Github user sryza commented on a diff in the pull request:
https://github.com/apache/spark/pull/2746#discussion_r18733008
--- Diff:
core/src/main/scala/org/apache/spark/scheduler/ExecutorScalingManager.scala ---
@@ -0,0 +1,324 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.scheduler
+
+import java.util.{Timer, TimerTask}
+
+import scala.collection.mutable
+
+import org.apache.spark.{Logging, SparkException}
+import org.apache.spark.scheduler.cluster.CoarseGrainedSchedulerBackend
+
+/**
+ * An agent that dynamically scales the number of executors based on the
workload.
+ *
+ * The add policy depends on the number of pending tasks. If the queue of
pending tasks has not
+ * been drained for N seconds, then new executors are added. If the queue
persists for another M
+ * seconds, then more executors are added and so on. The number added in
each round increases
+ * exponentially from the previous round until an upper bound on the
number of executors has
+ * been reached.
+ *
+ * The rationale for the exponential increase is twofold: (1) Executors
should be added slowly
+ * in the beginning in case the number of extra executors needed turns out
to be small. Otherwise,
+ * we may add more executors than we need just to remove them later. (2)
Executors should be added
+ * quickly over time in case the maximum number of executors is very high.
Otherwise, it will take
+ * a long time to ramp up under heavy workloads.
+ *
+ * The remove policy is simpler: If an executor has been idle, meaning it
has not been scheduled
+ * to run any tasks, for K seconds, then it is removed. This requires
starting a timer on each
+ * executor instead of just starting a global one as in the add case.
+ *
+ * The relevant Spark properties include the following:
+ * spark.dynamicAllocation.enabled - Whether this feature is enabled
+ * spark.dynamicAllocation.minExecutors - Lower bound on the number of
executors
+ * spark.dynamicAllocation.maxExecutors - Upper bound on the number of
executors
+ * spark.dynamicAllocation.addExecutorThreshold - How long before new
executors are added (N)
+ * spark.dynamicAllocation.addExecutorInterval - How often to add new
executors (M)
+ * spark.dynamicAllocation.removeExecutorThreshold - How long before an
executor is removed (K)
+ *
+ * Synchronization: Because the schedulers in Spark are single-threaded,
contention only arises
+ * if the application itself runs multiple jobs concurrently. Under normal
circumstances, however,
+ * synchronizing each method on this class should not be expensive
assuming biased locking is
+ * enabled in the JVM (on by default for Java 6+). Tighter locks are also
used where possible.
+ *
+ * Note: This is part of a larger implementation (SPARK-3174) and
currently does not actually
+ * request to add or remove executors. The mechanism to actually do this
will be added separately,
+ * e.g. in SPARK-3822 for Yarn.
+ */
+private[scheduler] class ExecutorScalingManager(scheduler:
TaskSchedulerImpl) extends Logging {
+ private val conf = scheduler.conf
+
+ // Lower and upper bounds on the number of executors. These are required.
+ private val minNumExecutors =
conf.getInt("spark.dynamicAllocation.minExecutors", -1)
+ private val maxNumExecutors =
conf.getInt("spark.dynamicAllocation.maxExecutors", -1)
+ if (minNumExecutors < 0 || maxNumExecutors < 0) {
+ throw new SparkException("spark.dynamicAllocation.{min/max}Executors
must be set!")
+ }
+
+ // How frequently to add and remove executors
+ private val addExecutorThreshold =
+ conf.getLong("spark.dynamicAllocation.addExecutorThreshold", 60) // s
+ private val addExecutorInterval =
+ conf.getLong("spark.dynamicAllocation.addExecutorInterval",
addExecutorThreshold) // s
+ private val removeExecutorThreshold =
+ conf.getLong("spark.dynamicAllocation.removeExecutorThreshold", 300)
// s
+
+ // Timers that keep track of when to add and remove executors
+ private var addExecutorTimer: Option[Timer] = None
+ private val removeExecutorTimers: mutable.Map[String, Timer] = new
mutable.HashMap[String, Timer]
+
+ // Number of executors to add in the next round
+ private var numExecutorsToAdd = 1
+
+ // The number of pending executors that have not actually been
added/removed yet
+ private var numExecutorsPendingToAdd = 0
+ private var numExecutorsPendingToRemove = 0
+
+ // Keep track of all executors here to decouple us from the logic in
TaskSchedulerImpl
+ private val executorIds = new mutable.HashSet[String] ++=
scheduler.executorIdToHost.keys
+
+ // Start idle timer for all new executors
+ synchronized { executorIds.foreach(startRemoveExecutorTimer) }
+
+ /**
+ * Start the add executor timer if it does not already exist.
+ * This is called when a new pending task is added. The add is then
triggered
+ * if the pending tasks queue is not drained in `addExecutorThreshold`
seconds.
+ */
+ def startAddExecutorTimer(): Unit =
startAddExecutorTimer(addExecutorThreshold)
+
+ /**
+ * Restart the add executor timer.
+ * This is called when the previous add executor timer has expired but
not canceled. The add
+ * is then triggered again if all pending executors from the previous
round have registered,
+ * and the pending tasks queue is still not drained in
`addExecutorInterval` seconds.
+ */
+ private def restartAddExecutorTimer(): Unit =
startAddExecutorTimer(addExecutorInterval)
+
+ /**
+ * Start the add executor timer using the given delay if the timer does
not already exist.
+ */
+ private def startAddExecutorTimer(timerDelaySeconds: Long): Unit = {
+ addExecutorTimer.synchronized {
+ if (addExecutorTimer.isEmpty) {
+ logDebug(s"Starting add executor timer (to expire in
$timerDelaySeconds seconds)")
+ addExecutorTimer = Some(new Timer)
+ addExecutorTimer.get.schedule(
+ new AddExecutorTimerTask(numExecutorsToAdd), timerDelaySeconds *
1000)
+ }
+ }
+ }
+
+ /**
+ * Start a timer to remove the given executor if the timer does not
already exist.
