15.1.2. Large Data with configuration options

As it has been observed in Large Data Mode, LARGE_DATA builtin transports option offers an easy and efficient way to improve performance when working with large data. Nonetheless, custom configuration can help to enhance the performance even further.

Fast DDS provides configuration options to adjust the behavior of the builtin transports. This becomes particularly relevant when using the LARGE_DATA mode, as it enables increasing the maximum message size beyond 65500 KB and prevents fragmentation, leveraging the TCP and SHM transports.

All builtin transports can be configured by adjusting the following parameters:

max_msg_size: Message maximum size that can be sent over the transport. Sending messages larger than this size will result in fragmentation. Its maximum value is (2^32)-1 B for TCP and SHM and 65500 KB for UDP.
sockets_size: Size of the send and receive socket buffers. This value must be higher or equal than the max_msg_size to obtain a valid configuration. It also defines the size of the shared memory segment, calculated as twice the value set. Its maximum value is (2^32)-1 B.
non_blocking: If set to true, the transport will use non-blocking sockets. This can be useful to avoid blocking the application if the socket buffers are full. However, some messages will be lost. Its default value is false.
tcp_negotiation_timeout: It specifies the timeout duration for logical port negotiation. This parameter is useful for ensuring the availability of the logical port before data transmission, thus preventing message loss during the negotiation process. Conversely, it can delay the discovery process. The default value is 0, implying that discovery will occur as soon as possible, but the initial messages might be lost if reliability is set to BEST_EFFORT_RELIABILITY_QOS. This parameter is only valid for the LARGE_DATA mode.

Adjusting the maximum message size and the socket buffer sizes to a large enough value to accommodate the data to be sent can help improve the performance with large messages, as well as setting the transport to non-blocking mode.

In this way, it is possible to take advantage of the TCP transport to avoid fragmentation and use the non-blocking mode to avoid blocking the application when the socket buffers are full. This configuration can be used, for example, when streaming video, which will result in a significant increase in fluidity.

Note that even when using the LARGE_DATA mode within the same machine, the configuration options can prove useful for improving performance, as they affect the SHM transport. It is highly recommended to set a shared memory segment size large enough to accommodate the data to be sent. To achieve this, the sockets_size parameter must be set to a value at least half of the message size.

The following snippets show how to configure the LARGE_DATA mode:

export FASTDDS_BUILTIN_TRANSPORTS=LARGE_DATA?max_msg_size=1MB&sockets_size=1MB&non_blocking=true&tcp_negotiation_timeout=50

<?xml version="1.0" encoding="UTF-8" ?>
<dds xmlns="http://www.eprosima.com/XMLSchemas/fastRTPS_Profiles">
    <profiles>
        <participant profile_name="large_data_builtin_transports_options" is_default_profile="true">
            <rtps>
                <builtinTransports max_msg_size="200KB" sockets_size="200KB" non_blocking="true" tcp_negotiation_timeout="50">LARGE_DATA</builtinTransports>
            </rtps>
        </participant>
    </profiles>
</dds>

eprosima::fastdds::dds::DomainParticipantQos pqos = PARTICIPANT_QOS_DEFAULT;

/* Transports configuration */
// UDPv4 transport for PDP over multicast and SHM / TCPv4 transport for EDP and application data
// Message Size and Sockets sizes of 200 KB + Non-blocking send + 50ms negotiation timeout
eprosima::fastdds::rtps::BuiltinTransportsOptions large_data_options;
large_data_options.maxMessageSize = 200000;
large_data_options.sockets_buffer_size = 200000;
large_data_options.non_blocking_send = true;
large_data_options.tcp_negotiation_timeout = 50;
pqos.setup_transports(eprosima::fastdds::rtps::BuiltinTransports::LARGE_DATA, large_data_options);

/* Create participant as usual */
eprosima::fastdds::dds::DomainParticipant* participant =
        eprosima::fastdds::dds::DomainParticipantFactory::get_instance()->create_participant(0, pqos);

Note

To learn how to check and modify the default maximum system value for the socket buffers size, please refer to Finding out system maximum values.

Warning

Setting a max_msg_size value higher than 65500 KB with the DEFAULT or UDP modes will result in an error and the participant creation will fail.