Reported by Jun Rao:
For over-partitioned topics, each broker could be the leader for at least 1 partition. In the producer, we randomly select a partition to send the data. Pretty soon, each producer will establish a connection to each of the n brokers. Effectively, we increased the # of socket connections by a factor of n, compared to 0.7.
The increased number of socket connections increases the number of open file handles, this could come pretty close to the OS limit.