squah-confluent commented on code in PR #23733: URL: https://github.com/apache/kafka/pull/23733#discussion_r4266124033
########## group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/uniform2/AssignRemainingExtraPartitions.java: ########## @@ -0,0 +1,265 @@ +/* + * Licensed to the Apache Software Foundation (ASF) under one or more + * contributor license agreements. See the NOTICE file distributed with + * this work for additional information regarding copyright ownership. + * The ASF licenses this file to You under the Apache License, Version 2.0 + * (the "License"); you may not use this file except in compliance with + * the License. You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ +package org.apache.kafka.coordinator.group.assignor.uniform2; + +import org.apache.kafka.coordinator.group.assignor.uniform2.util.IntPair; +import org.apache.kafka.coordinator.group.assignor.uniform2.util.LongHeap; + +import java.util.Arrays; + +/** + * The second step deciding the {@link Quotas}: assigns the extra partitions that no owner kept, + * one at a time, to subscribers not having one of the topic yet, among the ones with the smallest + * assignments. + * + * <p>The topics with the fewest subscribers go first, so that they get the smallest of their few + * candidates: when the subscriptions are nested, this alone balances the assignment sizes. The + * extra partition goes to the cohort holding the smallest assignment, and among its members those + * with the smallest assignment are served round robin in member order. The member with the + * smallest assignment may already have an extra partition of the topic, and another member of the + * cohort then gets it: the balance step moves it if it has to. Without owners, a homogeneous group + * is served round robin. + * + * <p>On exit every topic has exactly its number of extra partitions, on distinct subscribers. + */ +final class AssignRemainingExtraPartitions { + /** + * The state of a cohort while the extra partitions are handed out. + */ + private static final class CohortState { + /** + * The members of the cohort. + */ + final int[] members; + + /** + * The position in the members of the next member to serve, round robin. + */ + int cursor; + + /** + * The lowest number of extra partitions of the members. + */ + int lowest; + + /** + * How many members have the lowest number of extra partitions. + */ + int atLowest; + + /** + * How many members have an extra partition of the topic being handed out. + */ + int holders; + + CohortState(int[] members) { + this.members = members; + } + } + + /** + * The members and topics of the group. + */ + private final IndexedGroup group; + + /** + * The quotas, which the step completes. + */ + private final Quotas quotas; + + /** + * The cohorts of the topic being handed out which can still take an extra partition of it, by + * increasing smallest assignment size among their members, whether that member has an extra + * partition of the topic or not, then in cohort order, see {@link #sizeAndCohort(int)}. + */ + private LongHeap candidates; + + /** + * Per cohort, its state. + */ + private CohortState[] cohortStates; + + /** + * Per member, whether it has an extra partition of the topic being handed out. + */ + private boolean[] hasExtra; + + /** + * @param group The members and topics of the group. + * @param quotas The quotas, as the first step left them, which the step completes. + */ + AssignRemainingExtraPartitions(IndexedGroup group, Quotas quotas) { + this.group = group; + this.quotas = quotas; + } + + /** + * @return The key of the cohort in the heap of candidates: the pair of the smallest assignment + * size among its members and the cohort, so that the heap orders the cohorts by that + * assignment size, then cohort order. That assignment size does not change while the + * cohort is a candidate, since only the members of a polled cohort get an extra + * partition. + */ + private long sizeAndCohort(int cohort) { + return IntPair.pack(quotas.minAssignmentSize(cohort) + cohortStates[cohort].lowest, cohort); + } + + /** + * Hands out the extra partitions that no owner kept. When the owners kept them all, as in a + * stable group, returns at once without allocating anything. + */ + void run() { + if (!hasExtraPartitionsToHandOut()) { + return; + } + cohortStates = new CohortState[group.cohortCount()]; + for (int cohort = 0; cohort < cohortStates.length; cohort++) { + cohortStates[cohort] = new CohortState(group.membersOf(cohort)); + findLowest(cohortStates[cohort]); + } + hasExtra = new boolean[group.memberCount()]; + candidates = new LongHeap(16); + for (int topic : topicsToHandOut()) { + handOutTopic(topic); + } + } + + /** + * @return {@code true} if a topic has extra partitions