--experimental-memory-mlock support

The flag protects etcd memory from being swapped out to disk.
This can happen in memory constrained systems where mmaped bbolt
area is natural condidate for swapping out.

This flag should provide better tail latency on the cost of higher RSS
ram usage. If the experiment is successful, the logic should get moved
into bbolt layer, where we can protect specific bbolt instances
(e.g. avoid protecting both during defragmentation).

server/etcdserver/server.go

@@ -316,6 +316,14 @@ func NewServer(cfg ServerConfig) (srv *EtcdServer, err error) {
 		)
 	}
 
+	if cfg.ExperimentalMemoryMlock {
+		cfg.Logger.Info("mlocking memory")
+		err := MlockAll()
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	if terr := fileutil.TouchDirAll(cfg.DataDir); terr != nil {
 		return nil, fmt.Errorf("cannot access data directory: %v", terr)
 	}