Github user shivaram commented on the pull request:
https://github.com/apache/spark/pull/4783#issuecomment-76138824
@advancedxy -- Thanks for the detailed writeup and the gist. I can see the
problem but I am not sure this is the complete fix. You are right that
shellSizes don't need to be aligned to 8 bytes for superclasses and the
alignment only needs to happen on object boundaries. However I am not sure
this completely extends all the way -- i.e. when the super class has say a 1
byte field then I don't think we can start counting from 13 (12 for Object + 1
for byte).
While looking around I found this very nice tool called the Java Object
Layout [1] which prints out the layout with alignment, gaps given an object
name. You can see an example of the alignment gaps I am talking about in [2]
where super-classes with boolean fields end up being aligned to 4 bytes by
hotspot (I have no idea what the IBM / Oracle JVMs do).
I'll try to think if there is a more elegant fix and if not we can go with
this.
cc @rxin
[1] http://openjdk.java.net/projects/code-tools/jol/
[2]
http://hg.openjdk.java.net/code-tools/jol/file/6df1a9b2e5da/jol-samples/src/main/java/org/openjdk/jol/samples/JOLSample_06_Gaps.java
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