diff --git a/src/collective/communicator.cu b/src/collective/communicator.cu
index 8bd10382def5..8cdb7f2fd3bd 100644
--- a/src/collective/communicator.cu
+++ b/src/collective/communicator.cu
@@ -30,12 +30,12 @@ DeviceCommunicator* Communicator::GetDevice(int device_ordinal) {
     old_world_size = communicator_->GetWorldSize();
 #ifdef XGBOOST_USE_NCCL
     if (type_ != CommunicatorType::kFederated) {
-      device_communicator_.reset(new NcclDeviceCommunicator(device_ordinal, Get()));
+      device_communicator_.reset(new NcclDeviceCommunicator(device_ordinal));
     } else {
-      device_communicator_.reset(new DeviceCommunicatorAdapter(device_ordinal, Get()));
+      device_communicator_.reset(new DeviceCommunicatorAdapter(device_ordinal));
     }
 #else
-    device_communicator_.reset(new DeviceCommunicatorAdapter(device_ordinal, Get()));
+    device_communicator_.reset(new DeviceCommunicatorAdapter(device_ordinal));
 #endif
   }
   return device_communicator_.get();
diff --git a/src/collective/device_communicator_adapter.cuh b/src/collective/device_communicator_adapter.cuh
index 06637c5b4768..f8135fb9473f 100644
--- a/src/collective/device_communicator_adapter.cuh
+++ b/src/collective/device_communicator_adapter.cuh
@@ -11,21 +11,18 @@ namespace collective {
 
 class DeviceCommunicatorAdapter : public DeviceCommunicator {
  public:
-  DeviceCommunicatorAdapter(int device_ordinal, Communicator *communicator)
-      : device_ordinal_{device_ordinal}, communicator_{communicator} {
+  explicit DeviceCommunicatorAdapter(int device_ordinal)
+      : device_ordinal_{device_ordinal}, world_size_{GetWorldSize()}, rank_{GetRank()} {
     if (device_ordinal_ < 0) {
       LOG(FATAL) << "Invalid device ordinal: " << device_ordinal_;
     }
-    if (communicator_ == nullptr) {
-      LOG(FATAL) << "Communicator cannot be null.";
-    }
   }
 
   ~DeviceCommunicatorAdapter() override = default;
 
   void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
                  Operation op) override {
-    if (communicator_->GetWorldSize() == 1) {
+    if (world_size_ == 1) {
       return;
     }
 
@@ -33,37 +30,34 @@ class DeviceCommunicatorAdapter : public DeviceCommunicator {
     auto size = count * GetTypeSize(data_type);
     host_buffer_.reserve(size);
     dh::safe_cuda(cudaMemcpy(host_buffer_.data(), send_receive_buffer, size, cudaMemcpyDefault));
-    communicator_->AllReduce(host_buffer_.data(), count, data_type, op);
+    Allreduce(host_buffer_.data(), count, data_type, op);
     dh::safe_cuda(cudaMemcpy(send_receive_buffer, host_buffer_.data(), size, cudaMemcpyDefault));
   }
 
   void AllGatherV(void const *send_buffer, size_t length_bytes, std::vector<std::size_t> *segments,
                   dh::caching_device_vector<char> *receive_buffer) override {
-    if (communicator_->GetWorldSize() == 1) {
+    if (world_size_ == 1) {
       return;
     }
 
     dh::safe_cuda(cudaSetDevice(device_ordinal_));
-    int const world_size = communicator_->GetWorldSize();
-    int const rank = communicator_->GetRank();
 
     segments->clear();
-    segments->resize(world_size, 0);
-    segments->at(rank) = length_bytes;
-    communicator_->AllReduce(segments->data(), segments->size(), DataType::kUInt64,
-                             Operation::kMax);
+    segments->resize(world_size_, 0);
+    segments->at(rank_) = length_bytes;
+    Allreduce(segments->data(), segments->size(), DataType::kUInt64, Operation::kMax);
     auto total_bytes = std::accumulate(segments->cbegin(), segments->cend(), 0UL);
     receive_buffer->resize(total_bytes);
 
