SERVER-49677 RoutingTableHistory::makeUpdated should skip copying chunkMap if it will result in no diffs

2 files changed, 27 insertions, 0 deletions

diff --git a/src/mongo/s/chunk_manager.cpp b/src/mongo/s/chunk_manager.cpp
index b138a5c14d7..49ba0907017 100644
--- a/src/mongo/s/chunk_manager.cpp
+++ b/src/mongo/s/chunk_manager.cpp
@@ -720,6 +720,12 @@ std::shared_ptr<RoutingTableHistory> RoutingTableHistory::makeNew(
 std::shared_ptr<RoutingTableHistory> RoutingTableHistory::makeUpdated(
     const std::vector<ChunkType>& changedChunks) {
 
+    // It's possible for there to be one chunk in changedChunks without the routing table having
+    // changed. We skip copying the ChunkMap when this happens.
+    if (changedChunks.size() == 1 && changedChunks[0].getVersion() == getVersion()) {
+        return shared_from_this();
+    }
+
     auto changedChunkInfos = flatten(changedChunks);
     auto chunkMap = _chunkMap.createMerged(changedChunkInfos);
 
diff --git a/src/mongo/s/chunk_manager_refresh_bm.cpp b/src/mongo/s/chunk_manager_refresh_bm.cpp
index be3490e08a1..750b4d67da5 100644
--- a/src/mongo/s/chunk_manager_refresh_bm.cpp
+++ b/src/mongo/s/chunk_manager_refresh_bm.cpp
@@ -102,6 +102,27 @@ MONGO_COMPILER_NOINLINE auto runIncrementalUpdate(const CollectionMetadata& cm,
                                                 ShardId("shard0"));
 }
 
+void BM_IncrementalRefreshWithNoChange(benchmark::State& state) {
+    const int nShards = state.range(0);
+    const int nChunks = state.range(1);
+    auto cm = makeChunkManagerWithOptimalBalancedDistribution(nShards, nChunks);
+
+    auto postMoveVersion = cm->getChunkManager()->getVersion();
+    const auto collName = NamespaceString(cm->getChunkManager()->getns());
+    std::vector<ChunkType> newChunks;
+    newChunks.emplace_back(
+        collName, getRangeForChunk(1, nChunks), postMoveVersion, ShardId("shard0"));
+
+    for (auto keepRunning : state) {
+        benchmark::DoNotOptimize(runIncrementalUpdate(*cm, newChunks));
+    }
+}
+
+BENCHMARK(BM_IncrementalRefreshWithNoChange)
+    ->Args({2, 50000})
+    ->Args({2, 250000})
+    ->Args({2, 500000});
+
 void BM_IncrementalRefreshOfPessimalBalancedDistribution(benchmark::State& state) {
     const int nShards = state.range(0);
     const int nChunks = state.range(1);