agrawaldevesh commented on a change in pull request #28708:
URL: https://github.com/apache/spark/pull/28708#discussion_r439183738



##########
File path: 
core/src/main/scala/org/apache/spark/storage/BlockManagerDecommissioner.scala
##########
@@ -0,0 +1,265 @@
+/*
+ * 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.storage
+
+import java.util.concurrent.ExecutorService
+
+import scala.collection.JavaConverters._
+import scala.collection.mutable
+import scala.util.control.NonFatal
+
+import org.apache.spark._
+import org.apache.spark.internal.Logging
+import org.apache.spark.internal.config
+import org.apache.spark.shuffle.MigratableResolver
+import org.apache.spark.storage.BlockManagerMessages.ReplicateBlock
+import org.apache.spark.util.ThreadUtils
+
+/**
+ * Class to handle block manager decommissioning retries.
+ * It creates a Thread to retry offloading all RDD cache and Shuffle blocks
+ */
+private[storage] class BlockManagerDecommissioner(
+  conf: SparkConf,
+  bm: BlockManager) extends Logging {
+
+  private val maxReplicationFailuresForDecommission =
+    conf.get(config.STORAGE_DECOMMISSION_MAX_REPLICATION_FAILURE_PER_BLOCK)
+
+  private class ShuffleMigrationRunnable(peer: BlockManagerId) extends 
Runnable {
+    @volatile var running = true
+    override def run(): Unit = {
+      var migrating: Option[(Int, Long)] = None
+      logInfo(s"Starting migration thread for ${peer}")
+      // Once a block fails to transfer to an executor stop trying to transfer 
more blocks
+      try {
+        while (running && !Thread.interrupted()) {
+          val migrating = Option(shufflesToMigrate.poll())
+          migrating match {
+            case None =>
+              logInfo("Nothing to migrate")
+              // Nothing to do right now, but maybe a transfer will fail or a 
new block
+              // will finish being committed.
+              val SLEEP_TIME_SECS = 1
+              Thread.sleep(SLEEP_TIME_SECS * 1000L)
+            case Some((shuffleId, mapId)) =>
+              logInfo(s"Trying to migrate shuffle ${shuffleId},${mapId} to 
${peer}")
+              val blocks =
+                bm.migratableResolver.getMigrationBlocks(shuffleId, mapId)
+              logInfo(s"Got migration sub-blocks ${blocks}")
+              blocks.foreach { case (blockId, buffer) =>
+                logInfo(s"Migrating sub-block ${blockId}")
+                bm.blockTransferService.uploadBlockSync(
+                  peer.host,
+                  peer.port,
+                  peer.executorId,
+                  blockId,
+                  buffer,
+                  StorageLevel.DISK_ONLY,
+                  null)// class tag, we don't need for shuffle
+                logInfo(s"Migrated sub block ${blockId}")
+              }
+              logInfo(s"Migrated ${shuffleId},${mapId} to ${peer}")
+          }
+        }
+        // This catch is intentionally outside of the while running block.
+        // if we encounter errors migrating to an executor we want to stop.
+      } catch {
+        case e: Exception =>
+          migrating match {
+            case Some(shuffleMap) =>
+              logError(s"Error ${e} during migration, adding ${shuffleMap} 
back to migration queue")
+              shufflesToMigrate.add(shuffleMap)
+            case None =>
+              logError(s"Error ${e} while waiting for block to migrate")
+          }
+      }
+    }
+  }
+
+  // Shuffles which are either in queue for migrations or migrated
+  private val migratingShuffles = mutable.HashSet[(Int, Long)]()
+
+  // Shuffles which are queued for migration
+  private[storage] val shufflesToMigrate =
+    new java.util.concurrent.ConcurrentLinkedQueue[(Int, Long)]()
+
+  @volatile private var stopped = false
