Move num_parallel_tree to model parameter. (#7751)

The size of forest should be a property of model itself instead of a training
hyper-parameter.

doc/model.schema

@@ -168,6 +168,9 @@
         "num_trees": {
           "type": "string"
         },
+        "num_parallel_tree": {
+          "type": "string"
+        },
         "size_leaf_vector": {
           "type": "string"
         }

python-package/xgboost/core.py

@@ -78,11 +78,14 @@ def from_cstr_to_pystr(data: CStrPptr, length: c_bst_ulong) -> List[str]:
     return res
 
 
+IterRange = TypeVar("IterRange", Optional[Tuple[int, int]], Tuple[int, int])
+
+
 def _convert_ntree_limit(
     booster: "Booster",
     ntree_limit: Optional[int],
-    iteration_range: Optional[Tuple[int, int]]
+    iteration_range: IterRange
-) -> Optional[Tuple[int, int]]:
+) -> IterRange:
     if ntree_limit is not None and ntree_limit != 0:
         warnings.warn(
             "ntree_limit is deprecated, use `iteration_range` or model "
@@ -1292,16 +1295,23 @@ def _get_booster_layer_trees(model: "Booster") -> Tuple[int, int]:
         num_parallel_tree = 0
     elif booster == "dart":
         num_parallel_tree = int(
-            config["learner"]["gradient_booster"]["gbtree"]["gbtree_train_param"][
+            config["learner"]["gradient_booster"]["gbtree"]["gbtree_model_param"][
                 "num_parallel_tree"
             ]
         )
     elif booster == "gbtree":
-        num_parallel_tree = int(
+        try:
-            config["learner"]["gradient_booster"]["gbtree_train_param"][
+            num_parallel_tree = int(
-                "num_parallel_tree"
+                config["learner"]["gradient_booster"]["gbtree_model_param"][
-            ]
+                    "num_parallel_tree"
-        )
+                ]
+            )
+        except KeyError:
+            num_parallel_tree = int(
+                config["learner"]["gradient_booster"]["gbtree_train_param"][
+                    "num_parallel_tree"
+                ]
+            )
     else:
         raise ValueError(f"Unknown booster: {booster}")
     num_groups = int(config["learner"]["learner_model_param"]["num_class"])

src/c_api/c_api_utils.h

@@ -129,18 +129,16 @@ inline uint32_t GetIterationFromTreeLimit(uint32_t ntree_limit, Learner *learner
 
     Json config{Object()};
     learner->SaveConfig(&config);
-    auto const &booster =
+    auto const &booster = get<String const>(config["learner"]["gradient_booster"]["name"]);
-        get<String const>(config["learner"]["gradient_booster"]["name"]);
     if (booster == "gblinear") {
       num_parallel_tree = 0;
     } else if (booster == "dart") {
-      num_parallel_tree = std::stoi(
+      num_parallel_tree =
-          get<String const>(config["learner"]["gradient_booster"]["gbtree"]
+          std::stoi(get<String const>(config["learner"]["gradient_booster"]["gbtree"]
-                                  ["gbtree_train_param"]["num_parallel_tree"]));
+                                            ["gbtree_model_param"]["num_parallel_tree"]));
     } else if (booster == "gbtree") {
       num_parallel_tree = std::stoi(get<String const>(
-          (config["learner"]["gradient_booster"]["gbtree_train_param"]
+          (config["learner"]["gradient_booster"]["gbtree_model_param"]["num_parallel_tree"])));
-                 ["num_parallel_tree"])));
     } else {
       LOG(FATAL) << "Unknown booster:" << booster;
     }

src/common/quantile.cc

@@ -429,9 +429,11 @@ void SketchContainerImpl<WQSketch>::AllReduce(
   this->GatherSketchInfo(reduced, &worker_segments, &sketches_scan, &global_sketches);
 
