Major Description
As part of SAMZA-126, we introduced a Thread.sleep call in BrokerProxy's fetchMessages method. The goal was to skip fetch requests on SimpleConsumer when the topicAndPartitionsToFetch variable was empty. Since we had no topic/partitions to fetch, we slowed the thread down by calling Thread.sleep(sleepMSWhileNoTopicPartitions), which defaults to 1000ms.
We now see that we are only getting about 1mb/s when restoring changelogs. This is very slow. Upon investigation, it appears that the BrokerProxy thread is sleeping 90% of the time during restore, and the main SamzaContainer thread is polling for more messages about 60% of the time.
The reason for the poor restore performance is that the BrokerProxy sleeps for 1 second every time the message queue for the restore topic is not empty. Effectively, the proxy starts throttling the reads. If I comment out the Thread.sleep line in the BrokerProxy, I get about 64mb/s network usage on my loopback (one broker running locally), 10mb/s disk read, and 70mb/s disk write on my MacBook Air SSD--the write appears to be the bottleneck (since we're writing all the values to the LevelDB store). This is much much faster than before.