     host_buffer_.reserve(total_bytes);
     size_t offset = 0;
-    for (int32_t i = 0; i < world_size; ++i) {
+    for (int32_t i = 0; i < world_size_; ++i) {
       size_t as_bytes = segments->at(i);
-      if (i == rank) {
-        dh::safe_cuda(cudaMemcpy(host_buffer_.data() + offset, send_buffer, segments->at(rank),
+      if (i == rank_) {
+        dh::safe_cuda(cudaMemcpy(host_buffer_.data() + offset, send_buffer, segments->at(rank_),
                                  cudaMemcpyDefault));
       }
-      communicator_->Broadcast(host_buffer_.data() + offset, as_bytes, i);
+      Broadcast(host_buffer_.data() + offset, as_bytes, i);
       offset += as_bytes;
     }
     dh::safe_cuda(cudaMemcpy(receive_buffer->data().get(), host_buffer_.data(), total_bytes,
@@ -76,7 +70,8 @@ class DeviceCommunicatorAdapter : public DeviceCommunicator {
 
  private:
   int const device_ordinal_;
-  Communicator *communicator_;
+  int const world_size_;
+  int const rank_;
   /// Host buffer used to call communicator functions.
   std::vector<char> host_buffer_{};
 };
diff --git a/src/collective/nccl_device_communicator.cu b/src/collective/nccl_device_communicator.cu
index 631193db4d86..57419b947656 100644
--- a/src/collective/nccl_device_communicator.cu
+++ b/src/collective/nccl_device_communicator.cu
@@ -7,31 +7,24 @@
 namespace xgboost {
 namespace collective {
 
-NcclDeviceCommunicator::NcclDeviceCommunicator(int device_ordinal, Communicator *communicator)
-    : device_ordinal_{device_ordinal}, communicator_{communicator} {
+NcclDeviceCommunicator::NcclDeviceCommunicator(int device_ordinal)
+    : device_ordinal_{device_ordinal}, world_size_{GetWorldSize()}, rank_{GetRank()} {
   if (device_ordinal_ < 0) {
     LOG(FATAL) << "Invalid device ordinal: " << device_ordinal_;
   }
-  if (communicator_ == nullptr) {
-    LOG(FATAL) << "Communicator cannot be null.";
-  }
-
-  int32_t const rank = communicator_->GetRank();
-  int32_t const world = communicator_->GetWorldSize();
-
-  if (world == 1) {
+  if (world_size_ == 1) {
     return;
   }
 
-  std::vector<uint64_t> uuids(world * kUuidLength, 0);
+  std::vector<uint64_t> uuids(world_size_ * kUuidLength, 0);
   auto s_uuid = xgboost::common::Span<uint64_t>{uuids.data(), uuids.size()};
-  auto s_this_uuid = s_uuid.subspan(rank * kUuidLength, kUuidLength);
+  auto s_this_uuid = s_uuid.subspan(rank_ * kUuidLength, kUuidLength);
   GetCudaUUID(s_this_uuid);
 
   // TODO(rongou): replace this with allgather.
-  communicator_->AllReduce(uuids.data(), uuids.size(), DataType::kUInt64, Operation::kSum);
+  Allreduce(uuids.data(), uuids.size(), DataType::kUInt64, Operation::kSum);
 
-  std::vector<xgboost::common::Span<uint64_t, kUuidLength>> converted(world);
+  std::vector<xgboost::common::Span<uint64_t, kUuidLength>> converted(world_size_);
   size_t j = 0;
   for (size_t i = 0; i < uuids.size(); i += kUuidLength) {
     converted[j] = xgboost::common::Span<uint64_t, kUuidLength>{uuids.data() + i, kUuidLength};
@@ -41,18 +34,18 @@ NcclDeviceCommunicator::NcclDeviceCommunicator(int device_ordinal, Communicator
   auto iter = std::unique(converted.begin(), converted.end());
   auto n_uniques = std::distance(converted.begin(), iter);
 
-  CHECK_EQ(n_uniques, world)
+  CHECK_EQ(n_uniques, world_size_)
       << "Multiple processes within communication group running on same CUDA "
       << "device is not supported. " << PrintUUID(s_this_uuid) << "\n";
 
   nccl_unique_id_ = GetUniqueId();
   dh::safe_cuda(cudaSetDevice(device_ordinal_));
-  dh::safe_nccl(ncclCommInitRank(&nccl_comm_, world, nccl_unique_id_, rank));
+  dh::safe_nccl(ncclCommInitRank(&nccl_comm_, world_size_, nccl_unique_id_, rank_));
   dh::safe_cuda(cudaStreamCreate(&cuda_stream_));
 }
 
