Optimizing Subscriber-Side Delays in ROS 2: Modifying .em Files to Address std::vector::resize() in FastCDR

Question

I am working on optimizing subscriber-side delays in ROS 2. When handling large data samples, I noticed that most delays occur during the std::vector::resize() operation in the FastCDR library while subscribing to data of type std_msgs::msg::ByteMultiArray. After inspecting the data structure in the ROS 2 build folder, I believe initializing the vector with a fixed size could help reduce delays caused by runtime memory allocation. I want to experiment with the std::vector::reserve() operation for this specific type.

The below code represents the ByteMultiArray_ in byte_multi_array__struct.hpp file

// message struct
template<class ContainerAllocator>
struct ByteMultiArray_
{
  using Type = ByteMultiArray_<ContainerAllocator>;

  explicit ByteMultiArray_(rosidl_runtime_cpp::MessageInitialization _init = rosidl_runtime_cpp::MessageInitialization::ALL)
  : layout(_init)
  {
    (void)_init;
  }

  explicit ByteMultiArray_(const ContainerAllocator & _alloc, rosidl_runtime_cpp::MessageInitialization _init = rosidl_runtime_cpp::MessageInitialization::ALL)
  : layout(_alloc, _init)
  {
    (void)_init;
  }

  // field types and members
  using _layout_type =
    std_msgs::msg::MultiArrayLayout_<ContainerAllocator>;
  _layout_type layout;
  using _data_type =
    std::vector<unsigned char, typename std::allocator_traits<ContainerAllocator>::template rebind_alloc<unsigned char>>;
  _data_type data;
   
  // setters for named parameter idiom
  Type & set__layout(
    const std_msgs::msg::MultiArrayLayout_<ContainerAllocator> & _arg)
  {
    this->layout = _arg;
    return *this;
  }

The msg__struct.hpp.em file in rosidl_generator_cpp which used to create the file byte_multi_array__struct.hpp

  explicit @(message.structure.namespaced_type.name)_(const ContainerAllocator & _alloc, rosidl_runtime_cpp::MessageInitialization _init = rosidl_runtime_cpp::MessageInitialization::ALL)
@[if alloc_list]@
  : @(',\n    '.join(alloc_list))
@[end if]@
  {
@[if not member_list]@
    (void)_init;
@[end if]@
@[if not alloc_list]@
    (void)_alloc;
@[end if]@
@[if default_value_members]@
    if (rosidl_runtime_cpp::MessageInitialization::ALL == _init ||
      rosidl_runtime_cpp::MessageInitialization::DEFAULTS_ONLY == _init)
    {
@[  for membset in default_value_members]@
@[    for line in generate_default_string(membset)]@
      @(line)
@[    end for]@
@[  end for]@
@[  if zero_value_members]@
    } else if (rosidl_runtime_cpp::MessageInitialization::ZERO == _init) {
@[    for membset in zero_value_members]@
@[      for line in generate_zero_string(membset, '_alloc, _init')]@
      @(line)
@[      end for]@
@[    end for]@
@[  end if]@
    }
@[end if]@
@[if non_defaulted_zero_initialized_members]@
    if (rosidl_runtime_cpp::MessageInitialization::ALL == _init ||
      rosidl_runtime_cpp::MessageInitialization::ZERO == _init)
    {
@[  for membset in non_defaulted_zero_initialized_members]@
@[    for line in generate_zero_string(membset, '_alloc, _init')]@
      @(line)
@[    end for]@
@[  end for]@
    }
@[end if]@
  }

  // field types and members
@[for member in message.structure.members]@
  using _@(member.name)_type =
    @(msg_type_to_cpp(member.type));
  _@(member.name)_type @(member.name);
@[end for]@c

How can I include this modification? Do I need to update the.em file, and if so, how should I proceed?

Is there other efficient methods for this optimization in FastCDR?