   std::vector<typename WQSketch::SummaryContainer> final_sketches(n_columns);
-  QuantileAllreduce<typename WQSketch::Entry> allreduce_result{global_sketches, worker_segments,
+
-                                                               sketches_scan, n_columns};
   ParallelFor(n_columns, n_threads_, [&](auto fidx) {
+    // gcc raises subobject-linkage warning if we put allreduce_result as lambda capture
+    QuantileAllreduce<typename WQSketch::Entry> allreduce_result{global_sketches, worker_segments,
+                                                                 sketches_scan, n_columns};
     int32_t intermediate_num_cuts = num_cuts[fidx];
     auto nbytes = WQSketch::SummaryContainer::CalcMemCost(intermediate_num_cuts);
     if (IsCat(feature_types_, fidx)) {

src/gbm/gbtree.cc

@@ -323,7 +323,7 @@ void GBTree::BoostNewTrees(HostDeviceVector<GradientPair>* gpair,
   std::vector<RegTree*> new_trees;
   ret->clear();
   // create the trees
-  for (int i = 0; i < tparam_.num_parallel_tree; ++i) {
+  for (int i = 0; i < model_.param.num_parallel_tree; ++i) {
     if (tparam_.process_type == TreeProcessType::kDefault) {
       CHECK(!updaters_.front()->CanModifyTree())
           << "Updater: `" << updaters_.front()->Name() << "` "
@@ -347,7 +347,7 @@ void GBTree::BoostNewTrees(HostDeviceVector<GradientPair>* gpair,
           << "boosting rounds can not exceed previous training rounds";
       // move an existing tree from trees_to_update
       auto t = std::move(model_.trees_to_update[model_.trees.size() +
-                                                bst_group * tparam_.num_parallel_tree + i]);
+                                                bst_group * model_.param.num_parallel_tree + i]);
       new_trees.push_back(t.get());
       ret->push_back(std::move(t));
     }
@@ -414,6 +414,10 @@ void GBTree::SaveConfig(Json* p_out) const {
   // e.g. updating a model, then saving and loading it would result in an empty
   // model
   out["gbtree_train_param"]["process_type"] = String("default");
+  // Duplicated from SaveModel so that user can get `num_parallel_tree` without parsing
+  // the model. We might remove this once we can deprecate `best_ntree_limit` so that the
+  // language binding doesn't need to know about the forest size.
+  out["gbtree_model_param"] = ToJson(model_.param);
 
   out["updater"] = Object();
 
@@ -460,6 +464,7 @@ void GBTree::Slice(int32_t layer_begin, int32_t layer_end, int32_t step,
   std::vector<int32_t> &out_trees_info = out_model.tree_info;
   out_trees_info.resize(layer_trees * n_layers);
   out_model.param.num_trees = out_model.trees.size();
+  out_model.param.num_parallel_tree = model_.param.num_parallel_tree;
   if (!this->model_.trees_to_update.empty()) {
     CHECK_EQ(this->model_.trees_to_update.size(), this->model_.trees.size())
         << "Not all trees are updated, "
@@ -512,8 +517,7 @@ void GBTree::PredictBatch(DMatrix* p_fmat,
   }
 
   uint32_t tree_begin, tree_end;
-  std::tie(tree_begin, tree_end) =
+  std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
-      detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
   CHECK_LE(tree_end, model_.trees.size()) << "Invalid number of trees.";
   if (tree_end > tree_begin) {
     predictor->PredictBatch(p_fmat, out_preds, model_, tree_begin, tree_end);
@@ -723,8 +727,7 @@ class Dart : public GBTree {
                                   model_);
     p_out_preds->version = 0;
     uint32_t tree_begin, tree_end;
-    std::tie(tree_begin, tree_end) =
+    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
-        detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
     auto n_groups = model_.learner_model_param->num_output_group;
 
