Github user andrewor14 commented on a diff in the pull request:
https://github.com/apache/spark/pull/2746#discussion_r19230787
--- Diff:
core/src/main/scala/org/apache/spark/scheduler/ExecutorAllocationManager.scala
---
@@ -0,0 +1,345 @@
+/*
+ * 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 scala.collection.mutable
+
+import org.apache.spark.{Logging, SparkException}
+import org.apache.spark.scheduler.cluster.CoarseGrainedSchedulerBackend
+
+/**
+ * An agent that dynamically allocates and removes executors based on the
workload.
+ *
+ * The add policy depends on the number of pending tasks. If the queue of
pending tasks is not
+ * drained in 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 for K
seconds (meaning it has not
+ * been scheduled to run any tasks), then it is removed. This requires
starting a timer on each
+ * executor instead of just starting a global one as in the add case.
+ *
+ * There is no retry logic in either case. Because the requests to the
cluster manager are
+ * asynchronous, this class does not know whether a request has been
granted until later. For
+ * this reason, both add and remove are treated as best-effort only.
+ *
+ * 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.addExecutorThresholdSeconds - How long before
new executors are added
+ * spark.dynamicAllocation.addExecutorIntervalSeconds - How often to add
new executors
+ * spark.dynamicAllocation.removeExecutorThresholdSeconds - How long
before an executor is removed
+ *
+ * Synchronization: Because the schedulers in Spark are single-threaded,
contention should only
+ * arise when new executors register or when existing executors are
removed, both of which are
+ * relatively rare events with respect to task scheduling. Thus,
synchronizing each method on the
+ * same lock should not be expensive assuming biased locking is enabled in
the JVM (on by default
+ * for Java 6+). This may not be true, however, if the application itself
runs multiple jobs
+ * concurrently.
+ *
+ * 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 ExecutorAllocationManager(scheduler:
TaskSchedulerImpl) extends Logging {
+ import ExecutorAllocationManager._
+
+ 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 (seconds)
+ private val addThresholdSeconds =
+ conf.getLong("spark.dynamicAllocation.addExecutorThresholdSeconds", 60)
+ private val addIntervalSeconds =
+ conf.getLong("spark.dynamicAllocation.addExecutorIntervalSeconds",
addThresholdSeconds)
+ private val removeThresholdSeconds =
+ conf.getLong("spark.dynamicAllocation.removeExecutorThresholdSeconds",
600)
+
+ // Number of executors to add in the next round
+ private var numExecutorsToAdd = 1
+
+ // Number of executors that have been requested but have not registered
yet
+ private var numExecutorsPendingToAdd = 0
+
+ // Executors that have been requested to be removed but have not been
killed yet
+ private val executorsPendingToRemove = new mutable.HashSet[String]
+
+ // Keep track of all executors here to decouple us from the logic in
TaskSchedulerImpl
+ private val executorIds = new mutable.HashSet[String]
+
+ // A timestamp of when the add timer should be triggered, or NOT_STARTED
if the timer is not
+ // started. This timer is started when there are pending tasks built up,
and canceled when
+ // there are no more pending tasks.
+ private var addTime = NOT_STARTED
+
+ // A timestamp for each executor of when the remove timer for that
executor should be triggered.
+ // Each remove timer is started when the executor first registers or
when the executor finishes
+ // running a task, and canceled when the executor is scheduled to run a
new task.
+ private val removeTimes = new mutable.HashMap[String, Long]
+
+ // A timestamp of when all pending add requests should expire
+ private var pendingAddExpirationTime = NOT_STARTED
+
+ // A timestamp for each executor of when the pending remove request for
the executor should expire
+ private val pendingRemoveExpirationTimes = new mutable.HashMap[String,
Long]
+
+ // How long before expiring pending requests to add or remove executors
(seconds)
+ private val pendingAddTimeoutSeconds = 300 // 5 min
+ private val pendingRemoveTimeoutSeconds = 300
+
+ // Polling loop interval (ms)
+ private val intervalMillis = 100
+
+ // Scheduler backend through which requests to add/remove executors are
made
+ // Note that this assumes the backend has already initialized when this
is first used
+ // Otherwise, an appropriate exception is thrown
+ private lazy val backend = scheduler.backend match {
+ case b: CoarseGrainedSchedulerBackend => b
+ case null =>
+ throw new SparkException("Scheduler backend not initialized yet!")
