Enable running objectives with 0 GPU. (#3878)

* Enable running objectives with 0 GPU.

* Enable 0 GPU for objectives.
* Add doc for GPU objectives.
* Fix some objectives defaulted to running on all GPUs.

doc/gpu/index.rst

@@ -78,6 +78,49 @@ The GPU algorithms currently work with CLI, Python and R packages. See :doc:`/bu
   param['max_bin'] = 16
   param['tree_method'] = 'gpu_hist'
 
+Objective functions
+===================
+Most of the objective functions implemented in XGBoost can be run on GPU.  Following table shows current support status.
+
+.. |tick| unicode:: U+2714
+.. |cross| unicode:: U+2718
+
++-----------------+-------------+
+| Objectives      | GPU support |
++-----------------+-------------+
+| reg:linear      | |tick|      |
++-----------------+-------------+
+| reg:logistic    | |tick|      |
++-----------------+-------------+
+| binary:logistic | |tick|      |
++-----------------+-------------+
+| binary:logitraw | |tick|      |
++-----------------+-------------+
+| binary:hinge    | |tick|      |
++-----------------+-------------+
+| count:poisson   | |tick|      |
++-----------------+-------------+
+| reg:gamma       | |tick|      |
++-----------------+-------------+
+| reg:tweedie     | |tick|      |
++-----------------+-------------+
+| multi:softmax   | |tick|      |
++-----------------+-------------+
+| multi:softprob  | |tick|      |
++-----------------+-------------+
+| survival:cox    | |cross|     |
++-----------------+-------------+
+| rank:pairwise   | |cross|     |
++-----------------+-------------+
+| rank:ndcg       | |cross|     |
++-----------------+-------------+
+| rank:map        | |cross|     |
++-----------------+-------------+
+
+For multi-gpu support, objective functions also honor the ``n_gpus`` parameter,
+which, by default is set to 1.  To disable running objectives on GPU, just set
+``n_gpus`` to 0.
+
 Benchmarks
 ==========
 You can run benchmarks on synthetic data for binary classification:
@@ -118,4 +161,3 @@ Authors
 * ... and the rest of the H2O.ai and NVIDIA team.
 
 Please report bugs to the user forum https://discuss.xgboost.ai/.
-

doc/parameter.rst

@@ -283,9 +283,6 @@ Specify the learning task and the corresponding learning objective. The objectiv
   - ``binary:logistic``: logistic regression for binary classification, output probability
   - ``binary:logitraw``: logistic regression for binary classification, output score before logistic transformation
   - ``binary:hinge``: hinge loss for binary classification. This makes predictions of 0 or 1, rather than producing probabilities.
-  - ``gpu:reg:linear``, ``gpu:reg:logistic``, ``gpu:binary:logistic``, ``gpu:binary:logitraw``: versions
-    of the corresponding objective functions evaluated on the GPU; note that like the GPU histogram algorithm,
-    they can only be used when the entire training session uses the same dataset
   - ``count:poisson`` --poisson regression for count data, output mean of poisson distribution
 
     - ``max_delta_step`` is set to 0.7 by default in poisson regression (used to safeguard optimization)

src/common/common.h

@@ -165,7 +165,7 @@ class GPUSet {
     CHECK_GE(n_gpus, -1) << "n_gpus must be >= -1.";
 
     GpuIdType const n_devices_visible = AllVisible().Size();
-    if (n_devices_visible == 0) { return Empty(); }
+    if (n_devices_visible == 0 || n_gpus == 0) { return Empty(); }
 
     GpuIdType const n_available_devices = n_devices_visible - gpu_id;
 

src/objective/hinge.cu

@@ -22,7 +22,7 @@ struct HingeObjParam : public dmlc::Parameter<HingeObjParam> {
   int n_gpus;
   int gpu_id;
   DMLC_DECLARE_PARAMETER(HingeObjParam) {
-    DMLC_DECLARE_FIELD(n_gpus).set_default(1).set_lower_bound(-1)
+    DMLC_DECLARE_FIELD(n_gpus).set_default(1).set_lower_bound(GPUSet::kAll)
         .describe("Number of GPUs to use for multi-gpu algorithms.");
     DMLC_DECLARE_FIELD(gpu_id)
         .set_lower_bound(0)

src/objective/multiclass_obj.cu

@@ -31,7 +31,7 @@ struct SoftmaxMultiClassParam : public dmlc::Parameter<SoftmaxMultiClassParam> {
   DMLC_DECLARE_PARAMETER(SoftmaxMultiClassParam) {
     DMLC_DECLARE_FIELD(num_class).set_lower_bound(1)
         .describe("Number of output class in the multi-class classification.");
-    DMLC_DECLARE_FIELD(n_gpus).set_default(1).set_lower_bound(-1)
+    DMLC_DECLARE_FIELD(n_gpus).set_default(1).set_lower_bound(GPUSet::kAll)
         .describe("Number of GPUs to use for multi-gpu algorithms.");
     DMLC_DECLARE_FIELD(gpu_id)
         .set_lower_bound(0)
@@ -49,7 +49,6 @@ class SoftmaxMultiClassObj : public ObjFunction {
   }
   void Configure(const std::vector<std::pair<std::string, std::string> >& args) override {
     param_.InitAllowUnknown(args);
-    CHECK(param_.n_gpus != 0) << "Must have at least one device";  // Default is -1
     devices_ = GPUSet::All(param_.gpu_id, param_.n_gpus);
     label_correct_.Resize(devices_.IsEmpty() ? 1 : devices_.Size());
   }

