Github user aarondav commented on the pull request:
https://github.com/apache/spark/pull/397#issuecomment-40320307
Apparently the JVM implements "biased locking"
(http://www.oracle.com/technetwork/java/6-performance-137236.html#2.1.1) when
an object's monitor is uncontended, which eliminates all synchronization
overhead (at the cost of an extremely expensive "bias revocation" operation if
any contention appears). So we would expect there to be no relevant performance
loss of this synchronization in this case, except perhaps for very small
streams which are completed before being biased.
However, the API of `def toByteArray: Array[Byte]` and `def toArray:
(Array[Byte], Int)` with different performance characteristics is pretty
strange. I think the current solution is probably fine, as the implementation
is pretty straightforward. I'll eat those words when the first JIRA comes back
with a bug introduced from this patch, though.
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