[breaking] Remove the predictor param, allow fallback to prediction…

… using `DMatrix`. (#9129)


- A `DeviceOrd` struct is implemented to indicate the device. It will eventually replace the `gpu_id` parameter.
- The `predictor` parameter is removed.
- Fallback to `DMatrix` when `inplace_predict` is not available.
- The heuristic for choosing a predictor is only used during training.

doc/gpu/index.rst

@@ -45,7 +45,7 @@ XGBoost makes use of `GPUTreeShap <https://github.com/rapidsai/gputreeshap>`_ as
 
 .. code-block:: python
 
-  model.set_param({"predictor": "gpu_predictor"})
+  model.set_param({"gpu_id": "0", "tree_method": "gpu_hist"})
   shap_values = model.predict(dtrain, pred_contribs=True)
   shap_interaction_values = model.predict(dtrain, pred_interactions=True)
 

doc/parameter.rst

@@ -199,18 +199,6 @@ Parameters for Tree Booster
   - Maximum number of discrete bins to bucket continuous features.
   - Increasing this number improves the optimality of splits at the cost of higher computation time.
 
-* ``predictor``, [default= ``auto``]
-
-  - The type of predictor algorithm to use. Provides the same results but allows the use of GPU or CPU.
-
-    - ``auto``: Configure predictor based on heuristics.
-    - ``cpu_predictor``: Multicore CPU prediction algorithm.
-    - ``gpu_predictor``: Prediction using GPU.  Used when ``tree_method`` is ``gpu_hist``.
-      When ``predictor`` is set to default value ``auto``, the ``gpu_hist`` tree method is
-      able to provide GPU based prediction without copying training data to GPU memory.
-      If ``gpu_predictor`` is explicitly specified, then all data is copied into GPU, only
-      recommended for performing prediction tasks.
-
 * ``num_parallel_tree``, [default=1]
 
   - Number of parallel trees constructed during each iteration. This option is used to support boosted random forest.

doc/prediction.rst

@@ -87,15 +87,6 @@ with the native Python interface :py:meth:`xgboost.Booster.predict` and
 behavior.  Also the ``save_best`` parameter from :py:obj:`xgboost.callback.EarlyStopping`
 might be useful.
 
-*********
-Predictor
-*********
-
-There are 2 predictors in XGBoost (3 if you have the one-api plugin enabled), namely
-``cpu_predictor`` and ``gpu_predictor``.  The default option is ``auto`` so that XGBoost
-can employ some heuristics for saving GPU memory during training.  They might have slight
-different outputs due to floating point errors.
-
 
 ***********
 Base Margin
@@ -134,15 +125,6 @@ it.  Be aware that the output of in-place prediction depends on input data type,
 input is on GPU data output is :py:obj:`cupy.ndarray`, otherwise a :py:obj:`numpy.ndarray`
 is returned.
 
-****************
-Categorical Data
-****************
-
-Other than users performing encoding, XGBoost has experimental support for categorical
-data using ``gpu_hist`` and ``gpu_predictor``.  No special operation needs to be done on
-input test data since the information about categories is encoded into the model during
-training.
-
 *************
 Thread Safety
 *************
@@ -159,7 +141,6 @@ instance we might accidentally call ``clf.set_params()`` inside a predict functi
 
     def predict_fn(clf: xgb.XGBClassifier, X):
         X = preprocess(X)
-        clf.set_params(predictor="gpu_predictor")  # NOT safe!
         clf.set_params(n_jobs=1)  # NOT safe!
         return clf.predict_proba(X, iteration_range=(0, 10))
 

doc/tutorials/dask.rst

@@ -148,8 +148,8 @@ Also for inplace prediction:
 
 .. code-block:: python
 
-  booster.set_param({'predictor': 'gpu_predictor'})
-  # where X is a dask DataFrame or dask Array containing cupy or cuDF backed data.
+  # where X is a dask DataFrame or dask Array backed by cupy or cuDF.
+  booster.set_param({"gpu_id": "0"})
   prediction = xgb.dask.inplace_predict(client, booster, X)
 
