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More efficient sort in tryRelocateShard (#128063)

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No need to do this via an allocation-heavy `Stream`, we can just put the
objects straight into an array, sort them in-place, and keep hold of the
array to avoid having to allocate anything on the next iteration.

Also slims down `BY_DESCENDING_SHARD_ID`: it's always sorting the same
index so we don't need to look at `ShardId#index` in the comparison, nor
do we really need multiple layers of vtable lookups, we can just compare
the shard IDs directly.

Relates #128021
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David Turner 2025-05-19 20:45:31 +01:00 • committed by GitHub
parent 923a8e1c6f
commit a84dff876e
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5
docs/changelog/128063.yaml Normal file
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@ -0,0 +1,5 @@
pr: 128063
summary: More efficient sort in `tryRelocateShard`
area: Allocation
type: enhancement
issues: []

30
server/src/main/java/org/elasticsearch/cluster/routing/allocation/allocator/BalancedShardsAllocator.java
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@ -54,7 +54,6 @@ import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.BiFunction;
import java.util.stream.StreamSupport;
import static org.elasticsearch.cluster.metadata.SingleNodeShutdownMetadata.Type.REPLACE;
import static org.elasticsearch.cluster.routing.ExpectedShardSizeEstimator.getExpectedShardSize;
@ -1091,7 +1090,13 @@ public class BalancedShardsAllocator implements ShardsAllocator {
return AllocateUnassignedDecision.fromDecision(decision, minNode != null ? minNode.routingNode.node() : null, nodeDecisions);
}
private static final Comparator<ShardRouting> BY_DESCENDING_SHARD_ID = Comparator.comparing(ShardRouting::shardId).reversed();
private static final Comparator<ShardRouting> BY_DESCENDING_SHARD_ID = (s1, s2) -> Integer.compare(s2.id(), s1.id());
/**
* Scratch space for accumulating/sorting the {@link ShardRouting} instances when contemplating moving the shards away from a node
* in {@link #tryRelocateShard} - re-used to avoid extraneous allocations etc.
*/
private ShardRouting[] shardRoutingsOnMaxWeightNode;
/**
* Tries to find a relocation from the max node to the minimal node for an arbitrary shard of the given index on the
@ -1102,13 +1107,24 @@ public class BalancedShardsAllocator implements ShardsAllocator {
final ModelIndex index = maxNode.getIndex(idx);
if (index != null) {
logger.trace("Try relocating shard of [{}] from [{}] to [{}]", idx, maxNode.getNodeId(), minNode.getNodeId());
final Iterable<ShardRouting> shardRoutings = StreamSupport.stream(index.spliterator(), false)
.filter(ShardRouting::started) // cannot rebalance unassigned, initializing or relocating shards anyway
.sorted(BY_DESCENDING_SHARD_ID) // check in descending order of shard id so that the decision is deterministic
::iterator;
if (shardRoutingsOnMaxWeightNode == null || shardRoutingsOnMaxWeightNode.length < index.numShards()) {
shardRoutingsOnMaxWeightNode = new ShardRouting[index.numShards() * 2]; // oversized so reuse is more likely
}
int startedShards = 0;
for (final var shardRouting : index) {
if (shardRouting.started()) { // cannot rebalance unassigned, initializing or relocating shards anyway
shardRoutingsOnMaxWeightNode[startedShards] = shardRouting;
startedShards += 1;
}
}
// check in descending order of shard id so that the decision is deterministic
ArrayUtil.timSort(shardRoutingsOnMaxWeightNode, 0, startedShards, BY_DESCENDING_SHARD_ID);
final AllocationDeciders deciders = allocation.deciders();
for (ShardRouting shard : shardRoutings) {
for (int shardIndex = 0; shardIndex < startedShards; shardIndex++) {
final ShardRouting shard = shardRoutingsOnMaxWeightNode[shardIndex];
final Decision rebalanceDecision = deciders.canRebalance(shard, allocation);
if (rebalanceDecision.type() == Type.NO) {
continue;