     PredictionCacheEntry predts;  // temporary storage for prediction
@@ -779,7 +782,7 @@ class Dart : public GBTree {
                       float missing, PredictionCacheEntry *out_preds,
                       uint32_t layer_begin, unsigned layer_end) const override {
     uint32_t tree_begin, tree_end;
-    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
+    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
     std::vector<Predictor const *> predictors{
       cpu_predictor_.get(),
 #if defined(XGBOOST_USE_CUDA)
@@ -867,7 +870,7 @@ class Dart : public GBTree {
     DropTrees(false);
     auto &predictor = this->GetPredictor();
     uint32_t _, tree_end;
-    std::tie(_, tree_end) = detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
+    std::tie(_, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
     predictor->PredictInstance(inst, out_preds, model_, tree_end);
   }
 
@@ -877,7 +880,7 @@ class Dart : public GBTree {
                            unsigned) override {
     CHECK(configured_);
     uint32_t tree_begin, tree_end;
-    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
+    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
     cpu_predictor_->PredictContribution(p_fmat, out_contribs, model_,
                                         tree_end, &weight_drop_, approximate);
   }
@@ -887,9 +890,9 @@ class Dart : public GBTree {
       unsigned layer_begin, unsigned layer_end, bool approximate) override {
     CHECK(configured_);
     uint32_t tree_begin, tree_end;
-    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
+    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
-    cpu_predictor_->PredictInteractionContributions(p_fmat, out_contribs, model_,
+    cpu_predictor_->PredictInteractionContributions(p_fmat, out_contribs, model_, tree_end,
-                                                    tree_end, &weight_drop_, approximate);
+                                                    &weight_drop_, approximate);
   }
 
  protected:

src/gbm/gbtree.h

@@ -60,11 +60,6 @@ namespace gbm {
 
 /*! \brief training parameters */
 struct GBTreeTrainParam : public XGBoostParameter<GBTreeTrainParam> {
-  /*!
-   * \brief number of parallel trees constructed each iteration
-   *  use this option to support boosted random forest
-   */
-  int num_parallel_tree;
   /*! \brief tree updater sequence */
   std::string updater_seq;
   /*! \brief type of boosting process to run */
@@ -75,11 +70,6 @@ struct GBTreeTrainParam : public XGBoostParameter<GBTreeTrainParam> {
   TreeMethod tree_method;
   // declare parameters
   DMLC_DECLARE_PARAMETER(GBTreeTrainParam) {
-    DMLC_DECLARE_FIELD(num_parallel_tree)
-        .set_default(1)
-        .set_lower_bound(1)
-        .describe("Number of parallel trees constructed during each iteration."\
-                  " This option is used to support boosted random forest.");
     DMLC_DECLARE_FIELD(updater_seq)
         .set_default("grow_colmaker,prune")
         .describe("Tree updater sequence.");
@@ -156,12 +146,11 @@ struct DartTrainParam : public XGBoostParameter<DartTrainParam> {
 namespace detail {
 // From here on, layer becomes concrete trees.
 inline std::pair<uint32_t, uint32_t> LayerToTree(gbm::GBTreeModel const &model,
-                                                 GBTreeTrainParam const &tparam,
                                                  size_t layer_begin,
                                                  size_t layer_end) {
   bst_group_t groups = model.learner_model_param->num_output_group;
-  uint32_t tree_begin = layer_begin * groups * tparam.num_parallel_tree;
+  uint32_t tree_begin = layer_begin * groups * model.param.num_parallel_tree;
-  uint32_t tree_end = layer_end * groups * tparam.num_parallel_tree;
+  uint32_t tree_end = layer_end * groups * model.param.num_parallel_tree;
   if (tree_end == 0) {
     tree_end = static_cast<uint32_t>(model.trees.size());
   }
@@ -177,7 +166,7 @@ inline bool SliceTrees(int32_t layer_begin, int32_t layer_end, int32_t step,
                        GBTreeModel const &model, GBTreeTrainParam const &tparam,
                        uint32_t layer_trees, Func fn) {
   uint32_t tree_begin, tree_end;
-  std::tie(tree_begin, tree_end) = detail::LayerToTree(model, tparam, layer_begin, layer_end);
+  std::tie(tree_begin, tree_end) = detail::LayerToTree(model, layer_begin, layer_end);
   if (tree_end > model.trees.size()) {
     return true;
   }
@@ -249,7 +238,7 @@ class GBTree : public GradientBooster {
 
