Reduce thread contention in column split histogram test. (dmlc#10708)

Author: trivialfis

tests/cpp/tree/hist/test_histogram.cc

@@ -222,18 +222,17 @@ TEST(CPUHistogram, SyncHist) {
   TestSyncHist(false);
 }
 
-void TestBuildHistogram(bool is_distributed, bool force_read_by_column, bool is_col_split) {
+void TestBuildHistogram(Context const* ctx, bool is_distributed, bool force_read_by_column, bool is_col_split) {
   size_t constexpr kNRows = 8, kNCols = 16;
   int32_t constexpr kMaxBins = 4;
-  Context ctx;
   auto p_fmat =
       RandomDataGenerator(kNRows, kNCols, 0.8).Seed(3).GenerateDMatrix();
   if (is_col_split) {
     p_fmat = std::shared_ptr<DMatrix>{
         p_fmat->SliceCol(collective::GetWorldSize(), collective::GetRank())};
   }
   auto const &gmat =
-      *(p_fmat->GetBatches<GHistIndexMatrix>(&ctx, BatchParam{kMaxBins, 0.5}).begin());
+      *(p_fmat->GetBatches<GHistIndexMatrix>(ctx, BatchParam{kMaxBins, 0.5}).begin());
   uint32_t total_bins = gmat.cut.Ptrs().back();
 
   static double constexpr kEps = 1e-6;
@@ -244,7 +243,7 @@ void TestBuildHistogram(bool is_distributed, bool force_read_by_column, bool is_
   bst_node_t nid = 0;
   HistogramBuilder histogram;
   HistMakerTrainParam hist_param;
-  histogram.Reset(&ctx, total_bins, {kMaxBins, 0.5}, is_distributed, is_col_split, &hist_param);
+  histogram.Reset(ctx, total_bins, {kMaxBins, 0.5}, is_distributed, is_col_split, &hist_param);
 
   RegTree tree;
 
@@ -262,11 +261,11 @@ void TestBuildHistogram(bool is_distributed, bool force_read_by_column, bool is_
   histogram.AddHistRows(&tree, &nodes_to_build, &dummy_sub, false);
   common::BlockedSpace2d space{
       1, [&](std::size_t nidx_in_set) { return row_set_collection[nidx_in_set].Size(); }, 256};
-  for (auto const &gidx : p_fmat->GetBatches<GHistIndexMatrix>(&ctx, {kMaxBins, 0.5})) {
+  for (auto const &gidx : p_fmat->GetBatches<GHistIndexMatrix>(ctx, {kMaxBins, 0.5})) {
     histogram.BuildHist(0, space, gidx, row_set_collection, nodes_to_build,
-                        linalg::MakeTensorView(&ctx, gpair, gpair.size()), force_read_by_column);
+                        linalg::MakeTensorView(ctx, gpair, gpair.size()), force_read_by_column);
   }
-  histogram.SyncHistogram(&ctx, &tree, nodes_to_build, {});
+  histogram.SyncHistogram(ctx, &tree, nodes_to_build, {});
 
   // Check if number of histogram bins is correct
   ASSERT_EQ(histogram.Histogram()[nid].size(), gmat.cut.Ptrs().back());
@@ -292,16 +291,21 @@ void TestBuildHistogram(bool is_distributed, bool force_read_by_column, bool is_
 }
 
 TEST(CPUHistogram, BuildHist) {
-  TestBuildHistogram(true, false, false);
-  TestBuildHistogram(false, false, false);
-  TestBuildHistogram(true, true, false);
-  TestBuildHistogram(false, true, false);
+  Context ctx;
+  TestBuildHistogram(&ctx, true, false, false);
+  TestBuildHistogram(&ctx, false, false, false);
+  TestBuildHistogram(&ctx, true, true, false);
+  TestBuildHistogram(&ctx, false, true, false);
 }
 
-TEST(CPUHistogram, BuildHistColSplit) {
+TEST(CPUHistogram, BuildHistColumnSplit) {
   auto constexpr kWorkers = 4;
-  collective::TestDistributedGlobal(kWorkers, [] { TestBuildHistogram(true, true, true); });
-  collective::TestDistributedGlobal(kWorkers, [] { TestBuildHistogram(true, false, true); });
+  Context ctx;
+  std::int32_t n_total_threads = std::thread::hardware_concurrency();
+  auto n_threads = std::max(n_total_threads / kWorkers, 1);
+  ctx.UpdateAllowUnknown(Args{{"nthread", std::to_string(n_threads)}});
+  collective::TestDistributedGlobal(kWorkers, [&] { TestBuildHistogram(&ctx, true, true, true); });
+  collective::TestDistributedGlobal(kWorkers, [&] { TestBuildHistogram(&ctx, true, false, true); });
 }
 
 namespace {