diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 20e91a5d93f6..133e151e5e4f 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -174,7 +174,7 @@ jobs:
     - uses: actions/checkout@e2f20e631ae6d7dd3b768f56a5d2af784dd54791 # v2.5.0
       with:
         submodules: 'true'
-    - uses: actions/setup-python@7f80679172b057fc5e90d70d197929d454754a5a # v4.3.0
+    - uses: actions/setup-python@0a5c61591373683505ea898e09a3ea4f39ef2b9c # v5.0.0
       with:
         python-version: "3.8"
         architecture: 'x64'
diff --git a/.github/workflows/python_tests.yml b/.github/workflows/python_tests.yml
index 0fca76673962..3fbcc7a01acf 100644
--- a/.github/workflows/python_tests.yml
+++ b/.github/workflows/python_tests.yml
@@ -310,7 +310,7 @@ jobs:
           submodules: 'true'
 
       - name: Set up Python 3.8
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@0a5c61591373683505ea898e09a3ea4f39ef2b9c # v5.0.0
         with:
           python-version: 3.8
 
diff --git a/.github/workflows/python_wheels.yml b/.github/workflows/python_wheels.yml
index f46b772950c9..12ae8a244e4f 100644
--- a/.github/workflows/python_wheels.yml
+++ b/.github/workflows/python_wheels.yml
@@ -21,7 +21,7 @@ jobs:
       with:
         submodules: 'true'
     - name: Setup Python
-      uses: actions/setup-python@7f80679172b057fc5e90d70d197929d454754a5a # v4.3.0
+      uses: actions/setup-python@0a5c61591373683505ea898e09a3ea4f39ef2b9c # v5.0.0
       with:
         python-version: "3.8"
     - name: Build wheels
diff --git a/.github/workflows/r_tests.yml b/.github/workflows/r_tests.yml
index d004ab15ca15..ad6853281c1f 100644
--- a/.github/workflows/r_tests.yml
+++ b/.github/workflows/r_tests.yml
@@ -74,7 +74,7 @@ jobs:
         key: ${{ runner.os }}-r-${{ matrix.config.r }}-6-${{ hashFiles('R-package/DESCRIPTION') }}
         restore-keys: ${{ runner.os }}-r-${{ matrix.config.r }}-6-${{ hashFiles('R-package/DESCRIPTION') }}
 
