diff --git a/core/src/main/scala/org/apache/spark/Accumulators.scala b/core/src/main/scala/org/apache/spark/Accumulators.scala
index 30f0ccd73ccc..bcf832467f00 100644
--- a/core/src/main/scala/org/apache/spark/Accumulators.scala
+++ b/core/src/main/scala/org/apache/spark/Accumulators.scala
@@ -280,15 +280,24 @@ object AccumulatorParam {
 
 // TODO: The multi-thread support in accumulators is kind of lame; check
 // if there's a more intuitive way of doing it right
-private[spark] object Accumulators {
-  // Store a WeakReference instead of a StrongReference because this way accumulators can be
-  // appropriately garbage collected during long-running jobs and release memory
-  type WeakAcc = WeakReference[Accumulable[_, _]]
-  val originals = Map[Long, WeakAcc]()
-  val localAccums = new ThreadLocal[Map[Long, WeakAcc]]() {
-    override protected def initialValue() = Map[Long, WeakAcc]()
+private[spark] object Accumulators extends Logging {
+  /**
+   * This global map holds the original accumulator objects that are created on the driver.
+   * It keeps weak references to these objects so that accumulators can be garbage-collected
+   * once the RDDs and user-code that reference them are cleaned up.
+   */
+  val originals = Map[Long, WeakReference[Accumulable[_, _]]]()
+
+  /**
+   * This thread-local map holds per-task copies of accumulators; it is used to collect the set
+   * of accumulator updates to send back to the driver when tasks complete. After tasks complete,
+   * this map is cleared by `Accumulators.clear()` (see Executor.scala).
+   */
+  private val localAccums = new ThreadLocal[Map[Long, Accumulable[_, _]]]() {
+    override protected def initialValue() = Map[Long, Accumulable[_, _]]()
   }
-  var lastId: Long = 0
+
+  private var lastId: Long = 0
 
   def newId(): Long = synchronized {
     lastId += 1
@@ -297,16 +306,16 @@ private[spark] object Accumulators {
 
   def register(a: Accumulable[_, _], original: Boolean): Unit = synchronized {
     if (original) {
-      originals(a.id) = new WeakAcc(a)
+      originals(a.id) = new WeakReference[Accumulable[_, _]](a)
     } else {
-      localAccums.get()(a.id) = new WeakAcc(a)
+      localAccums.get()(a.id) = a
     }
   }
 
   // Clear the local (non-original) accumulators for the current thread
   def clear() {
     synchronized {
-      localAccums.get.clear
+      localAccums.get.clear()
     }
   }
 
@@ -320,12 +329,7 @@ private[spark] object Accumulators {
   def values: Map[Long, Any] = synchronized {
     val ret = Map[Long, Any]()
     for ((id, accum) <- localAccums.get) {
-      // Since we are now storing weak references, we must check whether the underlying data
-      // is valid.
-      ret(id) = accum.get match {
-        case Some(values) => values.localValue
-        case None => None
-      }
+      ret(id) = accum.localValue
     }
     return ret
   }
@@ -341,6 +345,8 @@ private[spark] object Accumulators {
           case None =>
             throw new IllegalAccessError("Attempted to access garbage collected Accumulator.")
         }
+      } else {
+        logWarning(s"Ignoring accumulator update for unknown accumulator id $id")
       }
     }
   }