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Clarify Data Sharing enabling (backport #719) (#723)
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* Clarify Data Sharing enabling (#719)

Signed-off-by: EduPonz <[email protected]>
(cherry picked from commit 8eeb955)

Co-authored-by: Tomoya Fujita <[email protected]>
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mergify[bot] and fujitatomoya authored Oct 5, 2023
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Expand Up @@ -27,11 +27,14 @@ You can however set it to `rmw_fastrtps_dynamic_cpp` using the environment varia

## Advance usage

ROS 2 only allows for the configuration of certain middleware QoS (see [ROS 2 QoS policies](https://docs.ros.org/en/foxy/Concepts/About-Quality-of-Service-Settings.html#qos-policies)).
In addition to ROS 2 QoS policies, `rmw_fastrtps` sets two more Fast DDS configurable parameters:
[//]: # (TODO sloretz - link ROS 2 discovery documentation when it's created)
ROS 2 only allows for the configuration of certain middleware features.
For example, see [ROS 2 QoS policies](https://docs.ros.org/en/foxy/Concepts/About-Quality-of-Service-Settings.html#qos-policies).
In addition to ROS 2 QoS policies, `rmw_fastrtps` sets the following Fast DDS configurable parameters:

* History memory policy: `PREALLOCATED_WITH_REALLOC_MEMORY_MODE`
* Publication mode: `ASYNCHRONOUS_PUBLISH_MODE`
* Publication mode: `SYNCHRONOUS_PUBLISH_MODE`
* Data Sharing: `OFF`

However, `rmw_fastrtps` offers the possibility to further configure Fast DDS:

Expand Down Expand Up @@ -82,12 +85,14 @@ Bear in mind that setting this environment variable but not setting either of th
[Fast DDS XML]: https://fast-dds.docs.eprosima.com/en/latest/fastdds/xml_configuration/xml_configuration.html
[history memory policy]: https://fast-dds.docs.eprosima.com/en/latest/fastdds/dds_layer/core/policy/eprosimaExtensions.html#rtpsendpointqos
[publication mode]: https://fast-dds.docs.eprosima.com/en/latest/fastdds/dds_layer/core/policy/eprosimaExtensions.html#publishmodeqospolicy
[datasharing]: https://fast-dds.docs.eprosima.com/en/latest/fastdds/transport/datasharing.html
Note: Setting `RMW_FASTRTPS_USE_QOS_FROM_XML` to 1 effectively overrides whatever configuration was set with `RMW_FASTRTPS_PUBLICATION_MODE`.
Furthermore, If `RMW_FASTRTPS_USE_QOS_FROM_XML` is set to 1, and [history memory policy] or [publication mode] are not specified in the XML, then the Fast DDS' default configurations will be used:

* [history memory policy] : `PREALLOCATED_MEMORY_MODE`.
* [publication mode] : `SYNCHRONOUS_PUBLISH_MODE`.
* [datasharing] : `AUTO`.

There are two ways of telling a ROS 2 application which XML to use:

Expand Down Expand Up @@ -154,7 +159,7 @@ If neither of the previous profiles exist, `rmw_fastrtps` attempts to load the `

#### Example

The following example configures Fast DDS to publish synchronously, and to have a pre-allocated history that can be expanded whenever it gets filled.
The following example configures Fast DDS to publish synchronously, to have a pre-allocated history that can be expanded whenever it gets filled, and to use Data Sharing if possible.

1. Create a Fast DDS XML file with:

Expand All @@ -168,12 +173,20 @@ The following example configures Fast DDS to publish synchronously, and to have
<publishMode>
<kind>SYNCHRONOUS</kind>
</publishMode>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
<historyMemoryPolicy>PREALLOCATED_WITH_REALLOC</historyMemoryPolicy>
</publisher>
<!-- Default subscriber profile -->
<subscriber profile_name="default subscriber profile" is_default_profile="true">
<qos>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
<historyMemoryPolicy>PREALLOCATED_WITH_REALLOC</historyMemoryPolicy>
</subscriber>
Expand All @@ -183,11 +196,19 @@ The following example configures Fast DDS to publish synchronously, and to have
<publishMode>
<kind>SYNCHRONOUS</kind>
</publishMode>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
</publisher>
<!-- Request subscriber profile for services -->
<subscriber profile_name="service">
<qos>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
<historyMemoryPolicy>PREALLOCATED_WITH_REALLOC</historyMemoryPolicy>
</subscriber>
Expand All @@ -197,16 +218,29 @@ The following example configures Fast DDS to publish synchronously, and to have
<publishMode>
<kind>ASYNCHRONOUS</kind>
</publishMode>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
</publisher>
<!-- Request subscriber profile for server of service "add_two_ints" -->
<subscriber profile_name="rq/add_two_intsRequest">
<qos>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
<historyMemoryPolicy>PREALLOCATED_WITH_REALLOC</historyMemoryPolicy>
</subscriber>
<!-- Reply subscriber profile for client of service "add_two_ints" -->
<subscriber profile_name="rr/add_two_intsReply">
<qos>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
<historyMemoryPolicy>PREALLOCATED_WITH_REALLOC</historyMemoryPolicy>
</subscriber>
</profiles>
Expand All @@ -228,37 +262,41 @@ The following example configures Fast DDS to publish synchronously, and to have