-    - uses: actions/setup-python@7f80679172b057fc5e90d70d197929d454754a5a # v4.3.0
+    - uses: actions/setup-python@0a5c61591373683505ea898e09a3ea4f39ef2b9c # v5.0.0
       with:
         python-version: "3.8"
         architecture: 'x64'
diff --git a/R-package/src/Makevars.in b/R-package/src/Makevars.in
index dd13983f5b59..0f4b3ac6f6a7 100644
--- a/R-package/src/Makevars.in
+++ b/R-package/src/Makevars.in
@@ -104,6 +104,7 @@ OBJECTS= \
     $(PKGROOT)/src/collective/broadcast.o \
     $(PKGROOT)/src/collective/comm.o \
     $(PKGROOT)/src/collective/coll.o \
+    $(PKGROOT)/src/collective/communicator-inl.o \
     $(PKGROOT)/src/collective/tracker.o \
     $(PKGROOT)/src/collective/communicator.o \
     $(PKGROOT)/src/collective/in_memory_communicator.o \
diff --git a/R-package/src/Makevars.win b/R-package/src/Makevars.win
index 46a862711dc6..0c2084de940c 100644
--- a/R-package/src/Makevars.win
+++ b/R-package/src/Makevars.win
@@ -104,6 +104,7 @@ OBJECTS= \
     $(PKGROOT)/src/collective/broadcast.o \
     $(PKGROOT)/src/collective/comm.o \
     $(PKGROOT)/src/collective/coll.o \
+    $(PKGROOT)/src/collective/communicator-inl.o \
     $(PKGROOT)/src/collective/tracker.o \
     $(PKGROOT)/src/collective/communicator.o \
     $(PKGROOT)/src/collective/in_memory_communicator.o \
diff --git a/src/collective/communicator-inl.cc b/src/collective/communicator-inl.cc
new file mode 100644
index 000000000000..4164855f1cef
--- /dev/null
+++ b/src/collective/communicator-inl.cc
@@ -0,0 +1,34 @@
+/**
+ * Copyright 2024, XGBoost contributors
+ */
+#include "communicator-inl.h"
+
+namespace xgboost::collective {
+[[nodiscard]] std::vector<std::vector<char>> VectorAllgatherV(
+    std::vector<std::vector<char>> const &input) {
+  auto n_inputs = input.size();
+  std::vector<std::int64_t> sizes(n_inputs);
+  std::transform(input.cbegin(), input.cend(), sizes.begin(),
+                 [](auto const &vec) { return vec.size(); });
+
+  std::vector<std::int64_t> global_sizes = AllgatherV(sizes);
+  std::vector<std::int64_t> offset(global_sizes.size() + 1);
+  offset[0] = 0;
+  for (std::size_t i = 1; i < offset.size(); i++) {
+    offset[i] = offset[i - 1] + global_sizes[i - 1];
+  }
+
+  std::vector<char> collected;
+  for (auto const &vec : input) {
+    collected.insert(collected.end(), vec.cbegin(), vec.cend());
+  }
+  auto out = AllgatherV(collected);
+
+  std::vector<std::vector<char>> result;
+  for (std::size_t i = 1; i < offset.size(); ++i) {
+    std::vector<char> local(out.cbegin() + offset[i - 1], out.cbegin() + offset[i]);
+    result.emplace_back(std::move(local));
+  }
+  return result;
+}
+}  // namespace xgboost::collective
diff --git a/src/collective/communicator-inl.h b/src/collective/communicator-inl.h
index 34212def2377..991e19f2c65a 100644
--- a/src/collective/communicator-inl.h
+++ b/src/collective/communicator-inl.h
@@ -1,5 +1,5 @@
 /**
- * Copyright 2022-2023 by XGBoost contributors
+ * Copyright 2022-2024, XGBoost contributors
  */
 #pragma once
 #include <string>
@@ -192,6 +192,18 @@ inline std::vector<T> AllgatherV(std::vector<T> const &input) {
   return result;
 }
 
+/**
+ * @brief Gathers variable-length data from all processes and distributes it to all processes.
+ *
+ * @param inputs All the inputs from the local worker. The number of inputs can vary
+ *               across different workers. Along with which, the size of each vector in
+ *               the input can also vary.
+ *
+ * @return The AllgatherV result, containing vectors from all workers.
+ */
+[[nodiscard]] std::vector<std::vector<char>> VectorAllgatherV(
+    std::vector<std::vector<char>> const &input);
+
 /**
  * @brief Gathers variable-length strings from all processes and distributes them to all processes.
  * @param input Variable-length list of variable-length strings.
@@ -294,38 +306,5 @@ template <Operation op>
 inline void Allreduce(double *send_receive_buffer, size_t count) {
   Communicator::Get()->AllReduce(send_receive_buffer, count, DataType::kDouble, op);
 }
-
-template <typename T>
-struct SpecialAllgatherVResult {
-  std::vector<std::size_t> offsets;
-  std::vector<std::size_t> sizes;
-  std::vector<T> result;
-};
-
-/**
- * @brief Gathers variable-length data from all processes and distributes it to all processes.
- *
- * We assume each worker has the same number of inputs, but each input may be of a different size.
- *
- * @param inputs All the inputs from the local worker.
- * @param sizes  Sizes of each input.
- */
-template <typename T>
-inline SpecialAllgatherVResult<T> SpecialAllgatherV(std::vector<T> const &inputs,
-                                                    std::vector<std::size_t> const &sizes) {
-  // Gather the sizes across all workers.
-  auto const all_sizes = Allgather(sizes);
-
-  // Calculate input offsets (std::exclusive_scan).
-  std::vector<std::size_t> offsets(all_sizes.size());
-  for (std::size_t i = 1; i < offsets.size(); i++) {
-    offsets[i] = offsets[i - 1] + all_sizes[i - 1];
-  }
-
-  // Gather all the inputs.
-  auto const all_inputs = AllgatherV(inputs);
-
-  return {offsets, all_sizes, all_inputs};
-}
 }  // namespace collective
 }  // namespace xgboost
diff --git a/src/tree/hist/evaluate_splits.h b/src/tree/hist/evaluate_splits.h
index bc534d351f17..d25a41cb0b3c 100644
--- a/src/tree/hist/evaluate_splits.h
+++ b/src/tree/hist/evaluate_splits.h
@@ -1,5 +1,5 @@
 /**
- * Copyright 2021-2023 by XGBoost Contributors
+ * Copyright 2021-2024, XGBoost Contributors
  */
 #ifndef XGBOOST_TREE_HIST_EVALUATE_SPLITS_H_
 #define XGBOOST_TREE_HIST_EVALUATE_SPLITS_H_
@@ -26,6 +26,47 @@
 #include "xgboost/linalg.h"            // for Constants, Vector
 