   // Number of trees per layer.
   auto LayerTrees() const {
-    auto n_trees = model_.learner_model_param->num_output_group * tparam_.num_parallel_tree;
+    auto n_trees = model_.learner_model_param->num_output_group * model_.param.num_parallel_tree;
     return n_trees;
   }
 
@@ -258,7 +247,7 @@ class GBTree : public GradientBooster {
              GradientBooster *out, bool* out_of_bound) const override;
 
   int32_t BoostedRounds() const override {
-    CHECK_NE(tparam_.num_parallel_tree, 0);
+    CHECK_NE(model_.param.num_parallel_tree, 0);
     CHECK_NE(model_.learner_model_param->num_output_group, 0);
     return model_.trees.size() / this->LayerTrees();
   }
@@ -271,8 +260,7 @@ class GBTree : public GradientBooster {
                       uint32_t layer_begin, unsigned layer_end) const override {
     CHECK(configured_);
     uint32_t tree_begin, tree_end;
-    std::tie(tree_begin, tree_end) =
+    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
-        detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
     CHECK_LE(tree_end, model_.trees.size()) << "Invalid number of trees.";
     std::vector<Predictor const *> predictors{
       cpu_predictor_.get(),
@@ -371,16 +359,15 @@ class GBTree : public GradientBooster {
                        uint32_t layer_begin, uint32_t layer_end) override {
     CHECK(configured_);
     uint32_t tree_begin, tree_end;
-    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
+    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
-    cpu_predictor_->PredictInstance(inst, out_preds, model_,
+    cpu_predictor_->PredictInstance(inst, out_preds, model_, tree_end);
-                                    tree_end);
   }
 
   void PredictLeaf(DMatrix* p_fmat,
                    HostDeviceVector<bst_float>* out_preds,
                    uint32_t layer_begin, uint32_t layer_end) override {
     uint32_t tree_begin, tree_end;
-    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
+    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
     CHECK_EQ(tree_begin, 0) << "Predict leaf supports only iteration end: (0, "
                                "n_iteration), use model slicing instead.";
     this->GetPredictor()->PredictLeaf(p_fmat, out_preds, model_, tree_end);
@@ -392,7 +379,7 @@ class GBTree : public GradientBooster {
                            int, unsigned) override {
     CHECK(configured_);
     uint32_t tree_begin, tree_end;
-    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
+    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
     CHECK_EQ(tree_begin, 0)
         << "Predict contribution supports only iteration end: (0, "
            "n_iteration), using model slicing instead.";
@@ -405,7 +392,7 @@ class GBTree : public GradientBooster {
       uint32_t layer_begin, uint32_t layer_end, bool approximate) override {
     CHECK(configured_);
     uint32_t tree_begin, tree_end;
-    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
+    std::tie(tree_begin, tree_end) = detail::LayerToTree(model_, layer_begin, layer_end);
     CHECK_EQ(tree_begin, 0)
         << "Predict interaction contribution supports only iteration end: (0, "
            "n_iteration), using model slicing instead.";

