Github user mengxr commented on the pull request:
https://github.com/apache/spark/pull/8182#issuecomment-132354472
No particular reason. Just want to have less hash collision while keeping
the computation cheap. 16 is certainly not sufficient. See
https://en.wikipedia.org/wiki/Java_hashCode()#The_java.lang.String_hash_function.
In practice, a sparse instance usually has less than a few hundred nonzeros. I
chose 64 for this reason, but I'm okay with 128, 256, 512, or 1024. However, a
value greater than 1024 would be too much.
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