 When input is ``da.Array`` object, output is always ``da.Array``.  However, if the input

doc/tutorials/saving_model.rst

@@ -173,7 +173,6 @@ Will print out something similar to (not actual output as it's too long for demo
         "gradient_booster": {
           "gbtree_train_param": {
             "num_parallel_tree": "1",
-            "predictor": "gpu_predictor",
             "process_type": "default",
             "tree_method": "gpu_hist",
             "updater": "grow_gpu_hist",

include/xgboost/base.h

@@ -10,6 +10,7 @@
 #include <dmlc/omp.h>
 
 #include <cmath>
+#include <cstdint>
 #include <iostream>
 #include <string>
 #include <utility>
@@ -112,7 +113,7 @@ using bst_row_t = std::size_t;   // NOLINT
 /*! \brief Type for tree node index. */
 using bst_node_t = std::int32_t;      // NOLINT
 /*! \brief Type for ranking group index. */
-using bst_group_t = std::uint32_t;      // NOLINT
+using bst_group_t = std::uint32_t;  // NOLINT
 /**
  * \brief Type for indexing into output targets.
  */
@@ -125,6 +126,10 @@ using bst_layer_t = std::int32_t;  // NOLINT
  * \brief Type for indexing trees.
  */
 using bst_tree_t = std::int32_t;  // NOLINT
+/**
+ * @brief Ordinal of a CUDA device.
+ */
+using bst_d_ordinal_t = std::int16_t;  // NOLINT
 
 namespace detail {
 /*! \brief Implementation of gradient statistics pair. Template specialisation

include/xgboost/c_api.h

@@ -1067,6 +1067,9 @@ XGB_DLL int XGBoosterPredictFromDMatrix(BoosterHandle handle, DMatrixHandle dmat
 /**
  * \brief Inplace prediction from CPU dense matrix.
  *
+ * \note If the booster is configured to run on a CUDA device, XGBoost falls back to run
+ *       prediction with DMatrix with a performance warning.
+ *
  * \param handle        Booster handle.
  * \param values        JSON encoded __array_interface__ to values.
  * \param config        See \ref XGBoosterPredictFromDMatrix for more info.
@@ -1091,6 +1094,9 @@ XGB_DLL int XGBoosterPredictFromDense(BoosterHandle handle, char const *values,
 /**
  * \brief Inplace prediction from CPU CSR matrix.
  *
+ * \note If the booster is configured to run on a CUDA device, XGBoost falls back to run
+ *       prediction with DMatrix with a performance warning.
+ *
  * \param handle        Booster handle.
  * \param indptr        JSON encoded __array_interface__ to row pointer in CSR.
  * \param indices       JSON encoded __array_interface__ to column indices in CSR.
@@ -1116,6 +1122,9 @@ XGB_DLL int XGBoosterPredictFromCSR(BoosterHandle handle, char const *indptr, ch
 /**
  * \brief Inplace prediction from CUDA Dense matrix (cupy in Python).
  *
+ * \note If the booster is configured to run on a CPU, XGBoost falls back to run
+ *       prediction with DMatrix with a performance warning.
+ *
  * \param handle        Booster handle
  * \param values        JSON encoded __cuda_array_interface__ to values.
  * \param config        See \ref XGBoosterPredictFromDMatrix for more info.
@@ -1137,6 +1146,9 @@ XGB_DLL int XGBoosterPredictFromCudaArray(BoosterHandle handle, char const *valu
 /**
  * \brief Inplace prediction from CUDA dense dataframe (cuDF in Python).
  *
+ * \note If the booster is configured to run on a CPU, XGBoost falls back to run
+ *       prediction with DMatrix with a performance warning.
+ *
  * \param handle        Booster handle
  * \param values        List of __cuda_array_interface__ for all columns encoded in JSON list.
  * \param config        See \ref XGBoosterPredictFromDMatrix for more info.

include/xgboost/context.h

@@ -1,20 +1,79 @@
-/*!
- * Copyright 2014-2022 by Contributors
+/**
+ * Copyright 2014-2023, XGBoost Contributors
  * \file context.h
  */
 #ifndef XGBOOST_CONTEXT_H_
 #define XGBOOST_CONTEXT_H_
 
-#include <xgboost/logging.h>
-#include <xgboost/parameter.h>
+#include <xgboost/base.h>       // for bst_d_ordinal_t
+#include <xgboost/logging.h>    // for CHECK_GE
+#include <xgboost/parameter.h>  // for XGBoostParameter
 
-#include <memory>  // std::shared_ptr
-#include <string>
+#include <cstdint>  // for int16_t, int32_t, int64_t
+#include <memory>   // for shared_ptr
+#include <string>   // for string, to_string
 
 namespace xgboost {
 
 struct CUDAContext;
 