 namespace xgboost::tree {
+/**
+ * @brief Gather the expand entries from all the workers.
+ * @param entries Local expand entries on this worker.
+ * @return Global expand entries gathered from all workers.
+ */
+template <typename ExpandEntry>
+std::enable_if_t<std::is_same_v<ExpandEntry, CPUExpandEntry> ||
+                     std::is_same_v<ExpandEntry, MultiExpandEntry>,
+                 std::vector<ExpandEntry>>
+AllgatherColumnSplit(std::vector<ExpandEntry> const &entries) {
+  auto const n_entries = entries.size();
+
+  // First, gather all the primitive fields.
+  std::vector<ExpandEntry> local_entries(n_entries);
+
+  // Collect and serialize all entries
+  std::vector<std::vector<char>> serialized_entries;
+  for (std::size_t i = 0; i < n_entries; ++i) {
+    Json jentry{Object{}};
+    entries[i].Save(&jentry);
+
+    std::vector<char> out;
+    Json::Dump(jentry, &out, std::ios::binary);
+
+    serialized_entries.emplace_back(std::move(out));
+  }
+  auto all_serialized = collective::VectorAllgatherV(serialized_entries);
+  CHECK_GE(all_serialized.size(), local_entries.size());
+
+  std::vector<ExpandEntry> all_entries(all_serialized.size());
+  std::transform(all_serialized.cbegin(), all_serialized.cend(), all_entries.begin(),
+                 [](std::vector<char> const &e) {
+                   ExpandEntry entry;
+                   auto je = Json::Load(StringView{e.data(), e.size()}, std::ios::binary);
+                   entry.Load(je);
+                   return entry;
+                 });
+
+  return all_entries;
+}
+
 class HistEvaluator {
  private:
   struct NodeEntry {
@@ -36,8 +77,8 @@ class HistEvaluator {
   };
 
  private:
-  Context const* ctx_;
-  TrainParam const* param_;
+  Context const *ctx_;
+  TrainParam const *param_;
   std::shared_ptr<common::ColumnSampler> column_sampler_;
   TreeEvaluator tree_evaluator_;
   bool is_col_split_{false};
@@ -202,7 +243,7 @@ class HistEvaluator {
       common::CatBitField cat_bits{best.cat_bits};
       bst_bin_t partition = d_step == 1 ? (best_thresh - it_begin + 1) : (best_thresh - f_begin);
       CHECK_GT(partition, 0);
-      std::for_each(sorted_idx.begin(), sorted_idx.begin() + partition, [&](size_t c) {
+      std::for_each(sorted_idx.begin(), sorted_idx.begin() + partition, [&](std::size_t c) {
         auto cat = cut_val[c + f_begin];
         cat_bits.Set(cat);
       });
@@ -285,57 +326,23 @@ class HistEvaluator {
     return left_sum;
   }
 