 NcclDeviceCommunicator::~NcclDeviceCommunicator() {
-  if (communicator_->GetWorldSize() == 1) {
+  if (world_size_ == 1) {
     return;
   }
   if (cuda_stream_) {
@@ -139,9 +132,8 @@ void RunBitwiseAllreduce(char *out_buffer, char const *device_buffer, Func func,
 
 void NcclDeviceCommunicator::BitwiseAllReduce(void *send_receive_buffer, std::size_t count,
                                               DataType data_type, Operation op) {
-  auto const world_size = communicator_->GetWorldSize();
   auto const size = count * GetTypeSize(data_type);
-  dh::caching_device_vector<char> buffer(size * world_size);
+  dh::caching_device_vector<char> buffer(size * world_size_);
   auto *device_buffer = buffer.data().get();
 
   // First gather data from all the workers.
@@ -152,15 +144,15 @@ void NcclDeviceCommunicator::BitwiseAllReduce(void *send_receive_buffer, std::si
   auto *out_buffer = static_cast<char *>(send_receive_buffer);
   switch (op) {
     case Operation::kBitwiseAND:
-      RunBitwiseAllreduce(out_buffer, device_buffer, thrust::bit_and<char>(), world_size, size,
+      RunBitwiseAllreduce(out_buffer, device_buffer, thrust::bit_and<char>(), world_size_, size,
                           cuda_stream_);
       break;
     case Operation::kBitwiseOR:
-      RunBitwiseAllreduce(out_buffer, device_buffer, thrust::bit_or<char>(), world_size, size,
+      RunBitwiseAllreduce(out_buffer, device_buffer, thrust::bit_or<char>(), world_size_, size,
                           cuda_stream_);
       break;
     case Operation::kBitwiseXOR:
-      RunBitwiseAllreduce(out_buffer, device_buffer, thrust::bit_xor<char>(), world_size, size,
+      RunBitwiseAllreduce(out_buffer, device_buffer, thrust::bit_xor<char>(), world_size_, size,
                           cuda_stream_);
       break;
     default:
@@ -170,7 +162,7 @@ void NcclDeviceCommunicator::BitwiseAllReduce(void *send_receive_buffer, std::si
 
 void NcclDeviceCommunicator::AllReduce(void *send_receive_buffer, std::size_t count,
                                        DataType data_type, Operation op) {
-  if (communicator_->GetWorldSize() == 1) {
+  if (world_size_ == 1) {
     return;
   }
 
@@ -189,24 +181,22 @@ void NcclDeviceCommunicator::AllReduce(void *send_receive_buffer, std::size_t co
 void NcclDeviceCommunicator::AllGatherV(void const *send_buffer, size_t length_bytes,
                                         std::vector<std::size_t> *segments,
                                         dh::caching_device_vector<char> *receive_buffer) {
-  if (communicator_->GetWorldSize() == 1) {
+  if (world_size_ == 1) {
     return;
   }
 
   dh::safe_cuda(cudaSetDevice(device_ordinal_));
-  int const world_size = communicator_->GetWorldSize();
-  int const rank = communicator_->GetRank();
 
   segments->clear();
-  segments->resize(world_size, 0);
-  segments->at(rank) = length_bytes;
-  communicator_->AllReduce(segments->data(), segments->size(), DataType::kUInt64, Operation::kMax);
+  segments->resize(world_size_, 0);
+  segments->at(rank_) = length_bytes;
+  Allreduce(segments->data(), segments->size(), DataType::kUInt64, Operation::kMax);
   auto total_bytes = std::accumulate(segments->cbegin(), segments->cend(), 0UL);
   receive_buffer->resize(total_bytes);
 
   size_t offset = 0;
   dh::safe_nccl(ncclGroupStart());
-  for (int32_t i = 0; i < world_size; ++i) {
+  for (int32_t i = 0; i < world_size_; ++i) {
     size_t as_bytes = segments->at(i);
     dh::safe_nccl(ncclBroadcast(send_buffer, receive_buffer->data().get() + offset, as_bytes,
                                 ncclChar, i, nccl_comm_, cuda_stream_));
@@ -216,7 +206,7 @@ void NcclDeviceCommunicator::AllGatherV(void const *send_buffer, size_t length_b
 }
 
 void NcclDeviceCommunicator::Synchronize() {
-  if (communicator_->GetWorldSize() == 1) {
+  if (world_size_ == 1) {
     return;
   }
   dh::safe_cuda(cudaSetDevice(device_ordinal_));
diff --git a/src/collective/nccl_device_communicator.cuh b/src/collective/nccl_device_communicator.cuh
index e5f76119d914..925603d21252 100644
--- a/src/collective/nccl_device_communicator.cuh
+++ b/src/collective/nccl_device_communicator.cuh
@@ -12,7 +12,7 @@ namespace collective {
 
