我正在对我的spark应用程序进行压力测试，该应用程序使用spark cassandra连接器和cassandra驱动程序。在我的应用程序中，我使用cassandra驱动程序从C* 表中选择最近的值。只要spark作业通过spark-job服务器一个接一个地提交，这就可以正常工作。但如果多个作业提交（请求数= 80）同时发生，则我将获得如下异常。

org.jboss.netty.channel.ChannelException: Failed to create a selector.
        at org.jboss.netty.channel.socket.nio.AbstractNioSelector.openSelector(AbstractNioSelector.java:343) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.AbstractNioSelector.<init>(AbstractNioSelector.java:100) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.AbstractNioWorker.<init>(AbstractNioWorker.java:52) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.NioWorker.<init>(NioWorker.java:45) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.NioWorkerPool.createWorker(NioWorkerPool.java:45) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.NioWorkerPool.createWorker(NioWorkerPool.java:28) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.AbstractNioWorkerPool.newWorker(AbstractNioWorkerPool.java:143) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.AbstractNioWorkerPool.init(AbstractNioWorkerPool.java:81) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.NioWorkerPool.<init>(NioWorkerPool.java:39) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.NioWorkerPool.<init>(NioWorkerPool.java:33) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.NioClientSocketChannelFactory.<init>(NioClientSocketChannelFactory.java:151) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.NioClientSocketChannelFactory.<init>(NioClientSocketChannelFactory.java:116) ~[netty-3.8.0.Final.jar:na]
        at com.datastax.driver.core.Connection$Factory.<init>(Connection.java:532) ~[cassandra-driver-core-2.1.5.jar:na]
        at com.datastax.driver.core.Cluster$Manager.<init>(Cluster.java:1201) ~[cassandra-driver-core-2.1.5.jar:na]
        at com.datastax.driver.core.Cluster$Manager.<init>(Cluster.java:1144) ~[cassandra-driver-core-2.1.5.jar:na]
        at com.datastax.driver.core.Cluster.<init>(Cluster.java:121) ~[cassandra-driver-core-2.1.5.jar:na]
        at com.datastax.driver.core.Cluster.<init>(Cluster.java:108) ~[cassandra-driver-core-2.1.5.jar:na]
        at com.datastax.driver.core.Cluster.buildFrom(Cluster.java:177) ~[cassandra-driver-core-2.1.5.jar:na]
        at com.datastax.driver.core.Cluster$Builder.build(Cluster.java:1109) ~[cassandra-driver-core-2.1.5.jar:na]
        ...
         Caused by: java.io.IOException: Too many open files
        at sun.nio.ch.IOUtil.makePipe(Native Method) ~[na:1.7.0_55]
        at sun.nio.ch.EPollSelectorImpl.<init>(EPollSelectorImpl.java:65) ~[na:1.7.0_55]
        at sun.nio.ch.EPollSelectorProvider.openSelector(EPollSelectorProvider.java:36) ~[na:1.7.0_55]
        at java.nio.channels.Selector.open(Selector.java:227) ~[na:1.7.0_55]
        at org.jboss.netty.channel.socket.nio.SelectorUtil.open(SelectorUtil.java:63) ~[netty-3.8.0.Final.jar:na]
        at org.jboss.netty.channel.socket.nio.AbstractNioSelector.openSelector(AbstractNioSelector.java:341) ~[netty-3.8.0.Final.jar:na]

我通过在spark job server中创建单个上下文来运行作业。
我的代码

val dateQuery = "SELECT st_date FROM %s limit 1"
 val queryString = dateQuery.format(tableName)
 val cluster = Cluster.builder().addContactPoints(cassandraHosts: _*)
  .withCredentials(username, password).build()
 val session = cluster.connect(keyspace)
 val queryResult = Try(session.execute(queryString).map(x => x.getDate("st_date")).head)
 cluster.close()

问题

代码中有我做错的地方吗？
如何解决这个问题？
我应该为整个应用程序创建一个单一集群对象并共享它吗？
我应该使用sc.cassandraTable方法而不是直接使用java驱动程序吗？