-  /**
-   * @brief Gather the expand entries from all the workers.
-   * @param entries Local expand entries on this worker.
-   * @return Global expand entries gathered from all workers.
-   */
-  std::vector<CPUExpandEntry> Allgather(std::vector<CPUExpandEntry> const &entries) {
-    auto const world = collective::GetWorldSize();
-    auto const num_entries = entries.size();
-
-    // First, gather all the primitive fields.
-    std::vector<CPUExpandEntry> local_entries(num_entries);
-    std::vector<uint32_t> cat_bits;
-    std::vector<std::size_t> cat_bits_sizes;
-    for (std::size_t i = 0; i < num_entries; i++) {
-      local_entries[i].CopyAndCollect(entries[i], &cat_bits, &cat_bits_sizes);
-    }
-    auto all_entries = collective::Allgather(local_entries);
-
-    // Gather all the cat_bits.
-    auto gathered = collective::SpecialAllgatherV(cat_bits, cat_bits_sizes);
-
-    common::ParallelFor(num_entries * world, ctx_->Threads(), [&] (auto i) {
-      // Copy the cat_bits back into all expand entries.
-      all_entries[i].split.cat_bits.resize(gathered.sizes[i]);
-      std::copy_n(gathered.result.cbegin() + gathered.offsets[i], gathered.sizes[i],
-                  all_entries[i].split.cat_bits.begin());
-    });
-
-    return all_entries;
-  }
-
  public:
   void EvaluateSplits(const BoundedHistCollection &hist, common::HistogramCuts const &cut,
                       common::Span<FeatureType const> feature_types, const RegTree &tree,
                       std::vector<CPUExpandEntry> *p_entries) {
     auto n_threads = ctx_->Threads();
-    auto& entries = *p_entries;
+    auto &entries = *p_entries;
     // All nodes are on the same level, so we can store the shared ptr.
-    std::vector<std::shared_ptr<HostDeviceVector<bst_feature_t>>> features(
-        entries.size());
+    std::vector<std::shared_ptr<HostDeviceVector<bst_feature_t>>> features(entries.size());
     for (size_t nidx_in_set = 0; nidx_in_set < entries.size(); ++nidx_in_set) {
       auto nidx = entries[nidx_in_set].nid;
-      features[nidx_in_set] =
-          column_sampler_->GetFeatureSet(tree.GetDepth(nidx));
+      features[nidx_in_set] = column_sampler_->GetFeatureSet(tree.GetDepth(nidx));
     }
     CHECK(!features.empty());
-    const size_t grain_size =
-        std::max<size_t>(1, features.front()->Size() / n_threads);
-    common::BlockedSpace2d space(entries.size(), [&](size_t nidx_in_set) {
-      return features[nidx_in_set]->Size();
-    }, grain_size);
+    const size_t grain_size = std::max<size_t>(1, features.front()->Size() / n_threads);
+    common::BlockedSpace2d space(
+        entries.size(), [&](size_t nidx_in_set) { return features[nidx_in_set]->Size(); },
+        grain_size);
 
     std::vector<CPUExpandEntry> tloc_candidates(n_threads * entries.size());
     for (size_t i = 0; i < entries.size(); ++i) {
@@ -344,7 +351,7 @@ class HistEvaluator {
       }
     }
     auto evaluator = tree_evaluator_.GetEvaluator();
-    auto const& cut_ptrs = cut.Ptrs();
+    auto const &cut_ptrs = cut.Ptrs();
 
     common::ParallelFor2d(space, n_threads, [&](size_t nidx_in_set, common::Range1d r) {
       auto tidx = omp_get_thread_num();
@@ -385,18 +392,16 @@ class HistEvaluator {
       }
     });
 
-    for (unsigned nidx_in_set = 0; nidx_in_set < entries.size();
-         ++nidx_in_set) {
+    for (unsigned nidx_in_set = 0; nidx_in_set < entries.size(); ++nidx_in_set) {
       for (auto tidx = 0; tidx < n_threads; ++tidx) {
-        entries[nidx_in_set].split.Update(
-            tloc_candidates[n_threads * nidx_in_set + tidx].split);
+        entries[nidx_in_set].split.Update(tloc_candidates[n_threads * nidx_in_set + tidx].split);
       }
     }
 
     if (is_col_split_) {
       // With column-wise data split, we gather the best splits from all the workers and update the
       // expand entries accordingly.
-      auto all_entries = Allgather(entries);
+      auto all_entries = AllgatherColumnSplit(entries);
       for (auto worker = 0; worker < collective::GetWorldSize(); ++worker) {
         for (std::size_t nidx_in_set = 0; nidx_in_set < entries.size(); ++nidx_in_set) {
           entries[nidx_in_set].split.Update(
@@ -407,7 +412,7 @@ class HistEvaluator {
   }
 