src/gbm/gbtree_model.h

@@ -31,7 +31,7 @@ struct GBTreeModelParam : public dmlc::Parameter<GBTreeModelParam> {
   /*! \brief number of trees */
   int32_t num_trees;
   /*! \brief (Deprecated) number of roots */
-  int32_t deprecated_num_roots;
+  int32_t num_parallel_tree;
   /*! \brief number of features to be used by trees */
   int32_t deprecated_num_feature;
   /*! \brief pad this space, for backward compatibility reason.*/
@@ -50,7 +50,7 @@ struct GBTreeModelParam : public dmlc::Parameter<GBTreeModelParam> {
     std::memset(this, 0, sizeof(GBTreeModelParam));  // FIXME(trivialfis): Why?
     static_assert(sizeof(GBTreeModelParam) == (4 + 2 + 2 + 32) * sizeof(int32_t),
                   "64/32 bit compatibility issue");
-    deprecated_num_roots = 1;
+    num_parallel_tree = 1;
   }
 
   // declare parameters, only declare those that need to be set.
@@ -59,6 +59,12 @@ struct GBTreeModelParam : public dmlc::Parameter<GBTreeModelParam> {
         .set_lower_bound(0)
         .set_default(0)
         .describe("Number of features used for training and prediction.");
+    DMLC_DECLARE_FIELD(num_parallel_tree)
+        .set_default(1)
+        .set_lower_bound(1)
+        .describe(
+            "Number of parallel trees constructed during each iteration."
+            " This option is used to support boosted random forest.");
     DMLC_DECLARE_FIELD(size_leaf_vector)
         .set_lower_bound(0)
         .set_default(0)
@@ -70,7 +76,7 @@ struct GBTreeModelParam : public dmlc::Parameter<GBTreeModelParam> {
   inline GBTreeModelParam ByteSwap() const {
     GBTreeModelParam x = *this;
     dmlc::ByteSwap(&x.num_trees, sizeof(x.num_trees), 1);
-    dmlc::ByteSwap(&x.deprecated_num_roots, sizeof(x.deprecated_num_roots), 1);
+    dmlc::ByteSwap(&x.num_parallel_tree, sizeof(x.num_parallel_tree), 1);
     dmlc::ByteSwap(&x.deprecated_num_feature, sizeof(x.deprecated_num_feature), 1);
     dmlc::ByteSwap(&x.pad_32bit, sizeof(x.pad_32bit), 1);
     dmlc::ByteSwap(&x.deprecated_num_pbuffer, sizeof(x.deprecated_num_pbuffer), 1);

tests/cpp/gbm/test_gbtree.cc

@@ -207,7 +207,7 @@ TEST(GBTree, JsonIO) {
   ASSERT_EQ(get<Integer>(get<Object>(get<Array>(gbtree_model["trees"]).front()).at("id")), 0);
   ASSERT_EQ(get<Array>(gbtree_model["tree_info"]).size(), 1ul);
 
-  auto j_train_param = model["config"]["gbtree_train_param"];
+  auto j_train_param = model["config"]["gbtree_model_param"];
   ASSERT_EQ(get<String>(j_train_param["num_parallel_tree"]), "1");
 }
 
@@ -337,6 +337,13 @@ std::pair<Json, Json> TestModelSlice(std::string booster) {
 
   Json sliced_config {Object()};
   sliced->SaveConfig(&sliced_config);
+  // Only num trees is changed
+  if (booster == "gbtree") {
+    sliced_config["learner"]["gradient_booster"]["gbtree_model_param"]["num_trees"] = String("60");
+  } else {
+    sliced_config["learner"]["gradient_booster"]["gbtree"]["gbtree_model_param"]["num_trees"] =
+        String("60");
+  }
   CHECK_EQ(sliced_config, config);
 
   auto get_trees = [&](Json const& model) {

tests/python/test_basic_models.py

@@ -8,8 +8,6 @@ import locale
 import tempfile
 
 dpath = os.path.join(tm.PROJECT_ROOT, 'demo/data/')
-dtrain = xgb.DMatrix(dpath + 'agaricus.txt.train')
-dtest = xgb.DMatrix(dpath + 'agaricus.txt.test')
 
 rng = np.random.RandomState(1994)
 
