[GitHub] [spark] cloud-fan commented on a change in pull request #26040: [SPARK-9853][Core] Optimize shuffle fetch of continuous partition IDs

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cloud-fan commented on a change in pull request #26040: [SPARK-9853][Core] Optimize shuffle fetch of continuous partition IDs
URL: https://github.com/apache/spark/pull/26040#discussion_r331999858
 
 

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 File path: common/network-shuffle/src/main/java/org/apache/spark/network/shuffle/protocol/FetchShuffleBlocks.java
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 @@ -36,19 +36,24 @@
   // it corresponds to the i-th int[] in reduceIds, which contains all reduce id for this map id.
   public final long[] mapIds;
   public final int[][] reduceIds;
+  // Indicates whether read continuous shuffle blocks in batch for IO reducing. When it is true,
+  // the reduceIds contains the range [startReduceId, endReduceId) for each ShuffleBlockBatchId.
 
 Review comment:
   I think it's clearer to document `reduceIds` instead of `batchFetchEnabled`. For example,
   ```
   when batchFetchEnabled=true, reduceIds[i] contains 2 elements: startReduceId (inclusive) and
   endReduceId (exclusive) for the mapper mapIds[i].
   when batchFetchEnabled=false, reduceIds[i] contains all the reduce IDs that mapper mapIds[i] needs to fetch.
   ```

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