+/**
+ * @brief A type for device ordinal. The type is packed into 32-bit for efficient use in
+ *        viewing types like `linalg::TensorView`.
+ */
+struct DeviceOrd {
+  enum Type : std::int16_t { kCPU = 0, kCUDA = 1 } device{kCPU};
+  // CUDA device ordinal.
+  bst_d_ordinal_t ordinal{-1};
+
+  [[nodiscard]] bool IsCUDA() const { return device == kCUDA; }
+  [[nodiscard]] bool IsCPU() const { return device == kCPU; }
+
+  DeviceOrd() = default;
+  constexpr DeviceOrd(Type type, bst_d_ordinal_t ord) : device{type}, ordinal{ord} {}
+
+  DeviceOrd(DeviceOrd const& that) = default;
+  DeviceOrd& operator=(DeviceOrd const& that) = default;
+  DeviceOrd(DeviceOrd&& that) = default;
+  DeviceOrd& operator=(DeviceOrd&& that) = default;
+
+  /**
+   * @brief Constructor for CPU.
+   */
+  [[nodiscard]] constexpr static auto CPU() { return DeviceOrd{kCPU, -1}; }
+  /**
+   * @brief Constructor for CUDA device.
+   *
+   * @param ordinal CUDA device ordinal.
+   */
+  [[nodiscard]] static auto CUDA(bst_d_ordinal_t ordinal) { return DeviceOrd{kCUDA, ordinal}; }
+
+  [[nodiscard]] bool operator==(DeviceOrd const& that) const {
+    return device == that.device && ordinal == that.ordinal;
+  }
+  [[nodiscard]] bool operator!=(DeviceOrd const& that) const { return !(*this == that); }
+  /**
+   * @brief Get a string representation of the device and the ordinal.
+   */
+  [[nodiscard]] std::string Name() const {
+    switch (device) {
+      case DeviceOrd::kCPU:
+        return "CPU";
+      case DeviceOrd::kCUDA:
+        return "CUDA:" + std::to_string(ordinal);
+      default: {
+        LOG(FATAL) << "Unknown device.";
+        return "";
+      }
+    }
+  }
+};
+
+static_assert(sizeof(DeviceOrd) == sizeof(std::int32_t));
+
+/**
+ * @brief Runtime context for XGBoost. Contains information like threads and device.
+ */
 struct Context : public XGBoostParameter<Context> {
  public:
   // Constant representing the device ID of CPU.
@@ -36,29 +95,59 @@ struct Context : public XGBoostParameter<Context> {
   // fail when gpu_id is invalid
   bool fail_on_invalid_gpu_id{false};
   bool validate_parameters{false};
-
-  /*!
-   * \brief Configure the parameter `gpu_id'.
+  /**
+   * @brief Configure the parameter `gpu_id'.
    *
-   * \param require_gpu  Whether GPU is explicitly required from user.
+   * @param require_gpu Whether GPU is explicitly required by the user through other
+   *                    configurations.
    */
   void ConfigureGpuId(bool require_gpu);
-  /*!
-   * Return automatically chosen threads.
+  /**
+   * @brief Returns the automatically chosen number of threads based on the `nthread`
+   *        parameter and the system settting.
    */
-  std::int32_t Threads() const;
-
-  bool IsCPU() const { return gpu_id == kCpuId; }
-  bool IsCUDA() const { return !IsCPU(); }
-
-  CUDAContext const* CUDACtx() const;
-  // Make a CUDA context based on the current context.
-  Context MakeCUDA(std::int32_t device = 0) const {
+  [[nodiscard]] std::int32_t Threads() const;
+  /**
+   * @brief Is XGBoost running on CPU?
+   */
+  [[nodiscard]] bool IsCPU() const { return gpu_id == kCpuId; }
+  /**
+   * @brief Is XGBoost running on a CUDA device?
+   */
+  [[nodiscard]] bool IsCUDA() const { return !IsCPU(); }
+  /**
+   * @brief Get the current device and ordinal.
+   */
+  [[nodiscard]] DeviceOrd Device() const {
+    return IsCPU() ? DeviceOrd::CPU() : DeviceOrd::CUDA(static_cast<bst_d_ordinal_t>(gpu_id));
+  }
+  /**
+   * @brief Get the CUDA device ordinal. -1 if XGBoost is running on CPU.
+   */
+  [[nodiscard]] bst_d_ordinal_t Ordinal() const { return this->gpu_id; }
+  /**
+   * @brief Name of the current device.
+   */
+  [[nodiscard]] std::string DeviceName() const { return Device().Name(); }
+  /**
+   * @brief Get a CUDA device context for allocator and stream.
+   */
+  [[nodiscard]] CUDAContext const* CUDACtx() const;
+  /**
+   * @brief Make a CUDA context based on the current context.
+   *
+   * @param ordinal The CUDA device ordinal.
+   */
+  [[nodiscard]] Context MakeCUDA(std::int32_t ordinal = 0) const {
     Context ctx = *this;
-    ctx.gpu_id = device;
+    CHECK_GE(ordinal, 0);
+    ctx.gpu_id = ordinal;
     return ctx;
   }
-  Context MakeCPU() const {
+  /**
+   * @brief Make a CPU context based on the current context.
+   */
+  [[nodiscard]] Context MakeCPU() const {
     Context ctx = *this;
     ctx.gpu_id = kCpuId;
     return ctx;
@@ -87,9 +176,9 @@ struct Context : public XGBoostParameter<Context> {
   }
 
