Add ability to monitor size of DataBox items

- Adds a function to observe the size in bytes of all non-reference items
  in a DataBox
- Adds an observer to gather the sizes in MB of each item (summed over all
  elements) for the DataBoxes of each component

src/DataStructures/DataBox/DataBox.hpp

@@ -10,11 +10,13 @@
 #include <exception>
 #include <functional>
 #include <initializer_list>
+#include <map>
 #include <ostream>
 #include <pup.h>
 #include <sstream>
 #include <string>
 #include <tuple>
+#include <unordered_map>
 #include <utility>
 
 #include "DataStructures/DataBox/Access.hpp"
@@ -320,6 +322,9 @@ class DataBox<tmpl::list<Tags...>> : public Access,
   template <bool PrintImmutableItems = true>
   std::string print_items() const;
 
+  /// The size in bytes of each item (excluding reference items)
+  std::map<std::string, size_t> size_of_items() const;
+
   /// Retrieve the tag `Tag`, should be called by the free function db::get
   template <typename Tag>
   const auto& get() const;
@@ -548,6 +553,23 @@ std::string DataBox<tmpl::list<Tags...>>::print_items() const {
   return os.str();
 }
 
+template <typename... Tags>
+std::map<std::string, size_t> DataBox<tmpl::list<Tags...>>::size_of_items()
+    const {
+  std::map<std::string, size_t> result{};
+  const auto add_item_size = [this, &result](auto tag_v) {
+    (void)this;
+    using tag = tmpl::type_from<decltype(tag_v)>;
+    if constexpr (not db::is_reference_tag_v<tag>) {
+      const auto& item = get_item<tag>();
+      result[pretty_type::get_name<tag>()] = size_of_object_in_bytes(item);
+    }
+  };
+  tmpl::for_each<mutable_item_creation_tags>(add_item_size);
+  tmpl::for_each<immutable_item_creation_tags>(add_item_size);
+  return result;
+}
+
 namespace detail {
 // This function exists so that the user can look at the template
 // arguments to find out what triggered the static_assert.

src/ParallelAlgorithms/Events/CMakeLists.txt

@@ -9,6 +9,7 @@ spectre_target_sources(
   ${LIBRARY}
   PRIVATE
   ObserveAdaptiveSteppingDiagnostics.cpp
+  ObserveDataBox.cpp
   ObserveNorms.cpp
   )
 
@@ -20,6 +21,7 @@ spectre_target_headers(
   Factory.hpp
   MonitorMemory.hpp
   ObserveAdaptiveSteppingDiagnostics.hpp
+  ObserveDataBox.hpp
   ObserveAtExtremum.hpp
   ObserveFields.hpp
   ObserveNorms.hpp

src/ParallelAlgorithms/Events/ObserveDataBox.cpp

@@ -0,0 +1,14 @@
+// Distributed under the MIT License.
+// See LICENSE.txt for details.
+
+#include "ParallelAlgorithms/Events/ObserveDataBox.hpp"
+
+#include <pup.h>
+
+namespace Events {
+ObserveDataBox::ObserveDataBox(CkMigrateMessage* /*m*/) {}
+
+void ObserveDataBox::pup(PUP::er& p) { Event::pup(p); }
+
+PUP::able::PUP_ID ObserveDataBox::my_PUP_ID = 0;  // NOLINT
+}  // namespace Events