 class NcclDeviceCommunicator : public DeviceCommunicator {
  public:
-  NcclDeviceCommunicator(int device_ordinal, Communicator *communicator);
+  explicit NcclDeviceCommunicator(int device_ordinal);
   ~NcclDeviceCommunicator() override;
   void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
                  Operation op) override;
@@ -49,11 +49,10 @@ class NcclDeviceCommunicator : public DeviceCommunicator {
   ncclUniqueId GetUniqueId() {
     static const int kRootRank = 0;
     ncclUniqueId id;
-    if (communicator_->GetRank() == kRootRank) {
+    if (rank_ == kRootRank) {
       dh::safe_nccl(ncclGetUniqueId(&id));
     }
-    communicator_->Broadcast(static_cast<void *>(&id), sizeof(ncclUniqueId),
-                             static_cast<int>(kRootRank));
+    Broadcast(static_cast<void *>(&id), sizeof(ncclUniqueId), static_cast<int>(kRootRank));
     return id;
   }
 
@@ -61,7 +60,8 @@ class NcclDeviceCommunicator : public DeviceCommunicator {
                         Operation op);
 
   int const device_ordinal_;
-  Communicator *communicator_;
+  int const world_size_;
+  int const rank_;
   ncclComm_t nccl_comm_{};
   cudaStream_t cuda_stream_{};
   ncclUniqueId nccl_unique_id_{};
diff --git a/tests/cpp/collective/test_nccl_device_communicator.cu b/tests/cpp/collective/test_nccl_device_communicator.cu
index 6ac861a55877..81dd3d46db0d 100644
--- a/tests/cpp/collective/test_nccl_device_communicator.cu
+++ b/tests/cpp/collective/test_nccl_device_communicator.cu
@@ -16,12 +16,7 @@ namespace xgboost {
 namespace collective {
 
 TEST(NcclDeviceCommunicatorSimpleTest, ThrowOnInvalidDeviceOrdinal) {
-  auto construct = []() { NcclDeviceCommunicator comm{-1, nullptr}; };
-  EXPECT_THROW(construct(), dmlc::Error);
-}
-
-TEST(NcclDeviceCommunicatorSimpleTest, ThrowOnInvalidCommunicator) {
-  auto construct = []() { NcclDeviceCommunicator comm{0, nullptr}; };
+  auto construct = []() { NcclDeviceCommunicator comm{-1}; };
   EXPECT_THROW(construct(), dmlc::Error);
 }
 
diff --git a/tests/cpp/plugin/helpers.h b/tests/cpp/plugin/helpers.h
index c4d303bb5437..20b4afc3026b 100644
--- a/tests/cpp/plugin/helpers.h
+++ b/tests/cpp/plugin/helpers.h
@@ -37,7 +37,14 @@ class ServerForTest {
   }
 
   ~ServerForTest() {
+    using namespace std::chrono_literals;
+    while (!server_) {
+      std::this_thread::sleep_for(100ms);
+    }
     server_->Shutdown();
+    while (!server_thread_) {
+      std::this_thread::sleep_for(100ms);
+    }
     server_thread_->join();
   }
 
@@ -56,7 +63,7 @@ class BaseFederatedTest : public ::testing::Test {
 
   void TearDown() override { server_.reset(nullptr); }
 
-  static int constexpr kWorldSize{3};
+  static int constexpr kWorldSize{2};
   std::unique_ptr<ServerForTest> server_;
 };
 
diff --git a/tests/cpp/plugin/test_federated_adapter.cu b/tests/cpp/plugin/test_federated_adapter.cu
index 3fb793fa7160..134446f11350 100644
--- a/tests/cpp/plugin/test_federated_adapter.cu
+++ b/tests/cpp/plugin/test_federated_adapter.cu
@@ -9,6 +9,7 @@
 #include <thread>
 
 #include "../../../plugin/federated/federated_communicator.h"
+#include "../../../src/collective/communicator-inl.cuh"
 #include "../../../src/collective/device_communicator_adapter.cuh"
 #include "./helpers.h"
 
@@ -17,67 +18,63 @@ namespace xgboost::collective {
 class FederatedAdapterTest : public BaseFederatedTest {};
 
 TEST(FederatedAdapterSimpleTest, ThrowOnInvalidDeviceOrdinal) {
-  auto construct = []() { DeviceCommunicatorAdapter adapter{-1, nullptr}; };
+  auto construct = []() { DeviceCommunicatorAdapter adapter{-1}; };
   EXPECT_THROW(construct(), dmlc::Error);
 }
 
