Currently each upstream task needs to broadcast to every single partition of intermediate streams in order to aggregate watermarks in the consumers. It's O(n^2). For 256 tasks, 256-partition intermediate stream this can easily result in 64k msg/s if we send watermark every second. To illustrate:
T1 T2 T3
| \ /\ | /\ /\ |
P1 P2 P3
A better way to do this is to have only one downstream consumer doing the aggregation, and then broadcast to all the partitions. This is safe as we can do a simple proof: if P1 received watermark of t from all T1, T2, and T3, all the messages before t have been published to (P1, P2, P3) already (might not be consumed yet). So P1 can safely broadcast the watermark t to P2 and P3. To illustrate:
T1 T2 T3
\ | /
This reduced the full message count from O(n^2) to O. The cost is that this might introduce a few milliseconds delay since we need to exchange the message twice. The benefit clearly wins. In practice, the aggregate consumer can be decided from the (topic.hash() % total partitions) to spread the aggregation if we have multiple intermediate streams.