ws: Memory leak?

my code:

var WebSocketServer = require('ws').Server
  , wss = new WebSocketServer({ port: 8081 });

wss.on('connection', function connection(ws) {
  ws.on('message', function incoming(message) {
    console.log('received: %s', message);
  });

  ws.send('something');
});

When 20000 clients connected, memory usage is 2.864 GB. Then close all clients, After a few minute, memory usage is 2.188GB. And, after 20000 new clients connected, memory usage is 4.137 GB. It looks like that memory did not free.