-TEST(FederatedAdapterSimpleTest, ThrowOnInvalidCommunicator) {
-  auto construct = []() { DeviceCommunicatorAdapter adapter{0, nullptr}; };
-  EXPECT_THROW(construct(), dmlc::Error);
+namespace {
+void VerifyAllReduceSum() {
+  auto const world_size = collective::GetWorldSize();
+  auto const rank = collective::GetRank();
+  int count = 3;
+  thrust::device_vector<double> buffer(count, 0);
+  thrust::sequence(buffer.begin(), buffer.end());
+  collective::AllReduce<collective::Operation::kSum>(rank, buffer.data().get(), count);
+  thrust::host_vector<double> host_buffer = buffer;
+  EXPECT_EQ(host_buffer.size(), count);
+  for (auto i = 0; i < count; i++) {
+    EXPECT_EQ(host_buffer[i], i * world_size);
+  }
 }
+}  // anonymous namespace
 
-TEST_F(FederatedAdapterTest, DeviceAllReduceSum) {
-  std::vector<std::thread> threads;
-  for (auto rank = 0; rank < kWorldSize; rank++) {
-    threads.emplace_back([rank, server_address = server_->Address()] {
-      FederatedCommunicator comm{kWorldSize, rank, server_address};
-      // Assign device 0 to all workers, since we run gtest in a single-GPU machine
-      DeviceCommunicatorAdapter adapter{0, &comm};
-      int count = 3;
-      thrust::device_vector<double> buffer(count, 0);
-      thrust::sequence(buffer.begin(), buffer.end());
-      adapter.AllReduce(buffer.data().get(), count, DataType::kDouble, Operation::kSum);
-      thrust::host_vector<double> host_buffer = buffer;
-      EXPECT_EQ(host_buffer.size(), count);
-      for (auto i = 0; i < count; i++) {
-        EXPECT_EQ(host_buffer[i], i * kWorldSize);
-      }
-    });
-  }
-  for (auto& thread : threads) {
-    thread.join();
+TEST_F(FederatedAdapterTest, MGPUAllReduceSum) {
+  auto const n_gpus = common::AllVisibleGPUs();
+  if (n_gpus <= 1) {
+    GTEST_SKIP() << "Skipping MGPUAllReduceSum test with # GPUs = " << n_gpus;
   }
+  RunWithFederatedCommunicator(kWorldSize, server_->Address(), &VerifyAllReduceSum);
 }
 
-TEST_F(FederatedAdapterTest, DeviceAllGatherV) {
-  std::vector<std::thread> threads;
-  for (auto rank = 0; rank < kWorldSize; rank++) {
-    threads.emplace_back([rank, server_address = server_->Address()] {
-      FederatedCommunicator comm{kWorldSize, rank, server_address};
-      // Assign device 0 to all workers, since we run gtest in a single-GPU machine
-      DeviceCommunicatorAdapter adapter{0, &comm};
+namespace {
+void VerifyAllGatherV() {
+  auto const world_size = collective::GetWorldSize();
+  auto const rank = collective::GetRank();
+  int const count = rank + 2;
+  thrust::device_vector<char> buffer(count, 0);
+  thrust::sequence(buffer.begin(), buffer.end());
+  std::vector<std::size_t> segments(world_size);
+  dh::caching_device_vector<char> receive_buffer{};
 
-      int const count = rank + 2;
-      thrust::device_vector<char> buffer(count, 0);
-      thrust::sequence(buffer.begin(), buffer.end());
-      std::vector<std::size_t> segments(kWorldSize);
-      dh::caching_device_vector<char> receive_buffer{};
+  collective::AllGatherV(rank, buffer.data().get(), count, &segments, &receive_buffer);
 
