[SPARK-13776][WEBUI] Limit the max number of acceptors and selectors for Jetty

## What changes were proposed in this pull request?

As each acceptor/selector in Jetty will use one thread, the number of threads should at least be the number of acceptors and selectors plus 1. Otherwise, the thread pool of Jetty server may be exhausted by acceptors/selectors and not be able to response any request.

To avoid wasting threads, the PR limits the max number of acceptors and selectors and also updates the max thread number if necessary.

## How was this patch tested?

Just make sure we don't break any existing tests

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #11615 from zsxwing/SPARK-13776.

core/src/main/scala/org/apache/spark/ui/JettyUtils.scala

@@ -25,7 +25,7 @@ import scala.collection.mutable.ArrayBuffer
 import scala.language.implicitConversions
 import scala.xml.Node
 
-import org.eclipse.jetty.server.{Connector, Request, Server}
+import org.eclipse.jetty.server.{AbstractConnector, Connector, Request, Server}
 import org.eclipse.jetty.server.handler._
 import org.eclipse.jetty.server.nio.SelectChannelConnector
 import org.eclipse.jetty.server.ssl.SslSelectChannelConnector
@@ -271,9 +271,24 @@ private[spark] object JettyUtils extends Logging {
 
       gzipHandlers.foreach(collection.addHandler)
       connectors.foreach(_.setHost(hostName))
+      // As each acceptor and each selector will use one thread, the number of threads should at
+      // least be the number of acceptors and selectors plus 1. (See SPARK-13776)
+      var minThreads = 1
+      connectors.foreach { c =>
+        // Currently we only use "SelectChannelConnector"
+        val connector = c.asInstanceOf[SelectChannelConnector]
+        // Limit the max acceptor number to 8 so that we don't waste a lot of threads
+        connector.setAcceptors(math.min(connector.getAcceptors, 8))
+        // The number of selectors always equals to the number of acceptors
+        minThreads += connector.getAcceptors * 2
+      }
       server.setConnectors(connectors.toArray)
 
       val pool = new QueuedThreadPool
+      if (serverName.nonEmpty) {
+        pool.setName(serverName)
+      }
+      pool.setMaxThreads(math.max(pool.getMaxThreads, minThreads))
       pool.setDaemon(true)
       server.setThreadPool(pool)
       val errorHandler = new ErrorHandler()