Use atomic operations and a read lock instead of a write lock

Author: shanson7

idx/memory/memory.go

@@ -7,6 +7,7 @@ import (
 	"sort"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/grafana/metrictank/errors"
@@ -213,20 +214,20 @@ func (m *MemoryIdx) Stop() {
 func (m *MemoryIdx) Update(point schema.MetricPoint, partition int32) (idx.Archive, int32, bool) {
 	pre := time.Now()
 
-	m.Lock()
-	defer m.Unlock()
+	m.RLock()
+	defer m.RUnlock()
 
 	existing, ok := m.defById[point.MKey]
 	if ok {
-		oldPart := existing.Partition
 		if LogLevel < 2 {
 			log.Debug("metricDef with id %v already in index", point.MKey)
 		}
 
 		if existing.LastUpdate < int64(point.Time) {
-			existing.LastUpdate = int64(point.Time)
+			atomic.SwapInt64(&existing.LastUpdate, int64(point.Time))
 		}
-		existing.Partition = partition
+
+		oldPart := atomic.SwapInt32(&existing.Partition, partition)
 		statUpdate.Inc()
 		statUpdateDuration.Value(time.Since(pre))
 		return *existing, oldPart, true