  private:
-  // mutable for lazy initialization for cuda context to avoid initializing CUDA at load.
-  // shared_ptr is used instead of unique_ptr as with unique_ptr it's difficult to define p_impl
-  // while trying to hide CUDA code from host compiler.
+  // mutable for lazy cuda context initialization. This avoids initializing CUDA at load.
+  // shared_ptr is used instead of unique_ptr as with unique_ptr it's difficult to define
+  // p_impl while trying to hide CUDA code from the host compiler.
   mutable std::shared_ptr<CUDAContext> cuctx_;
   // cached value for CFS CPU limit. (used in containerized env)
   std::int32_t cfs_cpu_count_;  // NOLINT

include/xgboost/gbm.h

@@ -149,18 +149,14 @@ class GradientBooster : public Model, public Configurable {
    * \param layer_begin Beginning of boosted tree layer used for prediction.
    * \param layer_end   End of booster layer. 0 means do not limit trees.
    * \param approximate use a faster (inconsistent) approximation of SHAP values
-   * \param condition condition on the condition_feature (0=no, -1=cond off, 1=cond on).
-   * \param condition_feature feature to condition on (i.e. fix) during calculations
    */
-  virtual void PredictContribution(DMatrix* dmat,
-                                   HostDeviceVector<bst_float>* out_contribs,
-                                   unsigned layer_begin, unsigned layer_end,
-                                   bool approximate = false, int condition = 0,
-                                   unsigned condition_feature = 0) = 0;
-
-  virtual void PredictInteractionContributions(
-      DMatrix *dmat, HostDeviceVector<bst_float> *out_contribs,
-      unsigned layer_begin, unsigned layer_end, bool approximate) = 0;
+  virtual void PredictContribution(DMatrix* dmat, HostDeviceVector<float>* out_contribs,
+                                   bst_layer_t layer_begin, bst_layer_t layer_end,
+                                   bool approximate = false) = 0;
+
+  virtual void PredictInteractionContributions(DMatrix* dmat, HostDeviceVector<float>* out_contribs,
+                                               bst_layer_t layer_begin, bst_layer_t layer_end,
+                                               bool approximate) = 0;
 
   /*!
    * \brief dump the model in the requested format

jvm-packages/xgboost4j-gpu/src/test/java/ml/dmlc/xgboost4j/gpu/java/BoosterTest.java

@@ -78,7 +78,6 @@ public void testBooster() throws XGBoostError {
         put("num_round", round);
         put("num_workers", 1);
         put("tree_method", "gpu_hist");
-        put("predictor", "gpu_predictor");
         put("max_bin", maxBin);
       }
     };

jvm-packages/xgboost4j-spark-gpu/src/main/scala/ml/dmlc/xgboost4j/scala/rapids/spark/GpuPreXGBoost.scala

@@ -281,7 +281,6 @@ object GpuPreXGBoost extends PreXGBoostProvider {
             // - predictor: Force to gpu predictor since native doesn't save predictor.
             val gpuId = if (!isLocal) XGBoost.getGPUAddrFromResources else 0
             booster.setParam("gpu_id", gpuId.toString)
-            booster.setParam("predictor", "gpu_predictor")
             logger.info("GPU transform on device: " + gpuId)
             boosterFlag.isGpuParamsSet = true;
           }