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https://issues.apache.org/jira/browse/KAFKA-2019?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14363625#comment-14363625
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Joseph Holsten commented on KAFKA-2019:
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[~becket_qin] could you provide an example of the worst case unbalance for this
code? I'm having trouble seeing how the hash-ordered round robit can have a
worst case more than number-of-consumer-threads.
Here's my reasoning:
With the current consumer-ordered sort, the threadAssignor will iterate over
the sequence:
- c0:t0
- c0:t1
- c0:t2
- c0:t3
- c1:t0
- c1:t1
- c1:t2
- c1:t3
With the hash-ordered sort, the sequence could have a best case of:
- c0:t0
- c1:t0
- c0:t1
- c1:t1
- c0:t2
- c1:t2
- c0:t3
- c1:t3
and a worst case identical to the consumer-ordered sort.
For a partition count of {{(n+1)*m}}, consumer-ordered will produce a
worst-case ordering every time, with {{c0}} always ending up with {{2m}}
partitions, and {{c1..cn}} ending up with m partitions. Hash-ordered will
produce this sort of worst case:
Likelyhood that {{c0}} will have all m threads selected:
{code}
C(n*m,m) / P(n*m,n*n) =
((n*m)!/m!((n*m)-(m))!) / ((n*m)!)
{code}
So the likelyhood of any consumer {{c0..cn} will have all its threads selected:
{code}
n * C(n*m,m) / P(n*m,n*n) =
n * ((n*m)!/m!((n*m)-(m))!) / ((n*m)!)
{code}
So the likelyhood that hash-ordering will be as unbalanced as consumer-ordered
with {{n=2}}, {{m=2}}:
{code}
2 * C(2*2,2) / P(2*2,2*2) =
2 * ((2*2)!/2!((2*2)-(2))!) / ((2*2)!) =
2 * (6) / (24) =
0.5
{code}
And with {{n=2}}, {{m=4}}:
{code}
2 * C(2*4,4) / P(2*4,2*2) =
2 * ((2*4)!/4!((2*4)-(4))!) / ((2*4)!) =
2 * 70 / 40320 =
~ 0.00347
{code}
I totally agree to the point about backward incompatibility, it would not be
pleasant to have different servers using different thread sequences. So this
code can't go in as a modification to the existing RoundRobinAssignor, perhaps
as a new assignor?
> RoundRobinAssignor clusters by consumer
> ---------------------------------------
>
> Key: KAFKA-2019
> URL: https://issues.apache.org/jira/browse/KAFKA-2019
> Project: Kafka
> Issue Type: Bug
> Components: consumer
> Reporter: Joseph Holsten
> Assignee: Neha Narkhede
> Priority: Minor
> Attachments: 0001-sort-consumer-thread-ids-by-hashcode.patch,
> KAFKA-2019.patch
>
>
> When rolling out a change today, I noticed that some of my consumers are
> "greedy", taking far more partitions than others.
> The cause is that the RoundRobinAssignor is using a list of ConsumerThreadIds
> sorted by toString, which is {{ "%s-%d".format(consumer, threadId)}}. This
> causes each consumer's threads to be adjacent to each other.
> One possible fix would be to define ConsumerThreadId.hashCode, and sort by
> that.
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