+ * This is called when the executor initially registers with the driver
or finishes running
+ * a task. The removal is then triggered if the executor stays idle
(i.e. not running a task)
+ * for `removeExecutorThreshold` seconds.
+ */
+ def startRemoveExecutorTimer(executorId: String): Unit = {
+ removeExecutorTimers.synchronized {
+ if (!removeExecutorTimers.contains(executorId)) {
+ logDebug(s"Starting idle timer for executor $executorId " +
+ s"(to expire in $removeExecutorThreshold seconds)")
+ removeExecutorTimers(executorId) = new Timer
+ removeExecutorTimers(executorId).schedule(
+ new RemoveExecutorTimerTask(executorId), removeExecutorThreshold
* 1000)
+ }
+ }
+ // Acquire a more general lock here because we might mutate
`executorId`
+ synchronized {
+ if (!executorIds.contains(executorId)) {
+ logWarning(s"Started idle timer for unknown executor $executorId.")
+ executorIds.add(executorId)
+ }
+ }
+ }
+
+ /**
+ * Cancel any existing timer that adds executors.
+ * This is called when the pending task queue is drained.
+ */
+ def cancelAddExecutorTimer(): Unit = addExecutorTimer.synchronized {
+ addExecutorTimer.foreach { timer =>
+ logDebug("Canceling add executor timer because task queue is
drained!")
+ timer.cancel()
+ numExecutorsToAdd = 1
+ addExecutorTimer = None
+ }
+ }
+
+ /**
+ * Cancel any existing timer that removes the given executor.
+ * This is called when the executor is no longer idle.
+ */
+ def cancelRemoveExecutorTimer(executorId: String): Unit =
removeExecutorTimers.synchronized {
+ if (removeExecutorTimers.contains(executorId)) {
+ logDebug(s"Canceling idle timer for executor $executorId.")
+ removeExecutorTimers(executorId).cancel()
+ removeExecutorTimers.remove(executorId)
+ }
+ }
+
+ /**
+ * Negotiate with the scheduler backend to add new executors.
+ * This ensures the resulting number of executors is correctly
constrained by the upper bound.
+ * Return the number of executors actually requested.
+ */
+ private def addExecutors(numExecutorsRequested: Int): Int = synchronized
{
+ val numExistingExecutors = executorIds.size + numExecutorsPendingToAdd
+ val numExecutorsToAdd =
+ if (numExistingExecutors + numExecutorsRequested <= maxNumExecutors)
{
+ numExecutorsRequested
+ } else {
+ // Add just enough to reach `maxNumExecutors`
+ maxNumExecutors - numExistingExecutors
+ }
+ val newNumExecutors = numExistingExecutors + numExecutorsToAdd
+
+ if (numExecutorsToAdd > 0) {
+ getCoarseGrainedBackend.foreach { backend =>
+ logInfo(s"Pending tasks are building up! " +
+ s"Adding $numExecutorsToAdd new executor(s) (new total is
$newNumExecutors).")
+ numExecutorsPendingToAdd += numExecutorsToAdd
+ backend.requestExecutors(numExecutorsToAdd)
+ return numExecutorsToAdd
+ }
+ } else {
+ logDebug(s"Not adding executors because there are already
$maxNumExecutors executors, " +
+ s"which is the limit.")
+ }
+ 0
+ }
+
+ /**
+ * Negotiate with the scheduler backend to remove existing executors.
+ * This ensures the resulting number of executors is correctly
constrained by the lower bound.
+ * Return whether the request to remove the executor is actually sent.
+ */
+ private def removeExecutor(executorId: String): Boolean = synchronized {
+ val numExistingExecutors = executorIds.size -
numExecutorsPendingToRemove
+ if (numExistingExecutors - 1 >= minNumExecutors) {
+ getCoarseGrainedBackend.foreach { backend =>
+ logInfo(s"Removing executor $executorId because it has been idle
for " +
+ s"$removeExecutorThreshold seconds (new total is
${numExistingExecutors - 1}).")
+ numExecutorsPendingToRemove += 1
+ backend.killExecutor(executorId)
+ return true
+ }
+ } else {
+ logDebug(s"Not removing idle executor $executorId because there are
only $minNumExecutors " +
+ "executor(s) left, which is the limit.")
+ }
+ false
+ }
+
+ /**
+ * Callback for the scheduler to signal that the given executor has been
added.
+ */
+ def executorAdded(executorId: String): Unit = synchronized {
+ if (!executorIds.contains(executorId)) {
+ logDebug(s"New executor $executorId has registered.")
+ if (numExecutorsPendingToAdd > 0) {
+ numExecutorsPendingToAdd -= 1
+ logDebug(s"Decrementing pending executors to add (now at
$numExecutorsPendingToAdd).")
+ }
+ executorIds.add(executorId)
+ startRemoveExecutorTimer(executorId)
+ }
+ }
+
+ /**
+ * Callback for the scheduler to signal that the given executor has been
removed.
+ */
+ def executorRemoved(executorId: String): Unit = synchronized {
+ if (executorIds.contains(executorId)) {
+ logDebug(s"Existing executor $executorId has been removed.")
+ executorIds.remove(executorId)
+ if (numExecutorsPendingToRemove > 0) {
+ numExecutorsPendingToRemove -= 1
+ logDebug(s"Decrementing pending executors to remove (now at
$numExecutorsPendingToRemove).")
+ }
+ } else {
+ logWarning(s"Not removing unknown executor $executorId")
--- End diff --
This is a little bit cryptic
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