+
+  private val migrationPeers =
+    mutable.HashMap[BlockManagerId, (ShuffleMigrationRunnable, 
ExecutorService)]()
+
+  private lazy val blockMigrationExecutor =
+    ThreadUtils.newDaemonSingleThreadExecutor("block-manager-decommission")
+
+  private val blockMigrationRunnable = new Runnable {
+    val sleepInterval = 
conf.get(config.STORAGE_DECOMMISSION_REPLICATION_REATTEMPT_INTERVAL)
+
+    override def run(): Unit = {
+      var failures = 0
+      while (!stopped && !Thread.interrupted() && failures < 20) {
+        logInfo("Iterating on migrating from the block manager.")
+        try {
+          // If enabled we migrate shuffle blocks first as they are more 
expensive.
+          if (conf.get(config.STORAGE_SHUFFLE_DECOMMISSION_ENABLED)) {
+            logDebug("Attempting to replicate all shuffle blocks")
+            offloadShuffleBlocks()
+            logInfo("Done starting workers to migrate shuffle blocks")
+          }
+          if (conf.get(config.STORAGE_RDD_DECOMMISSION_ENABLED)) {
+            logDebug("Attempting to replicate all cached RDD blocks")
+            decommissionRddCacheBlocks()
+            logInfo("Attempt to replicate all cached blocks done")
+          }
+          if (!conf.get(config.STORAGE_RDD_DECOMMISSION_ENABLED) &&
+            !conf.get(config.STORAGE_SHUFFLE_DECOMMISSION_ENABLED)) {
+            logWarning("Decommissioning, but no task configured set one or 
both:\n" +
+              "spark.storage.decommission.shuffle_blocks\n" +
+              "spark.storage.decommission.rdd_blocks")
+          }
+          logInfo(s"Waiting for ${sleepInterval} before refreshing 
migrations.")
+          Thread.sleep(sleepInterval)
+        } catch {
+          case e: InterruptedException =>
+            logInfo("Interrupted during migration, will not refresh 
migrations.")
+            stopped = true
+          case NonFatal(e) =>
+            failures += 1
+            logError("Error occurred while trying to replicate cached RDD 
blocks" +
+              s" for block manager decommissioning (failure count: 
$failures)", e)
+        }
+      }
+    }
+  }
+
+  /**
+   * Tries to offload all shuffle blocks that are registered with the shuffle 
service locally.
+   * Note: this does not delete the shuffle files in-case there is an 
in-progress fetch
+   * but rather shadows them.
+   * Requires an Indexed based shuffle resolver.
+   * Note: if called in testing please call stopOffloadingShuffleBlocks to 
avoid thread leakage.
+   */
+  private[storage] def offloadShuffleBlocks(): Unit = {
+    // Update the queue of shuffles to be migrated
+    logInfo("Offloading shuffle blocks")
+    val localShuffles = bm.migratableResolver.getStoredShuffles()
+    val newShufflesToMigrate = localShuffles.&~(migratingShuffles).toSeq
+    shufflesToMigrate.addAll(newShufflesToMigrate.asJava)
+    migratingShuffles ++= newShufflesToMigrate
+
+    // Update the threads doing migrations
+    // TODO: Sort & only start as many threads as min(||blocks||, ||targets||) 
using location pref
+    val livePeerSet = bm.getPeers(false).toSet
+    val currentPeerSet = migrationPeers.keys.toSet
+    val deadPeers = currentPeerSet.&~(livePeerSet)
+    val newPeers = livePeerSet.&~(currentPeerSet)
+    migrationPeers ++= newPeers.map { peer =>
+      logDebug(s"Starting thread to migrate shuffle blocks to ${peer}")
+      val executor = 
ThreadUtils.newDaemonSingleThreadExecutor(s"migrate-shuffle-to-${peer}")

Review comment:
       This will spawn a thread per peer .. which is like per executor. I think 
we should bound this using a thread pool or something, otherwise this would 
scale with the cluster size. 




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