# Nav2 composition

**URL:** <https://discourse.openrobotics.org/t/nav2-composition/22175>\
**Category:** Navigation\
**Created:** [September 6, 2021, 3:36am UTC](https://discourse.openrobotics.org/t/nav2-composition/22175 "2021-09-06T03:36:00Z")\
**Posts on this page:** 1\
**Showing post:** 1

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**Author:** ![gezp](https://sea2.discourse-cdn.com/flex022/user_avatar/discourse.openrobotics.org/gezp/32/11248_2.png) [@gezp](https://discourse.openrobotics.org/u/gezp)\
**Post date:** [September 6, 2021, 3:36am UTC](https://discourse.openrobotics.org/t/nav2-composition/22175/1 "2021-09-06T03:36:01Z")

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Hello guys,

I’m working on project [Reduce ROS2 Nodes and Determinism](https://navigation.ros.org/2021summerOfCode/projects/spinners.html) of OSPP2021 (under the mentor of Steve Macenski).

Currently, i’m trying to create a composition-based navigation package for embedded systems users that need to make optimizations due to harsh resource constraints. ( compose all Nav2 nodes in a single process instead of launching these nodes separately)

- first, i create multiple nodes in a single `main()` (manual composition).
- then, i create a multi-threaded executor to spin these nodes.
- in addition, i also use `option.use_intra_process_comms(true)` to get efficient intra-process communication.

There were some problems when i tried to implement these.

- The first problem: some QoS isn’t supported (for example `transient-local` ), so some nodes which use `transient-local` couldn’t enable intra-process communication.
- The second problem: it consumes higher cpu usage (increase 50%), which confuses me.

For the first problem, i noticed that intra-process comms of rclcpp are still kind of under development, you could find more details:

- [Allow intra-process communication to leverage full suite of QoS settings · Issue #1753 · ros2/rclcpp · GitHub](https://github.com/ros2/rclcpp/issues/1753)
- [Status of today's ROS 2 message delivery mechanisms · Issue #1750 · ros2/rclcpp · GitHub](https://github.com/ros2/rclcpp/issues/1750)

For the second problem, i made some experiments for performance (memory and cpu usage), and python library `psutil` was used to collect data.

- try a bunch of single threaded executors instead of 1 large multithreaded executor
- try to disable intra-process comms ( `option.use_intra_process_comms(false)` )

 ![test](https://us1.discourse-cdn.com/flex022/uploads/ros/original/2X/e/e09397782d112bab24e00d1c50f05e1eacd303d5.png)

in this table, the case `Manual composition with multiple single-threaded executors` looks good, it consumes lower cpu(saves ~20%) and lower memory(saves ~70%) than `Normal multi-process` . you could find source code here ([https://github.com/gezp/navigation2/blob/nav2\_composition/nav2\_composition/src/main.cpp](https://github.com/gezp/navigation2/blob/nav2_composition/nav2_composition/src/main.cpp))

we could look the cpu usage of these cases:

- Manual composition with multiple single-threaded executors : use a bunch of single-threaded executors for these nodes
- Manual composition with a multi-threaded executor: use a large multi-threaded executor for these nodes

It’s obvious that `a large multi-threaded executor` consumes higher cpu(increase 30%-50%) than `a bunch of single-threaded executors` , and i don’t know why, any discussion is welcome!

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