cleanup.

src/data/ellpack_page.cu

@@ -179,8 +179,8 @@ EllpackPageImpl::EllpackPageImpl(Context const* ctx, DMatrix* p_fmat, const Batc
   auto ft = p_fmat->Info().feature_types.ConstDeviceSpan();
   monitor_.Start("BinningCompression");
   CHECK(p_fmat->SingleColBlock());
-  for (const auto& batch : p_fmat->GetBatches<SparsePage>()) {
-    CreateHistIndices(ctx, batch, ft);
+  for (auto const& page : p_fmat->GetBatches<SparsePage>()) {
+    this->CreateHistIndices(ctx, page, ft);
   }
   monitor_.Stop("BinningCompression");
 }

src/data/sparse_page_source.h

@@ -277,7 +277,9 @@ class SparsePageSourceImpl : public BatchIteratorImpl<S>, public FormatStreamPol
     std::size_t fetch_it = count_;
 
     exce_.Rethrow();
-
+    // Clear out the existing page before loading new ones. This helps reduce memory usage
+    // when page is not loaded with mmap, in addition, it triggers necessary CUDA
+    // synchronizations by freeing memory.
     page_.reset();
 
     for (std::int32_t i = 0; i < n_prefetch_batches; ++i, ++fetch_it) {

tests/cpp/data/test_ellpack_page.cu

@@ -264,10 +264,10 @@ class EllpackPageTest : public testing::TestWithParam<float> {
       ASSERT_EQ(from_sparse_page->base_rowid, from_ghist->base_rowid);
       ASSERT_EQ(from_sparse_page->n_rows, from_ghist->n_rows);
       ASSERT_EQ(from_sparse_page->gidx_buffer.size(), from_ghist->gidx_buffer.size());
+      ASSERT_EQ(from_sparse_page->NumSymbols(), from_ghist->NumSymbols());
       std::vector<common::CompressedByteT> h_gidx_from_sparse, h_gidx_from_ghist;
       auto from_ghist_acc = from_ghist->GetHostAccessor(&gpu_ctx, &h_gidx_from_ghist);
       auto from_sparse_acc = from_sparse_page->GetHostAccessor(&gpu_ctx, &h_gidx_from_sparse);
-      ASSERT_EQ(from_sparse_page->NumSymbols(), from_ghist->NumSymbols());
       for (size_t i = 0; i < from_ghist->n_rows * from_ghist->row_stride; ++i) {
         EXPECT_EQ(from_ghist_acc.gidx_iter[i], from_sparse_acc.gidx_iter[i]);
       }