[GitHub] [incubator-mxnet] apeforest commented on a change in pull request #16735: Use single-bit for mask in dropout operator

[GitBox <gi...@apache.org>]

apeforest commented on a change in pull request #16735: Use single-bit for mask in dropout operator
URL: https://github.com/apache/incubator-mxnet/pull/16735#discussion_r379267501
 
 

 ##########
 File path: src/operator/nn/dropout-inl.h
 ##########
 @@ -152,15 +181,20 @@ class DropoutOp {
                           const std::vector<TBlob> &out_grad) {
     Stream<xpu> *s = ctx.get_stream<xpu>();
     Tensor<xpu, 2, DType> grad = out_grad[dropout::kOut].FlatTo2D<xpu, DType>(s);
-    Tensor<xpu, 2, DType> mask = out_data[dropout::kMask].FlatTo2D<xpu, DType>(s);
+    Tensor<xpu, 1, uint8_t> mask = out_data[dropout::kMask].FlatTo1D<xpu, uint8_t>(s);
     Tensor<xpu, 2, DType> gdata = in_grad[dropout::kData].FlatTo2D<xpu, DType>(s);
     DType *ingradptr = gdata.dptr_;
     const DType *outgradptr = grad.dptr_;
-    const DType *maskptr = mask.dptr_;
-    const int count = mask.shape_[0] * mask.shape_[1];
-#pragma omp parallel for num_threads(engine::OpenMP::Get()->GetRecommendedOMPThreadCount())
-    for (int i = 0; i < count; ++i) {
-      ingradptr[i] = outgradptr[i] * maskptr[i];
+    const uint8_t *maskptr = mask.dptr_;
+    const index_t count = grad.shape_[0] * grad.shape_[1];
+    const float pk_1 = 1.0f / this->pkeep_;
+    const int nthr = engine::OpenMP::Get()->GetRecommendedOMPThreadCount();
+#pragma omp parallel for num_threads(nthr) schedule(static, 8)
+    for (index_t i = 0; i < count; ++i) {
+      auto mask_idx = i >> 3;  // div 8;
+      uint8_t mask_offset = i & 7;  // mod 8
+      bool mask_val = maskptr[mask_idx] & (1U << mask_offset);
+      ingradptr[i] = outgradptr[i] * mask_val * pk_1;
 
 Review comment:
   Sure

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