[GitHub] [spark] squito commented on a change in pull request #25299: [SPARK-27651][Core] Avoid the network when shuffle blocks are fetched from the same host

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squito commented on a change in pull request #25299: [SPARK-27651][Core] Avoid the network when shuffle blocks are fetched from the same host
URL: https://github.com/apache/spark/pull/25299#discussion_r309769700
 
 

 ##########
 File path: core/src/main/scala/org/apache/spark/storage/ShuffleBlockFetcherIterator.scala
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 @@ -365,12 +387,46 @@ final class ShuffleBlockFetcherIterator(
     }
   }
 
+
+  /**
+   * Fetch the host-local blocks while we are fetching remote blocks. This is ok because
+   * `ManagedBuffer`'s memory is allocated lazily when we create the input stream, so all we
+   * track in-memory are the ManagedBuffer references themselves.
+   */
+  private[this] def fetchHostLocalBlocks() {
+    logDebug(s"Start fetching host-local blocks: ${hostLocalBlocks.mkString(", ")}")
+
+    val localDirsByExec =
+      blockManager.master.getHostLocalDirs(hostLocalBlocksByExecutor.keySet.toArray).localDirs
 
 Review comment:
   the only reason I can see for ever turning this feature off is the extra roundtrip to the driver required here.  But instead of doing this once-per-task, could we add a cache on each executor for this data?  It shouldn't change much, but I guess you would need to limit the size as you could in theory have execs come and go a ton.

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