[jira] [Commented] (KAFKA-12464) Enhance constrained sticky Assign algorithm

["Luke Chen (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-12464?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17301391#comment-17301391 ] 

Luke Chen commented on KAFKA-12464:
-----------------------------------

[~ableegoldman] [~guozhang], could you help review this enhancement and have some comments? Thank you.

> Enhance constrained sticky Assign algorithm
> -------------------------------------------
>
>                 Key: KAFKA-12464
>                 URL: https://issues.apache.org/jira/browse/KAFKA-12464
>             Project: Kafka
>          Issue Type: Improvement
>    Affects Versions: 2.7.0
>            Reporter: Luke Chen
>            Assignee: Luke Chen
>            Priority: Major
>
> In KAFKA-9987, we did a great improvement for the case when all consumers were subscribed to same set of topics. The algorithm contains 4 phases:
>  # Reassign as many previously owned partitions as possible, up to the maxQuota
>  # Fill remaining members up to minQuota
>  # If we ran out of unassigned partitions before filling all consumers, we need to start stealing partitions from the over-full consumers at max capacity
>  # Otherwise we may have run out of unfilled consumers before assigning all partitions, in which case we should just distribute one partition each to all consumers at min capacity
>  
> Take an example for better understanding:
> *example:*
> Current status: 2 consumers (C0, C1), and 10 topic partitions: t1p0, t1p1, ... t1p9
> Suppose, current assignment is:
> _C0: t1p0, t1p1, t1p2, t1p3, t1p4_
>  _C1: t1p5, t1p6, t1p7, t1p8, t1p9_
> Now, new consumer added: C2, so we'll do:
>  # Reassign as many previously owned partitions as possible, up to the maxQuota
>  After this phase, the assignment will be: (maxQuota will be 4)
> _C0: t1p0, t1p1, t1p2, t1p3_
>  _C1: t1p5, t1p6, t1p7, t1p8_
>  # Fill remaining members up to minQuota
>  After this phase, the assignment will be:
> _C0: t1p0, t1p1, t1p2, t1p3_
>  _C1: t1p5, t1p6, t1p7, t1p8_
>  _C2: t1p4, t1p9_
>  # If we ran out of unassigned partitions before filling all consumers, we need to start stealing partitions from the over-full consumers at max capacity
>  After this phase, the assignment will be:
> _C0: t1p0, t1p1, t1p2_ 
>  _C1: t1p5, t1p6, t1p7, t1p8_
>  _C2: t1p4, t1p9,_ _t1p3_
>  # Otherwise we may have run out of unfilled consumers before assigning all partitions, in which case we should just distribute one partition each to all consumers at min capacity
>  
>  
> As we can see, we need 3 phases to complete the assignment. But we can actually completed with 2 phases. Here's the updated algorithm:
>  # Reassign as many previously owned partitions as possible, up to the maxQuota, and also considering the numMaxQuota by the remainder of (Partitions / Consumers)
>  # Fill remaining members up to maxQuota if current maxQuotaMember < numMaxQuota, otherwise, to minQuota
>  
> By considering the numMaxQuota, the original step 1 won't be too aggressive to assign too many partitions to consumers, and the step 2 won't be too conservative to assign not enough partitions to consumers, so that we don't need step 3 and step 4 to balance them.
>  
> {{So, the updated Pseudo-code sketch of the algorithm:}}
> C_f := (P/N)_floor, the floor capacity 
>  C_c := (P/N)_ceil, the ceiling capacity
> *C_r := (P%N) the allowed number of members with C_c partitions assigned*
>  *num_max_capacity_members := current number of members with C_c partitions assigned (default to 0)*
> members := the sorted set of all consumers
>  partitions := the set of all partitions
>  unassigned_partitions := the set of partitions not yet assigned, initialized to be all partitions
>  unfilled_members := the set of consumers not yet at capacity, initialized to empty
>  -max_capacity_members := the set of members with exactly C_c partitions assigned, initialized to empty-
>  member.owned_partitions := the set of previously owned partitions encoded in the Subscription
> // Reassign as many previously owned partitions as possible, *by considering the num_max_capacity_members*
>  for member : members
>       remove any partitions that are no longer in the subscription from its owned partitions
>       remove all owned_partitions if the generation is old
>       if member.owned_partitions.size < C_f
>           assign all owned partitions to member and remove from unassigned_partitions
>           add member to unfilled_members
>       -else if member.owned_partitions.size == C_f-
>           -assign first C_f owned_partitions to member and remove from unassigned_partitions-
>       else if member.owned_partitions.size >= C_c *&& num_max_capacity_members < C_r*
>           *assign first C_c owned_partitions to member and remove from unassigned_partitions*            
>          *num_max_capacity_members++*
>           a-dd member to max_capacity_members-
>      *else*
>            *assign first C_f owned_partitions to member and remove from unassigned_partitions*
> sort unassigned_partitions in partition order, ie t0_p0, t1_p0, t2_p0, t0_p1, t1_p0 .... (for data parallelism)
>  sort unfilled_members by memberId (for determinism)
> // Fill remaining members *up to the C_r numbers of C_c, otherwise, to C_f*
>  for member : unfilled_members
>       compute the remaining capacity as -C = C_f - num_assigned_partitions-
>          if num_max_capacity_members < C_r:
>                 C = C_c - num_assigned_partitions
