Github user lirui-intel commented on the pull request:
https://github.com/apache/spark/pull/1313#issuecomment-49264857
If a TaskSet only contains no-pref tasks, there won't be delay because the
only valid level is ANY, so everything gets scheduled right away.
If a TaskSet contains process-local + no-pref tasks (I suppose it should be
process-local + node-local + no-pref actually since process-local task is also
node-local?), there'll be delay on the no-pref tasks, even when all the
process-local tasks have been launched. This is because valid levels are not
re-computed as task finishes. Not sure if there'll be too much overhead to do
so... (it should also be re-computed when executor is lost)
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