@@ -38,6 +36,8 @@ class TestModels:
         param = {'verbosity': 0, 'objective': 'binary:logistic',
                  'booster': 'gblinear', 'alpha': 0.0001, 'lambda': 1,
                  'nthread': 1}
+        dtrain = xgb.DMatrix(dpath + 'agaricus.txt.train')
+        dtest = xgb.DMatrix(dpath + 'agaricus.txt.test')
         watchlist = [(dtest, 'eval'), (dtrain, 'train')]
         num_round = 4
         bst = xgb.train(param, dtrain, num_round, watchlist)
@@ -124,7 +124,7 @@ class TestModels:
         predt_1 = bst.predict(margined)
 
         assert np.any(np.abs(predt_1 - predt_0) > 1e-6)
-
+        dtrain = xgb.DMatrix(dpath + 'agaricus.txt.train')
         bst = xgb.train({'tree_method': 'hist'}, dtrain, 2)
         predt_2 = bst.predict(dtrain)
         assert np.all(np.abs(predt_2 - predt_1) < 1e-6)
@@ -150,6 +150,8 @@ class TestModels:
             'objective': 'reg:logistic',
             "tree_method": tree_method
         }
+        dtrain = xgb.DMatrix(dpath + 'agaricus.txt.train')
+        dtest = xgb.DMatrix(dpath + 'agaricus.txt.test')
         watchlist = [(dtest, 'eval'), (dtrain, 'train')]
         num_round = 10
 
@@ -195,6 +197,8 @@ class TestModels:
         self.run_custom_objective()
 
     def test_multi_eval_metric(self):
+        dtrain = xgb.DMatrix(dpath + 'agaricus.txt.train')
+        dtest = xgb.DMatrix(dpath + 'agaricus.txt.test')
         watchlist = [(dtest, 'eval'), (dtrain, 'train')]
         param = {'max_depth': 2, 'eta': 0.2, 'verbosity': 1,
                  'objective': 'binary:logistic'}
@@ -216,6 +220,7 @@ class TestModels:
             param['scale_pos_weight'] = ratio
             return (dtrain, dtest, param)
 
+        dtrain = xgb.DMatrix(dpath + 'agaricus.txt.train')
         xgb.cv(param, dtrain, num_round, nfold=5,
                metrics={'auc'}, seed=0, fpreproc=fpreproc)
 
@@ -223,6 +228,7 @@ class TestModels:
         param = {'max_depth': 2, 'eta': 1, 'verbosity': 0,
                  'objective': 'binary:logistic'}
         num_round = 2
+        dtrain = xgb.DMatrix(dpath + 'agaricus.txt.train')
         xgb.cv(param, dtrain, num_round, nfold=5,
                metrics={'error'}, seed=0, show_stdv=False)
 
@@ -331,6 +337,7 @@ class TestModels:
         os.remove(model_path)
 
         try:
+            dtrain = xgb.DMatrix(dpath + 'agaricus.txt.train')
             xgb.train({'objective': 'foo'}, dtrain, num_boost_round=1)
         except ValueError as e:
             e_str = str(e)
@@ -422,68 +429,58 @@ class TestModels:
             assert cls.get_booster().best_ntree_limit == 2
             assert cls.best_ntree_limit == cls.get_booster().best_ntree_limit
 
