(ib.data)...)
 	mp.ph.lastItem = append(mp.ph.lastItem[:0], ib.items[len(ib.items)-1].String(ib.data)...)
 
-	fs.MustWriteData(&mp.itemsData, mp.sb.itemsData)
+	fs.MustWriteData(&mp.itemsData, sb.itemsData)
 	mp.bh.itemsBlockOffset = 0
-	mp.bh.itemsBlockSize = uint32(len(mp.sb.itemsData))
+	mp.bh.itemsBlockSize = uint32(len(mp.itemsData.B))
 
-	fs.MustWriteData(&mp.lensData, mp.sb.lensData)
+	fs.MustWriteData(&mp.lensData, sb.lensData)
 	mp.bh.lensBlockOffset = 0
-	mp.bh.lensBlockSize = uint32(len(mp.sb.lensData))
+	mp.bh.lensBlockSize = uint32(len(mp.lensData.B))
 
 	mp.unpackedIndexBlockBuf = mp.bh.Marshal(mp.unpackedIndexBlockBuf[:0])
 	mp.packedIndexBlockBuf = encoding.CompressZSTDLevel(mp.packedIndexBlockBuf[:0], mp.unpackedIndexBlockBuf, 0)