   // Add splits to tree, handles all statistic
-  void ApplyTreeSplit(CPUExpandEntry const& candidate, RegTree *p_tree) {
+  void ApplyTreeSplit(CPUExpandEntry const &candidate, RegTree *p_tree) {
     auto evaluator = tree_evaluator_.GetEvaluator();
     RegTree &tree = *p_tree;
 
@@ -437,8 +442,7 @@ class HistEvaluator {
     auto left_child = tree[candidate.nid].LeftChild();
     auto right_child = tree[candidate.nid].RightChild();
     tree_evaluator_.AddSplit(candidate.nid, left_child, right_child,
-                             tree[candidate.nid].SplitIndex(), left_weight,
-                             right_weight);
+                             tree[candidate.nid].SplitIndex(), left_weight, right_weight);
     evaluator = tree_evaluator_.GetEvaluator();
 
     snode_.resize(tree.GetNodes().size());
@@ -449,8 +453,7 @@ class HistEvaluator {
     snode_.at(right_child).root_gain =
         evaluator.CalcGain(candidate.nid, *param_, GradStats{candidate.split.right_sum});
 
-    interaction_constraints_.Split(candidate.nid,
-                                   tree[candidate.nid].SplitIndex(), left_child,
+    interaction_constraints_.Split(candidate.nid, tree[candidate.nid].SplitIndex(), left_child,
                                    right_child);
   }
 
@@ -571,53 +574,6 @@ class HistMultiEvaluator {
     return false;
   }
 
-  /**
-   * @brief Gather the expand entries from all the workers.
-   * @param entries Local expand entries on this worker.
-   * @return Global expand entries gathered from all workers.
-   */
-  std::vector<MultiExpandEntry> Allgather(std::vector<MultiExpandEntry> const &entries) {
-    auto const world = collective::GetWorldSize();
-    auto const num_entries = entries.size();
-
-    // First, gather all the primitive fields.
-    std::vector<MultiExpandEntry> local_entries(num_entries);
-    std::vector<uint32_t> cat_bits;
-    std::vector<std::size_t> cat_bits_sizes;
-    std::vector<GradientPairPrecise> gradients;
-    for (std::size_t i = 0; i < num_entries; i++) {
-      local_entries[i].CopyAndCollect(entries[i], &cat_bits, &cat_bits_sizes, &gradients);
-    }
-    auto all_entries = collective::Allgather(local_entries);
-
-    // Gather all the cat_bits.
-    auto gathered_cat_bits = collective::SpecialAllgatherV(cat_bits, cat_bits_sizes);
-
-    // Gather all the gradients.
-    auto const num_gradients = gradients.size();
-    auto const all_gradients = collective::Allgather(gradients);
-
-    auto const total_entries = num_entries * world;
-    auto const gradients_per_entry = num_gradients / num_entries;
-    auto const gradients_per_side = gradients_per_entry / 2;
-    common::ParallelFor(total_entries, ctx_->Threads(), [&] (auto i) {
-      // Copy the cat_bits back into all expand entries.
-      all_entries[i].split.cat_bits.resize(gathered_cat_bits.sizes[i]);
-      std::copy_n(gathered_cat_bits.result.cbegin() + gathered_cat_bits.offsets[i],
-                  gathered_cat_bits.sizes[i], all_entries[i].split.cat_bits.begin());
-
-      // Copy the gradients back into all expand entries.
-      all_entries[i].split.left_sum.resize(gradients_per_side);
-      std::copy_n(all_gradients.cbegin() + i * gradients_per_entry, gradients_per_side,
-                  all_entries[i].split.left_sum.begin());
-      all_entries[i].split.right_sum.resize(gradients_per_side);
-      std::copy_n(all_gradients.cbegin() + i * gradients_per_entry + gradients_per_side,
-                  gradients_per_side, all_entries[i].split.right_sum.begin());
-    });
-
-    return all_entries;
-  }
-
  public:
   void EvaluateSplits(RegTree const &tree, common::Span<const BoundedHistCollection *> hist,
                       common::HistogramCuts const &cut, std::vector<MultiExpandEntry> *p_entries) {
@@ -676,7 +632,7 @@ class HistMultiEvaluator {
     if (is_col_split_) {
       // With column-wise data split, we gather the best splits from all the workers and update the
       // expand entries accordingly.
-      auto all_entries = Allgather(entries);
+      auto all_entries = AllgatherColumnSplit(entries);
       for (auto worker = 0; worker < collective::GetWorldSize(); ++worker) {
         for (std::size_t nidx_in_set = 0; nidx_in_set < entries.size(); ++nidx_in_set) {
           entries[nidx_in_set].split.Update(
diff --git a/src/tree/hist/expand_entry.h b/src/tree/hist/expand_entry.h
index d6315877d1e0..fd16397e1193 100644
--- a/src/tree/hist/expand_entry.h
+++ b/src/tree/hist/expand_entry.h
@@ -90,7 +90,6 @@ struct ExpandEntryImpl {
     }
 