-    @pytest.mark.skipif(**tm.no_sklearn())
+    def run_slice(
-    @pytest.mark.parametrize('booster', ['gbtree', 'dart'])
+        self,
-    def test_slice(self, booster):
+        booster: xgb.Booster,
-        from sklearn.datasets import make_classification
+        dtrain: xgb.DMatrix,
-        num_classes = 3
+        num_parallel_tree: int,
-        X, y = make_classification(n_samples=1000, n_informative=5,
+        num_classes: int,
-                                   n_classes=num_classes)
+        num_boost_round: int
-        dtrain = xgb.DMatrix(data=X, label=y)
+    ):
-        num_parallel_tree = 4
-        num_boost_round = 16
-        total_trees = num_parallel_tree * num_classes * num_boost_round
-        booster = xgb.train({
-            'num_parallel_tree': 4, 'subsample': 0.5, 'num_class': 3, 'booster': booster,
-            'objective': 'multi:softprob'},
-                            num_boost_round=num_boost_round, dtrain=dtrain)
-        booster.feature_types = ["q"] * X.shape[1]
-
-        assert len(booster.get_dump()) == total_trees
         beg = 3
         end = 7
-        sliced: xgb.Booster = booster[beg: end]
+        sliced: xgb.Booster = booster[beg:end]
         assert sliced.feature_types == booster.feature_types
 
         sliced_trees = (end - beg) * num_parallel_tree * num_classes
         assert sliced_trees == len(sliced.get_dump())
 
         sliced_trees = sliced_trees // 2
-        sliced: xgb.Booster = booster[beg: end: 2]
+        sliced = booster[beg:end:2]
         assert sliced_trees == len(sliced.get_dump())
 
-        sliced: xgb.Booster = booster[beg: ...]
+        sliced = booster[beg: ...]
         sliced_trees = (num_boost_round - beg) * num_parallel_tree * num_classes
         assert sliced_trees == len(sliced.get_dump())
 
-        sliced: xgb.Booster = booster[beg:]
+        sliced = booster[beg:]
         sliced_trees = (num_boost_round - beg) * num_parallel_tree * num_classes
         assert sliced_trees == len(sliced.get_dump())
 
-        sliced: xgb.Booster = booster[:end]
+        sliced = booster[:end]
         sliced_trees = end * num_parallel_tree * num_classes
         assert sliced_trees == len(sliced.get_dump())
 
-        sliced: xgb.Booster = booster[...:end]
+        sliced = booster[...: end]
         sliced_trees = end * num_parallel_tree * num_classes
         assert sliced_trees == len(sliced.get_dump())
 
-        with pytest.raises(ValueError, match=r'>= 0'):
+        with pytest.raises(ValueError, match=r">= 0"):
-            booster[-1: 0]
+            booster[-1:0]
 
         # we do not accept empty slice.
         with pytest.raises(ValueError):
             booster[1:1]
         # stop can not be smaller than begin
-        with pytest.raises(ValueError, match=r'Invalid.*'):
+        with pytest.raises(ValueError, match=r"Invalid.*"):
             booster[3:0]
-        with pytest.raises(ValueError, match=r'Invalid.*'):
+        with pytest.raises(ValueError, match=r"Invalid.*"):
             booster[3:-1]
         # negative step is not supported.
-        with pytest.raises(ValueError, match=r'.*>= 1.*'):
+        with pytest.raises(ValueError, match=r".*>= 1.*"):
             booster[0:2:-1]
         # step can not be 0.
-        with pytest.raises(ValueError, match=r'.*>= 1.*'):
+        with pytest.raises(ValueError, match=r".*>= 1.*"):
             booster[0:2:0]
 
         trees = [_ for _ in booster]
@@ -492,12 +489,12 @@ class TestModels:
         with pytest.raises(TypeError):
             booster["wrong type"]
         with pytest.raises(IndexError):
-            booster[:num_boost_round+1]
+            booster[: num_boost_round + 1]
         with pytest.raises(ValueError):
-            booster[1, 2]       # too many dims
+            booster[1, 2]  # too many dims
         # setitem is not implemented as model is immutable during slicing.
         with pytest.raises(TypeError):
-            booster[...:end] = booster
+            booster[...: end] = booster
 
         sliced_0 = booster[1:3]
         np.testing.assert_allclose(
@@ -525,6 +522,44 @@ class TestModels:
         single = booster[1:7].predict(dtrain, output_margin=True)
         np.testing.assert_allclose(merged, single, atol=1e-6)
 
