[GitHub] [spark] maropu commented on a change in pull request #29498: [SPARK-32674][DOC] Add suggestion for parallel directory listing in tuning doc

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maropu commented on a change in pull request #29498:
URL: https://github.com/apache/spark/pull/29498#discussion_r474340280



##########
File path: docs/tuning.md
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@@ -264,6 +264,13 @@ parent RDD's number of partitions. You can pass the level of parallelism as a se
 or set the config property `spark.default.parallelism` to change the default.
 In general, we recommend 2-3 tasks per CPU core in your cluster.
 
+Sometimes you may also need to increase directory listing parallelism when job input has large number of directories,
+otherwise the process could take a very long time, especially when against object store like S3.
+If your job works on RDD with Hadoop input formats (e.g., via `SparkContext#sequenceFile`), the parallelism is
+controlled via `spark.hadoop.mapreduce.input.fileinputformat.list-status.num-threads` (default is 1). For other
+cases such as Spark SQL, you can tune `spark.sql.sources.parallelPartitionDiscovery.threshold` to improve the listing

Review comment:
       I think the last statement should be described in the SQL side: https://github.com/apache/spark/blob/master/docs/sql-performance-tuning.md




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