[SPARK-13803] restore the changes in SPARK-3411

## What changes were proposed in this pull request?

This patch contains the functionality to balance the load of the cluster-mode drivers among workers

This patch restores the changes in #1106 which was erased due to the merging of #731

## How was this patch tested?

test with existing test cases

Author: CodingCat <zhunansjtu@gmail.com>

Closes #11702 from CodingCat/SPARK-13803.

core/src/main/scala/org/apache/spark/deploy/master/Master.scala

@@ -727,15 +727,28 @@ private[deploy] class Master(
    * every time a new app joins or resource availability changes.
    */
   private def schedule(): Unit = {
-    if (state != RecoveryState.ALIVE) { return }
+    if (state != RecoveryState.ALIVE) {
+      return
+    }
     // Drivers take strict precedence over executors
-    val shuffledWorkers = Random.shuffle(workers) // Randomization helps balance drivers
-    for (worker <- shuffledWorkers if worker.state == WorkerState.ALIVE) {
-      for (driver <- waitingDrivers) {
+    val shuffledAliveWorkers = Random.shuffle(workers.toSeq.filter(_.state == WorkerState.ALIVE))
+    val numWorkersAlive = shuffledAliveWorkers.size
+    var curPos = 0
+    for (driver <- waitingDrivers.toList) { // iterate over a copy of waitingDrivers
+      // We assign workers to each waiting driver in a round-robin fashion. For each driver, we
+      // start from the last worker that was assigned a driver, and continue onwards until we have
+      // explored all alive workers.
+      var launched = false
+      var numWorkersVisited = 0
+      while (numWorkersVisited < numWorkersAlive && !launched) {
+        val worker = shuffledAliveWorkers(curPos)
+        numWorkersVisited += 1
         if (worker.memoryFree >= driver.desc.mem && worker.coresFree >= driver.desc.cores) {
           launchDriver(worker, driver)
           waitingDrivers -= driver
+          launched = true
         }
+        curPos = (curPos + 1) % numWorkersAlive
       }
     }
     startExecutorsOnWorkers()