+ case _ =>
+ throw new SparkException(
+ "Dynamic allocation of executors is not applicable to fine-grained
schedulers. " +
+ "Please set spark.dynamicAllocation.enabled to false.")
+ }
+
+ initialize()
+
+ /**
+ * Start the main polling thread that keeps track of when to add and
remove executors.
+ * During each loop interval, this thread checks if any of the timers
have timed out, and,
+ * if so, triggers the relevant timer actions.
+ */
+ def initialize(): Unit = {
+ val thread = new Thread {
+ override def run(): Unit = {
+ while (true) {
+ ExecutorAllocationManager.this.synchronized {
+ val now = System.currentTimeMillis
+ try {
+ // If the add timer has timed out, add executors and refresh
the timer
+ if (addTime != NOT_STARTED && now >= addTime) {
+ addExecutors()
+ logDebug(s"Restarting add executor timer " +
+ s"(to be triggered in $addIntervalSeconds seconds)")
+ addTime += addIntervalSeconds * 1000
+ }
+
+ // If a remove timer has timed out, remove the executor and
cancel the timer
+ removeTimes.foreach { case (executorId, triggerTime) =>
+ if (now > triggerTime) {
+ removeExecutor(executorId)
+ cancelRemoveTimer(executorId)
+ }
+ }
+
+ // Expire any outstanding pending add requests that have
timed out
+ if (pendingAddExpirationTime != NOT_STARTED && now >=
pendingAddExpirationTime) {
+ logDebug(s"Expiring all pending add requests because they
have " +
+ s"not been fulfilled after $pendingAddTimeoutSeconds
seconds")
+ numExecutorsPendingToAdd = 0
+ pendingAddExpirationTime = NOT_STARTED
+ }
+
+ // Expire any outstanding pending remove requests that have
timed out
+ pendingRemoveExpirationTimes.foreach { case (executorId,
expirationTime) =>
+ if (now > expirationTime) {
+ logDebug(s"Expiring pending request to remove executor
$executorId because " +
+ s"it has not been fulfilled after
$pendingRemoveTimeoutSeconds seconds")
+ executorsPendingToRemove.remove(executorId)
+ pendingRemoveExpirationTimes.remove(executorId)
+ }
+ }
+ } catch {
+ case e: Exception => logError("Exception in dynamic executor
allocation thread!", e)
+ }
+ }
+ Thread.sleep(intervalMillis)
+ }
+ }
+ }
+ thread.setName("spark-dynamic-executor-allocation")
+ thread.setDaemon(true)
+ thread.start()
+ }
+
+ /**
+ * Request a number of executors from the scheduler backend.
+ * If the cap on the number of executors is reached, give up and reset
the
+ * number of executors to add next round instead of continuing to double
it.
+ */
+ private def addExecutors(): Unit = synchronized {
+ // Do not request more executors if we have already reached the upper
bound
+ val numExistingExecutors = executorIds.size + numExecutorsPendingToAdd
+ if (numExistingExecutors >= maxNumExecutors) {
+ logDebug(s"Not adding executors because there are already " +
+ s"$maxNumExecutors executor(s), which is the limit")
+ numExecutorsToAdd = 1
+ return
+ }
+
+ // Request executors with respect to the upper bound
+ val actualNumExecutorsToAdd =
+ math.min(numExistingExecutors + numExecutorsToAdd, maxNumExecutors)
- numExistingExecutors
--- End diff --
I think it's even clearer if we avoid using `math.min`:
```
val actualNumExecutorsToAdd =
if (numExistingExecutors + numExecutorsToAdd > maxNumExecutors) {
maxNumExecutors - numExistingExecutors
} else {
numExecutorsToAdd
}
```
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