refine sgd_op (#13626)

test=develop

paddle/fluid/operators/sgd_op.cu

@@ -12,7 +12,7 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License. */
 
-#define EIGEN_USE_GPU
+#include <algorithm>
 #include "paddle/fluid/operators/sgd_op.h"
 #include "paddle/fluid/platform/cuda_primitives.h"
 
@@ -33,22 +33,21 @@ __global__ void SGDKernel(const T* g, const T* p, const T* learning_rate,
   }
 }
 
-template <typename T, int block_size>
+template <typename T>
 __global__ void SparseSGDFunctorKernel(const T* selected_rows,
                                        const int64_t* rows,
                                        const T* learning_rate, T* tensor_out,
-                                       int64_t row_numel) {
-  const int ty = blockIdx.y;
-  int tid = threadIdx.x;
-
-  selected_rows += ty * row_numel;
-  tensor_out += rows[ty] * row_numel;
-
-  for (int index = tid; index < row_numel; index += block_size) {
+                                       int64_t row_numel, int64_t limit) {
+  for (int64_t i = blockIdx.x; i < limit; i += gridDim.x) {
+    const T* selected_rows_ptr = selected_rows + i * row_numel;
+    T* tensor_out_ptr = tensor_out + rows[i] * row_numel;
+    for (int64_t index = threadIdx.x; index < row_numel; index += blockDim.x) {
       // Since index in rows of SelectedRows can be duplicate, we have to use
       // Atomic Operation to avoid concurrent write error.
       paddle::platform::CudaAtomicAdd(
-        tensor_out + index, -1.0 * learning_rate[0] * selected_rows[index]);
+          tensor_out_ptr + index,
+          -1.0 * learning_rate[0] * selected_rows_ptr[index]);
+    }
   }
 }
 }  // namespace
@@ -97,13 +96,15 @@ class SGDOpCUDAKernel : public framework::OpKernel<T> {
       auto* in_data = in_value.data<T>();
       auto* out_data = param_out->data<T>();
 
-      const int block_size = 256;
-      dim3 threads(block_size, 1);
-      dim3 grid(1, in_rows.size());
-      SparseSGDFunctorKernel<
-          T, 256><<<grid, threads, 0, ctx.cuda_device_context().stream()>>>(
+      const int kThreadsPerBlock = 256;
+      int thread_x = kThreadsPerBlock;
+      int max_threads = ctx.cuda_device_context().GetMaxPhysicalThreadCount();
+      int max_blocks = std::max(max_threads / kThreadsPerBlock, 1);
+
+      SparseSGDFunctorKernel<<<max_blocks, thread_x, 0,
+                               ctx.cuda_device_context().stream()>>>(
           in_data, in_rows.CUDAData(ctx.GetPlace()), learning_rate->data<T>(),
-          out_data, in_row_numel);
+          out_data, in_row_numel, in_rows.size());
 
     } else {
       PADDLE_THROW("Unsupported Variable Type of Grad");