[GitHub] [druid] clintropolis commented on a change in pull request #11257: Reduce method invocation of reservoir sampling

[GitBox <gi...@apache.org>]

clintropolis commented on a change in pull request #11257:
URL: https://github.com/apache/druid/pull/11257#discussion_r648885387



##########
File path: server/src/main/java/org/apache/druid/server/coordinator/duty/BalanceSegments.java
##########
@@ -180,22 +181,29 @@ private void balanceTier(
       int maxSegmentsToMove
   )
   {
+    if (maxSegmentsToMove <= 0) {
+      return new Pair<>(0, 0);
+    }
+
     final BalancerStrategy strategy = params.getBalancerStrategy();
     final int maxIterations = 2 * maxSegmentsToMove;
     final int maxToLoad = params.getCoordinatorDynamicConfig().getMaxSegmentsInNodeLoadingQueue();
     int moved = 0, unmoved = 0;
 
+    Iterator<BalancerSegmentHolder> segmetnsToMove = strategy.pickSegmentsToMove(

Review comment:
       nit: typo `segmetnsToMove` -> `segmentsToMove`

##########
File path: server/src/main/java/org/apache/druid/server/coordinator/ReservoirSegmentSampler.java
##########
@@ -31,6 +32,42 @@
 
   private static final EmittingLogger log = new EmittingLogger(ReservoirSegmentSampler.class);
 
+  static List<BalancerSegmentHolder> getRandomBalancerSegmentHolders(
+      final List<ServerHolder> serverHolders,
+      Set<String> broadcastDatasources,
+      int k

Review comment:
       does it make sense to retain the `percentOfSegmentsToConsider` functionality to allow short-circuiting iterateAllSegments across all servers? We update the docs https://github.com/apache/druid/blob/master/docs/configuration/index.md#dynamic-configuration accordingly either way

##########
File path: server/src/main/java/org/apache/druid/server/coordinator/BalancerStrategy.java
##########
@@ -71,13 +71,60 @@
    * @return {@link BalancerSegmentHolder} containing segment to move and server it currently resides on, or null if
    *         there are no segments to pick from (i. e. all provided serverHolders are empty).
    */
+  @Deprecated

Review comment:
       this doesn't really seem deprecated so much as no longer called at all, maybe we should remove it?

##########
File path: server/src/main/java/org/apache/druid/server/coordinator/ReservoirSegmentSampler.java
##########
@@ -31,6 +32,42 @@
 
   private static final EmittingLogger log = new EmittingLogger(ReservoirSegmentSampler.class);
 
+  static List<BalancerSegmentHolder> getRandomBalancerSegmentHolders(
+      final List<ServerHolder> serverHolders,
+      Set<String> broadcastDatasources,
+      int k
+  )
+  {
+    List<BalancerSegmentHolder> holders = new ArrayList<>(k);
+    int numSoFar = 0;
+
+    for (ServerHolder server : serverHolders) {
+      if (!server.getServer().getType().isSegmentReplicationTarget()) {
+        // if the server only handles broadcast segments (which don't need to be rebalanced), we have nothing to do
+        continue;
+      }
+
+      for (DataSegment segment : server.getServer().iterateAllSegments()) {
+        if (broadcastDatasources.contains(segment.getDataSource())) {
+          // we don't need to rebalance segments that were assigned via broadcast rules
+          continue;
+        }
+
+        if (numSoFar < k) {
+          holders.add(new BalancerSegmentHolder(server.getServer(), segment));
+          numSoFar++;
+          continue;
+        }
+        int randNum = ThreadLocalRandom.current().nextInt(numSoFar + 1);
+        if (randNum < k) {
+          holders.set(randNum, new BalancerSegmentHolder(server.getServer(), segment));
+        }

Review comment:
       i think it would be worth adding some code comments here to describe that this algorithm has 2 phases, where it picks the first `k` segments from the servers, in order, then iterates through the server list randomly replacing these picked segments with decreasing frequency the more segments have been iterated.
   
   Im also somewhat curious/nervous about random this pick method is compared to the previous one, though I'm not entirely sure how it would be measured.
   
   I think we should add a new coordinator dynamic config property, https://github.com/apache/druid/blob/master/server/src/main/java/org/apache/druid/server/coordinator/CoordinatorDynamicConfig.java,  to allow falling back to the old algorithm in case there are any unpredictable strange behavior caused by this new algorithm, maybe `useBatchedSegmentSampler` or .. i'm not sure, naming is hard.




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