     self->split.is_cat = get<Boolean const>(split["is_cat"]);
-
     self->LoadGrad(split);
   }
 };
@@ -106,8 +105,8 @@ struct CPUExpandEntry : public ExpandEntryImpl<CPUExpandEntry> {
   void SaveGrad(Json* p_out) const {
     auto& out = *p_out;
     auto save = [&](std::string const& name, GradStats const& sum) {
-      out[name] = F32Array{2};
-      auto& array = get<F32Array>(out[name]);
+      out[name] = F64Array{2};
+      auto& array = get<F64Array>(out[name]);
       array[0] = sum.GetGrad();
       array[1] = sum.GetHess();
     };
@@ -115,9 +114,9 @@ struct CPUExpandEntry : public ExpandEntryImpl<CPUExpandEntry> {
     save("right_sum", this->split.right_sum);
   }
   void LoadGrad(Json const& in) {
-    auto const& left_sum = get<F32Array const>(in["left_sum"]);
+    auto const& left_sum = get<F64Array const>(in["left_sum"]);
     this->split.left_sum = GradStats{left_sum[0], left_sum[1]};
-    auto const& right_sum = get<F32Array const>(in["right_sum"]);
+    auto const& right_sum = get<F64Array const>(in["right_sum"]);
     this->split.right_sum = GradStats{right_sum[0], right_sum[1]};
   }
 
@@ -173,8 +172,8 @@ struct MultiExpandEntry : public ExpandEntryImpl<MultiExpandEntry> {
   void SaveGrad(Json* p_out) const {
     auto& out = *p_out;
     auto save = [&](std::string const& name, std::vector<GradientPairPrecise> const& sum) {
-      out[name] = F32Array{sum.size() * 2};
-      auto& array = get<F32Array>(out[name]);
+      out[name] = F64Array{sum.size() * 2};
+      auto& array = get<F64Array>(out[name]);
       for (std::size_t i = 0, j = 0; i < sum.size(); i++, j += 2) {
         array[j] = sum[i].GetGrad();
         array[j + 1] = sum[i].GetHess();
@@ -185,7 +184,7 @@ struct MultiExpandEntry : public ExpandEntryImpl<MultiExpandEntry> {
   }
   void LoadGrad(Json const& in) {
     auto load = [&](std::string const& name, std::vector<GradientPairPrecise>* p_sum) {
-      auto const& array = get<F32Array const>(in[name]);
+      auto const& array = get<F64Array const>(in[name]);
       auto& sum = *p_sum;
       sum.resize(array.size() / 2);
       for (std::size_t i = 0, j = 0; i < sum.size(); ++i, j += 2) {
diff --git a/src/tree/updater_quantile_hist.cc b/src/tree/updater_quantile_hist.cc
index c2aaedafac95..cd60e6602cf7 100644
--- a/src/tree/updater_quantile_hist.cc
+++ b/src/tree/updater_quantile_hist.cc
@@ -1,5 +1,5 @@
 /**
- * Copyright 2017-2023, XGBoost Contributors
+ * Copyright 2017-2024, XGBoost Contributors
  * \file updater_quantile_hist.cc
  * \brief use quantized feature values to construct a tree
  * \author Philip Cho, Tianqi Checn, Egor Smirnov
@@ -149,9 +149,6 @@ class MultiTargetHistBuilder {
   }
 
   void InitData(DMatrix *p_fmat, RegTree const *p_tree) {
-    if (collective::IsDistributed()) {
-      LOG(FATAL) << "Distributed training for vector-leaf is not yet supported.";
-    }
     monitor_->Start(__func__);
 
     p_last_fmat_ = p_fmat;
diff --git a/tests/cpp/collective/test_rabit_communicator.cc b/tests/cpp/collective/test_rabit_communicator.cc
index ba22d8fdb84f..9711e1aede71 100644
--- a/tests/cpp/collective/test_rabit_communicator.cc
+++ b/tests/cpp/collective/test_rabit_communicator.cc
@@ -1,13 +1,12 @@
-/*!
- * Copyright 2022 XGBoost contributors
+/**
+ * Copyright 2022-2024, XGBoost contributors
  */
 #include <gtest/gtest.h>
 
