Github user vanzin commented on a diff in the pull request:
https://github.com/apache/spark/pull/18231#discussion_r120943066
--- Diff:
common/network-shuffle/src/main/java/org/apache/spark/network/shuffle/ExternalShuffleBlockHandler.java
---
@@ -209,4 +190,51 @@ private ShuffleMetrics() {
}
}
+ private class ManagedBufferIterator implements Iterator<ManagedBuffer> {
+
+ private int index = 0;
+ private String appId;
+ private String execId;
+ private String shuffleId;
+ // An array containing mapId and reduceId pairs.
+ private int[][] mapIdAndReduceIds;
--- End diff --
Actually, I mean a single array. e.g.
```
int[] mapIdAndReduceIds;
mapIdAndReduceIds = new int[blockIds.length * 2];
mapIdAndReduceIds[0] = mapId1;
mapIdAndReduceIds[1] = reduceId1;
mapIdAndReduceIds[2] = mapId2;
mapIdAndReduceIds[3] = reduceId2;
etc etc etc
```
Reason being that if you really have millions of these, each "child" array
in your two-dimensional array wastes 16 (or 20?) bytes (16 bytes of object
overhead + 4 bytes for the array length). Looking in jvisual vm, an empty array
actually consumes 24 bytes, so it seems the JVM is aligning things and wasting
an extra 4 bytes per array...
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