+    @pytest.mark.skipif(**tm.no_sklearn())
+    @pytest.mark.parametrize("booster", ["gbtree", "dart"])
+    def test_slice(self, booster):
+        from sklearn.datasets import make_classification
+
+        num_classes = 3
+        X, y = make_classification(
+            n_samples=1000, n_informative=5, n_classes=num_classes
+        )
+        dtrain = xgb.DMatrix(data=X, label=y)
+        num_parallel_tree = 4
+        num_boost_round = 16
+        total_trees = num_parallel_tree * num_classes * num_boost_round
+        booster = xgb.train(
+            {
+                "num_parallel_tree": num_parallel_tree,
+                "subsample": 0.5,
+                "num_class": num_classes,
+                "booster": booster,
+                "objective": "multi:softprob",
+            },
+            num_boost_round=num_boost_round,
+            dtrain=dtrain,
+        )
+        booster.feature_types = ["q"] * X.shape[1]
+
+        assert len(booster.get_dump()) == total_trees
+
+        self.run_slice(booster, dtrain, num_parallel_tree, num_classes, num_boost_round)
+
+        bytesarray = booster.save_raw(raw_format="ubj")
+        booster = xgb.Booster(model_file=bytesarray)
+        self.run_slice(booster, dtrain, num_parallel_tree, num_classes, num_boost_round)
+
+        bytesarray = booster.save_raw(raw_format="deprecated")
+        booster = xgb.Booster(model_file=bytesarray)
+        self.run_slice(booster, dtrain, num_parallel_tree, num_classes, num_boost_round)
+
     @pytest.mark.skipif(**tm.no_pandas())
     def test_feature_info(self):
         import pandas as pd

tests/python/test_with_dask.py

@@ -530,7 +530,7 @@ def test_dask_regressor(model: str, client: "Client") -> None:
     forest = int(
         json.loads(regressor.get_booster().save_config())["learner"][
             "gradient_booster"
-        ]["gbtree_train_param"]["num_parallel_tree"]
+        ]["gbtree_model_param"]["num_parallel_tree"]
     )
 
     if model == "boosting":
@@ -584,7 +584,7 @@ def run_dask_classifier(
     assert n_threads != 0 and n_threads != os.cpu_count()
 
     forest = int(
-        config["learner"]["gradient_booster"]["gbtree_train_param"]["num_parallel_tree"]
+        config["learner"]["gradient_booster"]["gbtree_model_param"]["num_parallel_tree"]
     )
     if model == "boosting":
         assert len(history["validation_0"][metric]) == 2

tests/python/test_with_sklearn.py

@@ -329,21 +329,27 @@ def test_select_feature():
 
 def test_num_parallel_tree():
     from sklearn.datasets import fetch_california_housing
-    reg = xgb.XGBRegressor(n_estimators=4, num_parallel_tree=4,
+
-                           tree_method='hist')
+    reg = xgb.XGBRegressor(n_estimators=4, num_parallel_tree=4, tree_method="hist")
     X, y = fetch_california_housing(return_X_y=True)
     bst = reg.fit(X=X, y=y)
-    dump = bst.get_booster().get_dump(dump_format='json')
+    dump = bst.get_booster().get_dump(dump_format="json")
     assert len(dump) == 16
 
     reg = xgb.XGBRFRegressor(n_estimators=4)
     bst = reg.fit(X=X, y=y)
-    dump = bst.get_booster().get_dump(dump_format='json')
+    dump = bst.get_booster().get_dump(dump_format="json")
     assert len(dump) == 4
 
     config = json.loads(bst.get_booster().save_config())
-    assert int(config['learner']['gradient_booster']['gbtree_train_param'][
+    assert (
-        'num_parallel_tree']) == 4
+        int(
+            config["learner"]["gradient_booster"]["gbtree_model_param"][
+                "num_parallel_tree"
+            ]
+        )
+        == 4
+    )
 
 
 def test_calif_housing_regression():