 #include "../../../src/collective/rabit_communicator.h"
+#include "../helpers.h"
 
-namespace xgboost {
-namespace collective {
-
+namespace xgboost::collective {
 TEST(RabitCommunicatorSimpleTest, ThrowOnWorldSizeTooSmall) {
   auto construct = []() { RabitCommunicator comm{0, 0}; };
   EXPECT_THROW(construct(), dmlc::Error);
@@ -35,5 +34,37 @@ TEST(RabitCommunicatorSimpleTest, IsNotDistributed) {
   EXPECT_FALSE(comm.IsDistributed());
 }
 
-}  // namespace collective
-}  // namespace xgboost
+namespace {
+void VerifyVectorAllgatherV() {
+  auto n_workers = collective::GetWorldSize();
+  ASSERT_EQ(n_workers, 3);
+  auto rank = collective::GetRank();
+  // Construct input that has different length for each worker.
+  std::vector<std::vector<char>> inputs;
+  for (std::int32_t i = 0; i < rank + 1; ++i) {
+    std::vector<char> in;
+    for (std::int32_t j = 0; j < rank + 1; ++j) {
+      in.push_back(static_cast<char>(j));
+    }
+    inputs.emplace_back(std::move(in));
+  }
+
+  auto outputs = VectorAllgatherV(inputs);
+
+  ASSERT_EQ(outputs.size(), (1 + n_workers) * n_workers / 2);
+  auto const& res = outputs;
+
+  for (std::int32_t i = 0; i < n_workers; ++i) {
+    std::int32_t k = 0;
+    for (auto v : res[i]) {
+      ASSERT_EQ(v, k++);
+    }
+  }
+}
+}  // namespace
+
+TEST(VectorAllgatherV, Basic) {
+  std::int32_t n_workers{3};
+  RunWithInMemoryCommunicator(n_workers, VerifyVectorAllgatherV);
+}
+}  // namespace xgboost::collective
diff --git a/tests/cpp/common/test_json.cc b/tests/cpp/common/test_json.cc
index 72163efd78cc..3ee041a339ed 100644
--- a/tests/cpp/common/test_json.cc
+++ b/tests/cpp/common/test_json.cc
@@ -1,5 +1,5 @@
 /**
- * Copyright 2019-2023, XGBoost Contributors
+ * Copyright 2019-2024, XGBoost Contributors
  */
 #include <gtest/gtest.h>
 
diff --git a/tests/cpp/tree/test_quantile_hist.cc b/tests/cpp/tree/test_quantile_hist.cc
index cf806536a861..4021c9959440 100644
--- a/tests/cpp/tree/test_quantile_hist.cc
+++ b/tests/cpp/tree/test_quantile_hist.cc
@@ -1,5 +1,5 @@
 /**
- * Copyright 2018-2023 by XGBoost Contributors
+ * Copyright 2018-2024, XGBoost Contributors
  */
 #include <gtest/gtest.h>
 #include <xgboost/host_device_vector.h>
@@ -18,7 +18,6 @@
 #include "xgboost/data.h"
 
 namespace xgboost::tree {
-
 namespace {
 template <typename ExpandEntry>
 void TestPartitioner(bst_target_t n_targets) {
@@ -253,5 +252,5 @@ void TestColumnSplit(bst_target_t n_targets) {
 
 TEST(QuantileHist, ColumnSplit) { TestColumnSplit(1); }
 
-TEST(QuantileHist, DISABLED_ColumnSplitMultiTarget) { TestColumnSplit(3); }
+TEST(QuantileHist, ColumnSplitMultiTarget) { TestColumnSplit(3); }
 }  // namespace xgboost::tree