Github user StephanEwen commented on the issue:
https://github.com/apache/flink/pull/2559
Actually, just discovered that the problem is different all together.
While the KafkaConsumer is polling for new data (with a timeout), it holds
the consumer lock. If no data comes in Kafka, the lock is not released before
the poll timeout is over.
During that time, neither a "commitSync" nor "commitAsync" call can be
fired off. The `notifyCheckpointComplete` method hence blocks until the poll
timeout is over and the lock is released.
We can fix this by making sure that the consumer is "woken up" to release
the lock, and by making sure the lock acquisition is fair, so the committer
will get it next.
For the sake of releasing the lock fast in the committer method, it should
still be an asynchronous commit.
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