Memberlist: Avoid checking for conflicting tokens if no tokens changed.

If we can assume that the current ring state does not have conflicting
tokens, then we only need to check for conflicting tokens if the tokens
for any ingester actually changed. This will be the case for systems
not currently scaling up or down, as only heartbeats will be sent.

ring/model.go

@@ -202,11 +202,15 @@ func (d *Desc) mergeWithTime(mergeable memberlist.Mergeable, localCAS bool, now
 	otherIngesterMap := other.Ingesters
 
 	var updated []string
+	tokensChanged := false
 
 	for name, oing := range otherIngesterMap {
 		ting := thisIngesterMap[name]
 		// ting.Timestamp will be 0, if there was no such ingester in our version
 		if oing.Timestamp > ting.Timestamp {
+			if !tokensEqual(ting.Tokens, oing.Tokens) {
+				tokensChanged = true
+			}
 			oing.Tokens = append([]uint32(nil), oing.Tokens...) // make a copy of tokens
 			thisIngesterMap[name] = oing
 			updated = append(updated, name)
@@ -241,7 +245,7 @@ func (d *Desc) mergeWithTime(mergeable memberlist.Mergeable, localCAS bool, now
 	}
 
 	// resolveConflicts allocates lot of memory, so if we can avoid it, do that.
-	if conflictingTokensExist(thisIngesterMap) {
+	if tokensChanged && conflictingTokensExist(thisIngesterMap) {
 		resolveConflicts(thisIngesterMap)
 	}
 
@@ -303,6 +307,19 @@ func normalizeIngestersMap(inputRing *Desc) {
 	}
 }
 
+// tokensEqual checks for equality of two slices. Assumes the slices are sorted.
+func tokensEqual(lhs, rhs []uint32) bool {
+	if len(lhs) != len(rhs) {
+		return false
+	}
+	for i := 0; i < len(lhs); i++ {
+		if lhs[i] != rhs[i] {
+			return false
+		}
+	}
+	return true
+}
+
 var tokenMapPool = sync.Pool{New: func() interface{} { return make(map[uint32]struct{}) }}
 
 func conflictingTokensExist(normalizedIngesters map[string]InstanceDesc) bool {