[jira] [Created] (KAFKA-3493) Replica fetcher load is not balanced over fetcher threads

["Maysam Yabandeh (JIRA)" <ji...@apache.org>]

Maysam Yabandeh created KAFKA-3493:
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             Summary: Replica fetcher load is not balanced over fetcher threads
                 Key: KAFKA-3493
                 URL: https://issues.apache.org/jira/browse/KAFKA-3493
             Project: Kafka
          Issue Type: Improvement
    Affects Versions: 0.9.0.1
            Reporter: Maysam Yabandeh
             Fix For: 0.10.0.1


The replicas are not evenly distributed among the fetcher threads. This has caused some fetcher threads get overloaded and hence their requests timeout frequently. This is especially a big issue when a new node is added to the cluster and the fetch traffic is large. 

Here is an example run in a test cluster with 10 brokers and 6 fetcher threads (per source broker). A single topic consisting of 500+ partitions was assigned to have a replica for each parition on the newly added broker.

{code}[kafka-jetstream.canary]myabandeh@sjc8c-rl17-23b:~$ for i in `seq 0 5`; do grep ReplicaFetcherThread-$i- /var/log/kafka/server.log | grep "reset its fetch offset from 0" | wc -l; done
85
83
85
83
85
85
[kafka-jetstream.canary]myabandeh@sjc8c-rl17-23b:~$ for i in `seq 0 5`; do grep ReplicaFetcherThread-$i-22 /var/log/kafka/server.log | grep "reset its fetch offset from 0" | wc -l; done
15
1
13
1
14
1
{code}

The problem is that AbstractFetcherManager::getFetcherId method does not take the broker id into account:
{code}
  private def getFetcherId(topic: String, partitionId: Int) : Int = {
    Utils.abs(31 * topic.hashCode() + partitionId) % numFetchers
  }
{code}
Hence although the replicas are evenly distributed among the fetcher ids across all source brokers, this is not necessarily the case for each broker separately. 

I think a random function would do a much better job in distributing the load over the fetcher threads from each source broker.

Thoughts?



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