Modified Master.scala to use numWorkersVisited and numWorkersAlive in…

…stead of stopPos

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

@@ -489,28 +489,24 @@ private[spark] class Master(
     // First schedule drivers, they take strict precedence over applications
     // Randomization helps balance drivers
     val shuffledAliveWorkers = Random.shuffle(workers.toSeq.filter(_.state == WorkerState.ALIVE))
-    val aliveWorkerNum = shuffledAliveWorkers.size
-
-    if (aliveWorkerNum > 0) {
-      var curPos = 0
-      var stopPos = aliveWorkerNum
-      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
-        while (curPos != stopPos && !launched) {
-          val worker = shuffledAliveWorkers(curPos)
-          if (worker.memoryFree >= driver.desc.mem && worker.coresFree >= driver.desc.cores) {
-            launchDriver(worker, driver)
-            waitingDrivers -= driver
-            launched = true
-          }
-          curPos = (curPos + 1) % aliveWorkerNum
+    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 = (stopPos + 1) % aliveWorkerNum
-        stopPos = curPos + aliveWorkerNum
+        curPos = (curPos + 1) % numWorkersAlive
       }
     }