src/ParallelAlgorithms/Events/ObserveDataBox.hpp

@@ -0,0 +1,159 @@
+// Distributed under the MIT License.
+// See LICENSE.txt for details.
+
+#pragma once
+
+#include <map>
+#include <pup.h>
+#include <tuple>
+#include <vector>
+
+#include "DataStructures/DataBox/DataBox.hpp"
+#include "Domain/Structure/ElementId.hpp"
+#include "IO/Observer/ObserverComponent.hpp"
+#include "IO/Observer/ReductionActions.hpp"
+#include "Options/String.hpp"
+#include "Parallel/DistributedObject.hpp"
+#include "Parallel/GlobalCache.hpp"
+#include "Parallel/Invoke.hpp"
+#include "Parallel/Reduction.hpp"
+#include "ParallelAlgorithms/EventsAndTriggers/Event.hpp"
+#include "Utilities/PrettyType.hpp"
+#include "Utilities/TMPL.hpp"
+
+namespace Events {
+
+namespace detail {
+
+struct map_add {
+  std::map<std::string, size_t> operator()(
+      std::map<std::string, size_t> map_1,
+      const std::map<std::string, size_t>& map_2) {
+    for (const auto& [key, value] : map_2) {
+      map_1.at(key) += value;
+    }
+    return map_1;
+  }
+};
+
+using ReductionType = Parallel::ReductionData<
+    // Time
+    Parallel::ReductionDatum<double, funcl::AssertEqual<>>,
+    // Map of total mem usage in MB per item in DataBoxes
+    Parallel::ReductionDatum<std::map<std::string, size_t>, map_add>>;
+
+template <typename ContributingComponent>
+struct ReduceDataBoxSize {
+  template <typename ParallelComponent, typename DbTags, typename Metavariables,
+            typename ArrayIndex>
+  static void apply(db::DataBox<DbTags>& /*box*/,
+                    Parallel::GlobalCache<Metavariables>& cache,
+                    const ArrayIndex& /*array_index*/, const double time,
+                    const std::map<std::string, size_t>& item_sizes) {
+    auto& observer_writer_proxy = Parallel::get_parallel_component<
+        observers::ObserverWriter<Metavariables>>(cache);
+    const std::string subfile_name =
+        "/ObserveDataBox/" + pretty_type::name<ContributingComponent>();
+    std::vector<std::string> legend;
+    legend.reserve(item_sizes.size() + 1);
+    legend.emplace_back("Time");
+    std::vector<double> columns;
+    columns.reserve(item_sizes.size() + 1);
+    columns.emplace_back(time);
+    const double scaling = 1.0 / 1048576.0;  // so size is in MB
+    for (const auto& [name, size] : item_sizes) {
+      legend.emplace_back(name);
+      columns.emplace_back(scaling * static_cast<double>(size));
+    }
+    Parallel::threaded_action<
+        observers::ThreadedActions::WriteReductionDataRow>(
+        // Node 0 is always the writer
+        observer_writer_proxy[0], subfile_name, legend,
+        std::make_tuple(columns));
+  }
+};
+
+struct ContributeDataBoxSize {
+  template <typename ParallelComponent, typename DbTags, typename Metavariables,
+            typename ArrayIndex>
+  static void apply(db::DataBox<DbTags>& box,
+                    Parallel::GlobalCache<Metavariables>& cache,
+                    const ArrayIndex& array_index, const double time) {
+    const auto& my_proxy =
+        Parallel::get_parallel_component<ParallelComponent>(cache)[array_index];
+    auto& target_proxy = Parallel::get_parallel_component<
+        observers::ObserverWriter<Metavariables>>(cache);
+    const auto item_sizes = box.size_of_items();
+    if constexpr (Parallel::is_singleton_v<ParallelComponent>) {
+      Parallel::simple_action<ReduceDataBoxSize<ParallelComponent>>(
+          target_proxy[0], time, item_sizes);
+    } else {
+      Parallel::contribute_to_reduction<ReduceDataBoxSize<ParallelComponent>>(
+          ReductionType{time, item_sizes}, my_proxy, target_proxy[0]);
+    }
+  }
+};
+}  // namespace detail
+
+class ObserveDataBox : public Event {
+ public:
+  /// \cond
+  explicit ObserveDataBox(CkMigrateMessage* m);
+  using PUP::able::register_constructor;
+  WRAPPED_PUPable_decl_template(ObserveDataBox);  // NOLINT
+  /// \endcond
+
+  using options = tmpl::list<>;
+  static constexpr Options::String help = {
+      "Observe size (in MB) of each item (except reference items) in each "
+      "DataBox"};
+
+  ObserveDataBox() = default;
+
+  using compute_tags_for_observation_box = tmpl::list<>;
+
+  using return_tags = tmpl::list<>;
+  using argument_tags = tmpl::list<::Tags::DataBox>;
+
+  template <typename DataBoxType, typename ArrayIndex,
+            typename ParallelComponent, typename Metavariables>
+  void operator()(const DataBoxType& box,
+                  Parallel::GlobalCache<Metavariables>& /*cache*/,
+                  const ArrayIndex& array_index,
+                  const ParallelComponent* /*meta*/,
+                  const ObservationValue& observation_value) const;
+
+  using is_ready_argument_tags = tmpl::list<>;
+
+  template <typename Metavariables, typename ArrayIndex, typename Component>
+  bool is_ready(Parallel::GlobalCache<Metavariables>& /*cache*/,
+                const ArrayIndex& /*array_index*/,
+                const Component* const /*meta*/) const {
+    return true;
+  }
+
+  bool needs_evolved_variables() const override { return false; }
+
+  // NOLINTNEXTLINE(google-runtime-references)
+  void pup(PUP::er& p) override;
+};
+
+template <typename DataBoxType, typename ArrayIndex, typename ParallelComponent,
+          typename Metavariables>
+void ObserveDataBox::operator()(
+    const DataBoxType& /*box*/, Parallel::GlobalCache<Metavariables>& cache,
+    const ArrayIndex& array_index, const ParallelComponent* const /*meta*/,
+    const ObservationValue& observation_value) const {
+  if (is_zeroth_element(array_index)) {
+    using component_list =
+        tmpl::push_back<typename Metavariables::component_list>;
+    tmpl::for_each<component_list>([&observation_value,
+                                    &cache](auto component_v) {
+      using component = tmpl::type_from<decltype(component_v)>;
+      auto& target_proxy = Parallel::get_parallel_component<component>(cache);
+      Parallel::simple_action<detail::ContributeDataBoxSize>(
+          target_proxy, observation_value.value);
+    });
+  }
+}
+}  // namespace Events

tests/Unit/DataStructures/DataBox/Test_DataBox.cpp

@@ -3146,6 +3146,15 @@ void test_output() {
   remove_whitespace(output_stream);
   remove_whitespace(expected_stream);
   CHECK(output_stream == expected_stream);
+  const auto item_size = box.size_of_items();
+  CHECK(item_size.size() == 5);
+  CHECK(item_size.at("(anonymous namespace)::test_databox_tags::Tag0") == 8);
+  CHECK(item_size.at("(anonymous namespace)::test_databox_tags::Tag1") == 32);
+  CHECK(item_size.at("(anonymous namespace)::test_databox_tags::Tag2") == 24);
+  CHECK(item_size.at("(anonymous namespace)::test_databox_tags::Tag4Compute") ==
+        9);
+  CHECK(item_size.at("(anonymous namespace)::test_databox_tags::Tag5Compute") ==
+        29);
 }
 
 void test_exception_safety() {