Out Of Memory is seen frequently while client produced message too quickly and used all the direct memory.
This happens like this:

1. Client send message to broker;
2. Broker call bookkeeper client to write message to Bookie, and hold the direct memory until write success;
   If in the 2nd step the direct memory not get released, then the Broker will get OOM.

It would be great to have a way to detect the memory pressure, and do the publish rate limit if lack of memory; once the memory get back, cancel the rate limit.