Github user sryza commented on the pull request:
https://github.com/apache/spark/pull/663#issuecomment-42370189
By default, ThreadPoolExecutor doesn't start all the core threads at once.
Though I just learned that when it gets tasks, it will start new ones even if
others are idle, so I agree that 500 seems high. Uploading a patch that
defaults to 25 and makes it configurable.
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