src/objective/regression_obj.cu

@@ -34,7 +34,7 @@ struct RegLossParam : public dmlc::Parameter<RegLossParam> {
   DMLC_DECLARE_PARAMETER(RegLossParam) {
     DMLC_DECLARE_FIELD(scale_pos_weight).set_default(1.0f).set_lower_bound(0.0f)
       .describe("Scale the weight of positive examples by this factor");
-    DMLC_DECLARE_FIELD(n_gpus).set_default(1).set_lower_bound(-1)
+    DMLC_DECLARE_FIELD(n_gpus).set_default(1).set_lower_bound(GPUSet::kAll)
       .describe("Number of GPUs to use for multi-gpu algorithms.");
     DMLC_DECLARE_FIELD(gpu_id)
       .set_lower_bound(0)
@@ -53,7 +53,6 @@ class RegLossObj : public ObjFunction {
 
   void Configure(const std::vector<std::pair<std::string, std::string> >& args) override {
     param_.InitAllowUnknown(args);
-    CHECK(param_.n_gpus != 0) << "Must have at least one device";  // Default is -1
     devices_ = GPUSet::All(param_.gpu_id, param_.n_gpus);
     label_correct_.Resize(devices_.IsEmpty() ? 1 : devices_.Size());
   }
@@ -182,7 +181,7 @@ struct PoissonRegressionParam : public dmlc::Parameter<PoissonRegressionParam> {
     DMLC_DECLARE_FIELD(max_delta_step).set_lower_bound(0.0f).set_default(0.7f)
         .describe("Maximum delta step we allow each weight estimation to be." \
                   " This parameter is required for possion regression.");
-    DMLC_DECLARE_FIELD(n_gpus).set_default(-1).set_lower_bound(-1)
+    DMLC_DECLARE_FIELD(n_gpus).set_default(1).set_lower_bound(GPUSet::kAll)
         .describe("Number of GPUs to use for multi-gpu algorithms.");
     DMLC_DECLARE_FIELD(gpu_id)
         .set_lower_bound(0)
@@ -197,7 +196,6 @@ class PoissonRegression : public ObjFunction {
   // declare functions
   void Configure(const std::vector<std::pair<std::string, std::string> >& args) override {
     param_.InitAllowUnknown(args);
-    CHECK(param_.n_gpus != 0) << "Must have at least one device";  // Default is -1
     devices_ = GPUSet::All(param_.gpu_id, param_.n_gpus);
     label_correct_.Resize(devices_.IsEmpty() ? 1 : devices_.Size());
   }
@@ -364,7 +362,7 @@ struct GammaRegressionParam : public dmlc::Parameter<GammaRegressionParam> {
   int n_gpus;
   int gpu_id;
   DMLC_DECLARE_PARAMETER(GammaRegressionParam) {
-    DMLC_DECLARE_FIELD(n_gpus).set_default(-1).set_lower_bound(-1)
+    DMLC_DECLARE_FIELD(n_gpus).set_default(1).set_lower_bound(GPUSet::kAll)
         .describe("Number of GPUs to use for multi-gpu algorithms.");
     DMLC_DECLARE_FIELD(gpu_id)
         .set_lower_bound(0)
@@ -379,7 +377,6 @@ class GammaRegression : public ObjFunction {
   // declare functions
   void Configure(const std::vector<std::pair<std::string, std::string> >& args) override {
     param_.InitAllowUnknown(args);
-    CHECK(param_.n_gpus != 0) << "Must have at least one device";  // Default is -1
     devices_ = GPUSet::All(param_.gpu_id, param_.n_gpus);
     label_correct_.Resize(devices_.IsEmpty() ? 1 : devices_.Size());
   }
@@ -461,7 +458,7 @@ struct TweedieRegressionParam : public dmlc::Parameter<TweedieRegressionParam> {
   DMLC_DECLARE_PARAMETER(TweedieRegressionParam) {
     DMLC_DECLARE_FIELD(tweedie_variance_power).set_range(1.0f, 2.0f).set_default(1.5f)
       .describe("Tweedie variance power.  Must be between in range [1, 2).");
-    DMLC_DECLARE_FIELD(n_gpus).set_default(-1).set_lower_bound(-1)
+    DMLC_DECLARE_FIELD(n_gpus).set_default(1).set_lower_bound(GPUSet::kAll)
         .describe("Number of GPUs to use for multi-gpu algorithms.");
     DMLC_DECLARE_FIELD(gpu_id)
         .set_lower_bound(0)
@@ -476,7 +473,6 @@ class TweedieRegression : public ObjFunction {
   // declare functions
   void Configure(const std::vector<std::pair<std::string, std::string> >& args) override {
     param_.InitAllowUnknown(args);
-    CHECK(param_.n_gpus != 0) << "Must have at least one device";  // Default is -1
     devices_ = GPUSet::All(param_.gpu_id, param_.n_gpus);
     label_correct_.Resize(devices_.IsEmpty() ? 1 : devices_.Size());
   }