### Enable Zero Copy Data Sharing

ROS 2 provides [Loaned Messages](https://design.ros2.org/articles/zero_copy.html) that allows the user application to loan the message memory from the RMW implementation to eliminate the copy between the ROS 2 application and RMW implementation.
And Fast DDS `rmw_fastrtps_cpp` provides [Shared Memory Transport](https://fast-dds.docs.eprosima.com/en/latest/fastdds/transport/shared_memory/shared_memory.html) and [Data-sharing delivery](https://fast-dds.docs.eprosima.com/en/latest/fastdds/transport/datasharing.html) features to speed up the localhost communication.
Taking advantage of these features all together, it provides significant performance improvement to ROS 2 application.

By default, `rmw_fastrtps_cpp` tries to use [Shared Memory Transport](https://fast-dds.docs.eprosima.com/en/latest/fastdds/transport/shared_memory/shared_memory.html) and [Data-sharing delivery](https://fast-dds.docs.eprosima.com/en/latest/fastdds/transport/datasharing.html) for localhost communication along with network communication if the message data type is a bounded type (a fixed sized data object).

To enable [Loaned Messages](https://design.ros2.org/articles/zero_copy.html) with `rmw_fastrtps_cpp`, [Plain Old Data](https://en.wikipedia.org/wiki/Passive_data_structure) is the only requirement to `Iron Irwini` or later.
For `Humble Hawksbill`, the following XML file needs to be applied to set Fast-DDS `data_sharing` is explicitly enabled. (see more details for https://github.com/ros2/rmw_fastrtps/pull/568)

```xml
<?xml version="1.0" encoding="UTF-8" ?>
<profiles xmlns="http://www.eprosima.com/XMLSchemas/fastRTPS_Profiles">
<!-- Default publisher profile -->
<data_writer profile_name="default publisher profile" is_default_profile="true">
<qos>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
</data_writer>
<data_reader profile_name="default subscription profile" is_default_profile="true">
<qos>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
</data_reader>
</profiles>
```
ROS 2 provides [Loaned Messages](https://design.ros2.org/articles/zero_copy.html) that allow the user application to loan the messages memory from the RMW implementation to eliminate the data copy between the ROS 2 application and the RMW implementation.
Furthermore, `rmw_fastrtps`, through Fast DDS, provides both a [Shared Memory Transport](https://fast-dds.docs.eprosima.com/en/latest/fastdds/transport/shared_memory/shared_memory.html) and [Data Sharing delivery mechanism](https://fast-dds.docs.eprosima.com/en/latest/fastdds/transport/datasharing.html) to speed up the intra-host communication.
Combining these two features (message loaning and Data Sharing), it is possible to achieve a zero-copy message delivery pipeline, thus bringing significant performance improvements to ROS 2 application.

By default, both `rmw_fastrtps_cpp` and `rmw_fastrtps_dynamic_cpp` use [Shared Memory Transport](https://fast-dds.docs.eprosima.com/en/latest/fastdds/transport/shared_memory/shared_memory.html) for intra-host communication, along with network based transports (UDPv4) for inter-host message delivery.

In order to achieve a Zero Copy message delivery, applications need to both enable Fast DDS Data Sharing mechanism, and use the [Loaned Messages](https://design.ros2.org/articles/zero_copy.html) API:

1. To enable Loaned Messages in `Iron Irwini` or later, the only requirement is for the data type to be [Plain Old Data](https://en.wikipedia.org/wiki/Passive_data_structure).
For `Humble Hawksbill`, in addition to POD types, enabling Fast DDS Data Sharing is also required.
1. To enable Fast DDS Data Sharing delivery mechanism, the following XML profiles need to be loaded, and environment variable `RMW_FASTRTPS_USE_QOS_FROM_XML` needs to be set to 1 (see [Full QoS configuration](#full-qos-configuration))

```xml
<?xml version="1.0" encoding="UTF-8" ?>
<profiles xmlns="http://www.eprosima.com/XMLSchemas/fastRTPS_Profiles">
<!-- Default publisher profile -->
<publisher profile_name="default publisher profile" is_default_profile="true">
<qos>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
</publisher>
<!-- Default subscription profile -->
<subscriber profile_name="default subscription profile" is_default_profile="true">
<qos>
<data_sharing>
<kind>AUTOMATIC</kind>
</data_sharing>
</qos>
</subscriber>
</profiles>
```

## Quality Declaration files

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