-      adapter.AllGatherV(buffer.data().get(), count, &segments, &receive_buffer);
-
-      EXPECT_EQ(segments[0], 2);
-      EXPECT_EQ(segments[1], 3);
-      thrust::host_vector<char> host_buffer = receive_buffer;
-      EXPECT_EQ(host_buffer.size(), 9);
-      int expected[] = {0, 1, 0, 1, 2, 0, 1, 2, 3};
-      for (auto i = 0; i < 9; i++) {
-        EXPECT_EQ(host_buffer[i], expected[i]);
-      }
-    });
+  EXPECT_EQ(segments[0], 2);
+  EXPECT_EQ(segments[1], 3);
+  thrust::host_vector<char> host_buffer = receive_buffer;
+  EXPECT_EQ(host_buffer.size(), 5);
+  int expected[] = {0, 1, 0, 1, 2};
+  for (auto i = 0; i < 5; i++) {
+    EXPECT_EQ(host_buffer[i], expected[i]);
   }
-  for (auto& thread : threads) {
-    thread.join();
+}
+}  // anonymous namespace
+
+TEST_F(FederatedAdapterTest, MGPUAllGatherV) {
+  auto const n_gpus = common::AllVisibleGPUs();
+  if (n_gpus <= 1) {
+    GTEST_SKIP() << "Skipping MGPUAllGatherV test with # GPUs = " << n_gpus;
   }
+  RunWithFederatedCommunicator(kWorldSize, server_->Address(), &VerifyAllGatherV);
 }
 
 }  // namespace xgboost::collective
diff --git a/tests/cpp/plugin/test_federated_communicator.cc b/tests/cpp/plugin/test_federated_communicator.cc
index 62f33d5ee29a..8b0e1039adff 100644
--- a/tests/cpp/plugin/test_federated_communicator.cc
+++ b/tests/cpp/plugin/test_federated_communicator.cc
@@ -31,7 +31,7 @@ class FederatedCommunicatorTest : public BaseFederatedTest {
 
  protected:
   static void CheckAllgather(FederatedCommunicator &comm, int rank) {
-    int buffer[kWorldSize] = {0, 0, 0};
+    int buffer[kWorldSize] = {0, 0};
     buffer[rank] = rank;
     comm.AllGather(buffer, sizeof(buffer));
     for (auto i = 0; i < kWorldSize; i++) {
@@ -42,7 +42,7 @@ class FederatedCommunicatorTest : public BaseFederatedTest {
   static void CheckAllreduce(FederatedCommunicator &comm) {
     int buffer[] = {1, 2, 3, 4, 5};
     comm.AllReduce(buffer, sizeof(buffer) / sizeof(buffer[0]), DataType::kInt32, Operation::kSum);
-    int expected[] = {3, 6, 9, 12, 15};
+    int expected[] = {2, 4, 6, 8, 10};
     for (auto i = 0; i < 5; i++) {
       EXPECT_EQ(buffer[i], expected[i]);
     }
diff --git a/tests/cpp/plugin/test_federated_data.cc b/tests/cpp/plugin/test_federated_data.cc
index c6efb84d5a77..6a8233a0fefe 100644
--- a/tests/cpp/plugin/test_federated_data.cc
+++ b/tests/cpp/plugin/test_federated_data.cc
@@ -30,7 +30,7 @@ void VerifyLoadUri() {
   std::string uri = path + "?format=csv";
   dmat.reset(DMatrix::Load(uri, false, DataSplitMode::kCol));
 
-  ASSERT_EQ(dmat->Info().num_col_, 8 * collective::GetWorldSize() + 3);
+  ASSERT_EQ(dmat->Info().num_col_, 8 * collective::GetWorldSize() + 1);
   ASSERT_EQ(dmat->Info().num_row_, kRows);
 
   for (auto const& page : dmat->GetBatches<SparsePage>()) {
diff --git a/tests/cpp/plugin/test_federated_server.cc b/tests/cpp/plugin/test_federated_server.cc
index 4dd2f3c4031a..633d64df10f8 100644
--- a/tests/cpp/plugin/test_federated_server.cc
+++ b/tests/cpp/plugin/test_federated_server.cc
@@ -39,7 +39,7 @@ class FederatedServerTest : public BaseFederatedTest {
 
  protected:
   static void CheckAllgather(federated::FederatedClient& client, int rank) {
-    int data[kWorldSize] = {0, 0, 0};
+    int data[kWorldSize] = {0, 0};
     data[rank] = rank;
     std::string send_buffer(reinterpret_cast<char const*>(data), sizeof(data));
     auto reply = client.Allgather(send_buffer);
@@ -54,7 +54,7 @@ class FederatedServerTest : public BaseFederatedTest {
     std::string send_buffer(reinterpret_cast<char const*>(data), sizeof(data));
     auto reply = client.Allreduce(send_buffer, federated::INT32, federated::SUM);
     auto const* result = reinterpret_cast<int const*>(reply.data());
-    int expected[] = {3, 6, 9, 12, 15};
+    int expected[] = {2, 4, 6, 8, 10};
     for (auto i = 0; i < 5; i++) {
       EXPECT_EQ(result[i], expected[i]);
     }