that no owner kept. + */ + private boolean hasExtraPartitionsToHandOut() { + for (int topic = 0; topic < group.topicCount(); topic++) { + if (quotas.membersWithExtraPartition(topic).size() < quotas.extraPartitions(topic)) { + return true; + } + } + return false; + } + + /** + * Hands out the extra partitions of the topic that no owner kept, one at a time, each to the + * next member of the cohort holding the smallest assignment among those still having a member + * without one. + */ + private void handOutTopic(int topic) { + // The members keeping an extra partition of the topic, which grows as the topic is handed + // out. + var members = quotas.membersWithExtraPartition(topic); + for (int i = 0; i < members.size(); i++) { + hasExtra[members.get(i)] = true; + cohortStates[group.cohortOf(members.get(i))].holders++; + } + // The cohorts of the topic with a member still without an extra partition of it, the only + // ones that can take one. + candidates.clear(); + for (int cohort : group.cohortsOf(topic)) { + var state = cohortStates[cohort]; + if (state.holders < state.members.length) { + candidates.add(sizeAndCohort(cohort)); + } + } + // The extra partitions that no owner kept, counted once since the members grow while they + // are handed out. There is at least one: topicsToHandOut only lists the topics with some. + for (int toHandOut = quotas.extraPartitions(topic) - members.size(); toHandOut > 0; toHandOut--) { + int cohort = IntPair.low(candidates.poll()); + var state = cohortStates[cohort]; + int member = nextMember(state); + int count = quotas.extraPartitionCount(member); + quotas.giveExtraPartition(member, topic); + hasExtra[member] = true; + // The last member of the cohort at the lowest number of extra partitions moves up. + if (count == state.lowest && --state.atLowest == 0) { + findLowest(state); + } + if (++state.holders < state.members.length) { + candidates.add(sizeAndCohort(cohort)); + } + } + // Resets the scratch state for the next topic, including the members just served. + for (int i = 0; i < members.size(); i++) { + hasExtra[members.get(i)] = false; + cohortStates[group.cohortOf(members.get(i))].holders = 0; + } + } + + /** + * @return The topics with extra partitions nobody kept, by increasing number of subscribers, + * then topic order. + */ + private int[] topicsToHandOut() { + // Pairs of the number of subscribers and the topic, which sort by subscribers, then topic. + var keys = new long[group.topicCount()]; + int count = 0; + for (int topic = 0; topic < group.topicCount(); topic++) { + if (quotas.extraPartitions(topic) > quotas.membersWithExtraPartition(topic).size()) { + keys[count++] = IntPair.pack(group.subscriberCount(topic), topic); + } + } + Arrays.sort(keys, 0, count); + var topics = new int[count]; + for (int i = 0; i < count; i++) { + topics[i] = IntPair.low(keys[i]); + } + return topics; + } + + /** + * Finds the lowest number of extra partitions of the members of the cohort, and how many of + * them have it. + */ + private void findLowest(CohortState state) { + int min = Integer.MAX_VALUE; + int count = 0; + for (int member : state.members) { + int extras = quotas.extraPartitionCount(member); + if (extras < min) { + min = extras; + count = 1; + } else if (extras == min) { + count++; + } + } + state.lowest = min; + state.atLowest = count; + } + + /** + * @return The next member of the cohort, in round robin order from the cursor, having the + * lowest number of extra partitions among its members and no extra partition of the + * topic. If every member having the lowest number has one, the first member without + * one having the fewest extra partitions. + */ + private int nextMember(CohortState state) { + var members = state.members; + for (int step = 0; step < members.length; step++) { + int index = (state.cursor + step) % members.length; + int member = members[index]; + if (quotas.extraPartitionCount(member) == state.lowest && !hasExtra[member]) { + state.cursor = (index + 1) % members.length; + return member; + } + } + // Every member at the lowest number has an extra partition of the topic. + int best = -1; + for (int member : members) { + if (!hasExtra[member] && (best < 0 || quotas.extraPartitionCount(member) < quotas.extraPartitionCount(best))) { + best = member; + } + } + return best; Review Comment: Something to think about later?: I'm a little suspicious of this part in terms of performance. If we ever fall into this case, we will iterate through the entire cohort's members for every extra partition of the topic we need to hand out. We will also never reorder the cohort in the min heap, so it's going to receive all the extra partitions of the topic until every member has one. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
