[SPARK-10543] [CORE] Peak Execution Memory Quantile should be Per-task Basis

Read `PEAK_EXECUTION_MEMORY` using `update` to get per task partial value instead of cumulative value.

I tested with this workload:

```scala
val size = 1000
val repetitions = 10
val data = sc.parallelize(1 to size, 5).map(x => (util.Random.nextInt(size / repetitions),util.Random.nextDouble)).toDF("key", "value")
val res = data.toDF.groupBy("key").agg(sum("value")).count
```

Before:
![image](https://cloud.githubusercontent.com/assets/4317392/9828197/07dd6874-58b8-11e5-9bd9-6ba927c38b26.png)

After:
![image](https://cloud.githubusercontent.com/assets/4317392/9828151/a5ddff30-58b7-11e5-8d31-eda5dc4eae79.png)

Tasks view:
![image](https://cloud.githubusercontent.com/assets/4317392/9828199/17dc2b84-58b8-11e5-92a8-be89ce4d29d1.png)

cc andrewor14 I appreciate if you can give feedback on this since I think you introduced display of this metric.

Author: Forest Fang <forest.fang@outlook.com>

Closes #8726 from saurfang/stagepage.

core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala

@@ -368,7 +368,7 @@ private[ui] class StagePage(parent: StagesTab) extends WebUIPage("stage") {
           val peakExecutionMemory = validTasks.map { case TaskUIData(info, _, _) =>
             info.accumulables
               .find { acc => acc.name == InternalAccumulator.PEAK_EXECUTION_MEMORY }
-              .map { acc => acc.value.toLong }
+              .map { acc => acc.update.getOrElse("0").toLong }
               .getOrElse(0L)
               .toDouble
           }

core/src/test/scala/org/apache/spark/ui/StagePageSuite.scala

@@ -23,7 +23,7 @@ import scala.xml.Node
 
 import org.mockito.Mockito.{mock, when, RETURNS_SMART_NULLS}
 
-import org.apache.spark.{LocalSparkContext, SparkConf, SparkFunSuite, Success}
+import org.apache.spark._
 import org.apache.spark.executor.TaskMetrics
 import org.apache.spark.scheduler._
 import org.apache.spark.ui.jobs.{JobProgressListener, StagePage, StagesTab}
@@ -47,6 +47,14 @@ class StagePageSuite extends SparkFunSuite with LocalSparkContext {
     assert(html3.contains(targetString))
   }
 
+  test("SPARK-10543: peak execution memory should be per-task rather than cumulative") {
+    val unsafeConf = "spark.sql.unsafe.enabled"
+    val conf = new SparkConf(false).set(unsafeConf, "true")
+    val html = renderStagePage(conf).toString().toLowerCase
+    // verify min/25/50/75/max show task value not cumulative values
+    assert(html.contains("<td>10.0 b</td>" * 5))
+  }
+
   /**
    * Render a stage page started with the given conf and return the HTML.
    * This also runs a dummy stage to populate the page with useful content.
@@ -67,12 +75,19 @@ class StagePageSuite extends SparkFunSuite with LocalSparkContext {
 
     // Simulate a stage in job progress listener
     val stageInfo = new StageInfo(0, 0, "dummy", 1, Seq.empty, Seq.empty, "details")
-    val taskInfo = new TaskInfo(0, 0, 0, 0, "0", "localhost", TaskLocality.ANY, false)
-    jobListener.onStageSubmitted(SparkListenerStageSubmitted(stageInfo))
-    jobListener.onTaskStart(SparkListenerTaskStart(0, 0, taskInfo))
-    taskInfo.markSuccessful()
-    jobListener.onTaskEnd(
-      SparkListenerTaskEnd(0, 0, "result", Success, taskInfo, TaskMetrics.empty))
+    // Simulate two tasks to test PEAK_EXECUTION_MEMORY correctness
+    (1 to 2).foreach {
+      taskId =>
+        val taskInfo = new TaskInfo(taskId, taskId, 0, 0, "0", "localhost", TaskLocality.ANY, false)
+        val peakExecutionMemory = 10
+        taskInfo.accumulables += new AccumulableInfo(0, InternalAccumulator.PEAK_EXECUTION_MEMORY,
+          Some(peakExecutionMemory.toString), (peakExecutionMemory * taskId).toString, true)
+        jobListener.onStageSubmitted(SparkListenerStageSubmitted(stageInfo))
+        jobListener.onTaskStart(SparkListenerTaskStart(0, 0, taskInfo))
+        taskInfo.markSuccessful()
+        jobListener.onTaskEnd(
+          SparkListenerTaskEnd(0, 0, "result", Success, taskInfo, TaskMetrics.empty))
+    }
     jobListener.onStageCompleted(SparkListenerStageCompleted(stageInfo))
     page.render(request)
   }