3.9. `topic` section

topic-sections are used to specify per-topic settings. these include per-host scheduling priority, policy, cpu-affinities, endianess-swap-preferences and preferred networks for udp or tcp transport.

scheduling settings only affect ln_daemon threads. the actual publisher or subscriber threads of an application need to be controlled by other means (see the process priority setting for one option)

topic-sections are optional, usually only needed for more fine grained real-time tuning.

syntax:

TOPIC_SECTION := "topic " + TOPIC_NAME + "\n" + PER_HOST_SETTING*
PER_HOST_SETTING = HOST_NAME + ": " + SETTINGS + "\n"
SETTINGS := SETTING + [ ", " + SETTING ]*
SETTING := SETTING-NAME + "=" + SETTING-VALUE

variable substitution will be performed on TOPIC_NAME, HOST_NAME and SETTINGS.

multiple topic-sections for the same TOPIC_NAME can be used to add more per-host settings. if the same HOST_NAME is specified multiple times for the same topic, then only the last mentioned setting will have effect.

example:

topic robot_telemetry
localhost: policy=fifo, priority=55, affinity=0x04, preferred_network=rtnet, offer_endianess_swap
robot: priority=75, affinity=2, preferred_network=rtnet
view_node: priority=4, preferred_network=rtnet

topic robot_telemetry
view_node: priority=40

in this example for topic robot_telemetry a UDP transport from robot to localhost would use the rtnet. the receiver thread on localhost will have fifo prio 55 with affinity 0x04 (only cpu-id 3). the sender thread on robot will have fifo prio 75 on cpu-id 2. a subscriber on view_node would cause a UDP-receiver thread to have priority 40 WITHOUT any network-preferences (the 2nd mention of view_node overwrites the first one).

scheduling settings are also used for subscriptions where the publisher is on the same host, IFF the subscriber specifies a rate != -1. (only then ln_daemon needs to span a thread)

valid settings:

3.9.1. policy

which scheduling policy to use for ln_daemon-threads that work on this topic. typical values:

FIFO
RR
NORMAL
BATCH

ln_daemon threads are used for UDP/TCP-senders, -receivers and whenever a subscriber uses a subscription rate other than -1 (full rate).

3.9.2. priority

scheduling priority to be used for threads working on this topic. for linux and the FIFO-policy this is an integer value from 0 to 99 where 99 is the highest possible priority.

3.9.3. affinity

specifies which cpu affinity to use for threads that work for this topic. only if specified as hex-value it is interpreted as a bitmask (provide the 0x-prefix). other number formats are interpreted as list of cpu-id’s (first cpu has id 0).

bitmask examples::: 0x01: only cpu-id 0 0x02: only cpu-id 1 0x04: only cpu-id 3 0x08: only cpu-id 4 0x10: only cpu-id 5 0x12: cpu-id 5 or cpu-id 1

3.9.4. preferred_network

name of a defined/known network to use for transports of this topic. this is useful if there are multiple possible network connections between a publisher and subscriber.

3.9.5. offer_endianess_swap

if this flag is present then the named host will be used to do the endianess-swap if needed between hosts of different endianess. if not specified, ln_manager will decide which side does the endianess swap.

3.9.6. deny_endianess_swap

if this flag is present then the named host will NOT be used to do the endianess-swap if needed between hosts of different endianess. if not specified, ln_manager will decide which side does the endianess swap.

3.9. topic section