>                 num_max_capacity_members++
>           else
>                C = C_f - num_assigned_partitions
>  pop the first C partitions from unassigned_partitions and assign to member
> -// Steal partitions from members with max_capacity if necessary-
>  -if we run out of partitions before getting to the end of unfilled members:-
>  -for member : unfilled_members-
>        -poll for first member in max_capacity_members and remove one partition-
>         -assign this partition to the unfilled member-
> -// Distribute remaining partitions, one per consumer, to fill some up to C_c if necessary-
>  -if we run out of unfilled_members before assigning all partitions:-
>  -for partition : unassigned_partitions-
>          -assign to next member in members that is not in max_capacity_members (then add member to max_capacity_members)-
>  
>  
> {code:java}
> C_f := (P/N)_floor, the floor capacity
> C_c := (P/N)_ceil, the ceiling capacity
> C_r := (P%N) the allowed number of members with C_c partitions assigned
> num_max_capacity_members := current number of members with C_c partitions assigned (default to 0)
>   
>  members := the sorted set of all consumers
>  partitions := the set of all partitions
>  unassigned_partitions := the set of partitions not yet assigned, initialized to be all partitions
>  unfilled_members := the set of consumers not yet at capacity, initialized to empty
>  member.owned_partitions := the set of previously owned partitions encoded in the Subscription
>   
>  // Reassign as many previously owned partitions as possible, by considering the num_max_capacity_members 
>  for member : members
>          remove any partitions that are no longer in the subscription from its owned partitions
>          remove all owned_partitions if the generation is old
>          if member.owned_partitions.size < C_f
>              assign all owned partitions to member and remove from unassigned_partitions
>              add member to unfilled_members
>          else if member.owned_partitions.size >= C_c && num_max_capacity_members < C_r
>              assign first C_c owned_partitions to member and remove from unassigned_partitions
>              num_max_capacity_members++
>          else 
>              assign first C_f owned_partitions to member and remove from unassigned_partitions
>   
>  sort unassigned_partitions in partition order, ie t0_p0, t1_p0, t2_p0, t0_p1, t1_p0 .... (for data parallelism)
>  sort unfilled_members by memberId (for determinism)
>   
>  // Fill remaining members up to the C_r numbers of C_c, otherwise, to C_f
>  for member : unfilled_members
>      compute the remaining capacity as:
>       if num_max_capacity_members < C_r: 
>           C = C_c - num_assigned_partitions
>           num_max_capacity_members++
>       else 
>           C = C_f - num_assigned_partitions
>      pop the first C partitions from unassigned_partitions and assign to member{code}
>  
>  So, adopting the updated algorithm, the previous example will be:
> *example:*
> Current status: 2 consumers (C0, C1), and 10 topic partitions: t1p0, t1p1, ... t1p9
> Suppose, current assignment is:
> _C0: t1p0, t1p1, t1p2, t1p3, t1p4_
>  _C1: t1p5, t1p6, t1p7, t1p8, t1p9_
> Now, new consumer added: C2, so we'll do:
>  # Reassign as many previously owned partitions as possible, up to the maxQuota
>  After this phase, the assignment will be: (maxQuota will be 4, *and numAllowedWithMaxQuota will be 1)*
> _C0: t1p0, t1p1, t1p2, t1p3_
>  _C1: t1p5, t1p6, t1p7_
>  # Fill remaining members up to maxQuota if current maxQuotaMember < numMaxQuota, otherwise, to minQuota
>  After this phase, the assignment will be:
> _C0: t1p0, t1p1, t1p2, t1p3_
>  _C1: t1p5, t1p6, t1p7_
>  _C2: t1p4, t1p8,_ _t1p9_
>  
> *Another enhancement:*
> Currently, in phase 1, we loop through all consumerToOwnedPartitions 1 by 1, to do partition assignment or partition remove. However, we doesn't care each partition info, what we only care, is the partition number assigned. So, we can just use List.subList() to get the expected number of subList from the consumerToOwnedPartitions. It'll also improve the performance of the algorithm.
>  
> That is:
> {code:java}
> // Reassign as many previously owned partitions as possible
> for (Map.Entry<String, List<TopicPartition>> consumerEntry : consumerToOwnedPartitions.entrySet()) {
>     String consumer = consumerEntry.getKey();
>     List<TopicPartition> ownedPartitions = consumerEntry.getValue();
>     List<TopicPartition> consumerAssignment = assignment.get(consumer);
>     int i = 0;
>     // assign the first N partitions up to the max quota, and mark the remaining as being revoked
>     // originally, we loop through the partitions 1 by 1
> /*
>     for (TopicPartition tp : ownedPartitions) {
>         if (i < maxQuota) {
>             consumerAssignment.add(tp);
>             unassignedPartitions.remove(tp);
>         } else {
>             allRevokedPartitions.add(tp);
>         }
>         ++i;
>     }
> */
>     
>     // enhancement: since we only care the number of partitions assigned, we can use subList to assign the expected number, so no need to loop through them all
>     if (ownedPartitions.size() >= maxQuota) {
>         consumerAssignment.addAll(ownedPartitions.subList(0, maxQuota));
>         unassignedPartitions.removeAll(ownedPartitions.subList(maxQuota,ownedPartitions.size());
>     } ....
>      .....
> {code}
>  



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