app/vmselect/netstorage: reduce memory allocations when unpacking time series data by using a pool for unpackWork entries

This should slightly reduce load on GC when processing queries that touch big number of time series.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/646 according to the provided memory profile there.

app/vmselect/netstorage/netstorage.go

@@ -13,6 +13,7 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/bytesutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/decimal"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fasttime"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/storage"
 	"github.com/VictoriaMetrics/metrics"
 )
@@ -172,10 +173,37 @@ type unpackWork struct {
 	br        storage.BlockRef
 	tr        storage.TimeRange
 	fetchData bool
-	doneCh    chan error
 	sb        *sortBlock
+	doneCh    chan error
 }
 
+func (upw *unpackWork) reset() {
+	upw.br = storage.BlockRef{}
+	upw.tr = storage.TimeRange{}
+	upw.fetchData = false
+	upw.sb = nil
+	if n := len(upw.doneCh); n > 0 {
+		logger.Panicf("BUG: upw.doneCh must be empty; it contains %d items now", n)
+	}
+}
+
+func getUnpackWork() *unpackWork {
+	v := unpackWorkPool.Get()
+	if v != nil {
+		return v.(*unpackWork)
+	}
+	return &unpackWork{
+		doneCh: make(chan error, 1),
+	}
+}
+
+func putUnpackWork(upw *unpackWork) {
+	upw.reset()
+	unpackWorkPool.Put(upw)
+}
+
+var unpackWorkPool sync.Pool
+
 func init() {
 	for i := 0; i < gomaxprocs; i++ {
 		go unpackWorker()
@@ -206,12 +234,10 @@ func (pts *packedTimeseries) Unpack(dst *Result, tr storage.TimeRange, fetchData
 	// Feed workers with work
 	upws := make([]*unpackWork, len(pts.brs))
 	for i, br := range pts.brs {
-		upw := &unpackWork{
-			br:        br,
-			tr:        tr,
-			fetchData: fetchData,
-			doneCh:    make(chan error, 1),
-		}
+		upw := getUnpackWork()
+		upw.br = br
+		upw.tr = tr
+		upw.fetchData = fetchData
 		unpackWorkCh <- upw
 		upws[i] = upw
 	}
@@ -230,6 +256,7 @@ func (pts *packedTimeseries) Unpack(dst *Result, tr storage.TimeRange, fetchData
 		} else if upw.sb != nil {
 			putSortBlock(upw.sb)
 		}
+		putUnpackWork(upw)
 	}
 	if firstErr != nil {
 		return firstErr