More collective aggregators (#9060)

src/collective/aggregator.h

@@ -8,6 +8,7 @@
 #pragma once
 #include <xgboost/data.h>
 
+#include <limits>
 #include <string>
 #include <utility>
 #include <vector>
@@ -57,5 +58,72 @@ void ApplyWithLabels(MetaInfo const& info, T* buffer, size_t size, Function&& fu
     std::forward<Function>(function)(std::forward<Args>(args)...);
   }
 }
+
+/**
+ * @brief Find the global max of the given value across all workers.
+ *
+ * This only applies when the data is split row-wise (horizontally). When data is split
+ * column-wise (vertically), the local value is returned.
+ *
+ * @tparam T The type of the value.
+ * @param info MetaInfo about the DMatrix.
+ * @param value The input for finding the global max.
+ * @return The global max of the input.
+ */
+template <typename T>
+T GlobalMax(MetaInfo const& info, T value) {
+  if (info.IsRowSplit()) {
+    collective::Allreduce<collective::Operation::kMax>(&value, 1);
+  }
+  return value;
+}
+
+/**
+ * @brief Find the global sum of the given values across all workers.
+ *
+ * This only applies when the data is split row-wise (horizontally). When data is split
+ * column-wise (vertically), the original values are returned.
+ *
+ * @tparam T The type of the values.
+ * @param info MetaInfo about the DMatrix.
+ * @param values Pointer to the inputs to sum.
+ * @param size Number of values to sum.
+ */
+template <typename T>
+void GlobalSum(MetaInfo const& info, T* values, size_t size) {
+  if (info.IsRowSplit()) {
+    collective::Allreduce<collective::Operation::kSum>(values, size);
+  }
+}
+
+template <typename Container>
+void GlobalSum(MetaInfo const& info, Container* values) {
+  GlobalSum(info, values->data(), values->size());
+}
+
+/**
+ * @brief Find the global ratio of the given two values across all workers.
+ *
+ * This only applies when the data is split row-wise (horizontally). When data is split
+ * column-wise (vertically), the local ratio is returned.
+ *
+ * @tparam T The type of the values.
+ * @param info MetaInfo about the DMatrix.
+ * @param dividend The dividend of the ratio.
+ * @param divisor The divisor of the ratio.
+ * @return The global ratio of the two inputs.
+ */
+template <typename T>
+T GlobalRatio(MetaInfo const& info, T dividend, T divisor) {
+  std::array<T, 2> results{dividend, divisor};
+  GlobalSum(info, &results);
+  std::tie(dividend, divisor) = std::tuple_cat(results);
+  if (divisor <= 0) {
+    return std::numeric_limits<T>::quiet_NaN();
+  } else {
+    return dividend / divisor;
+  }
+}
+
 }  // namespace collective
 }  // namespace xgboost