nobleo / static_executor

Library that adds alternative(s) to the default executor in ROS2

static_executor

Library that adds alternative(s) to the default executor in ROS2

How to use

1. Add static_executor to your CMakeLists.txt and package.xml as a dependency.

2. Add

#include "static_executor/executors/static_single_threaded_executor.hpp"

at the top of your code.

3. You can now replace

rclcpp::executors::SingleThreadedExecutor exec;

with

rclcpp::executors::StaticSingleThreadedExecutor exec;

in your source code.

No other changes are required.

Depends on

rclcpp

static_executor's Issues

IPC causes spurious awakes of the executor

I'm investigating spurious awakes in both the standard and this executor.
See ros2/rclcpp#1021 for the standard executor

The problem mentioned in the ticket above does not appear in the static executor when I run that dummy executable with only 1 timer.
However, if I run a system made of 2 nodes (1 pub and 1 sub) in the same executor with IPC on, I see the exact same pattern.

I think that the root cause is how the static executor handles the waitable objects:
in each "iteration" of the static executor:

• timer makes the publisher to publish a message. It's intra-process only, so the message is pushed into the subscription queue and the condition variable is triggered.
• the executor checks waitable objects only through the is_ready() condition, that in this case consists in checking if the buffer is not empty. Since a message has just been pushed, the IPC subscription is invoked.
• the executor goes back to wait
• the executor wakes up immediately due to the condition variable triggered by the publisher above
• there is no work to do
• the executor goes back to wait

@MartinCornelis2 what do you think? I think that this problem with IPC is not present with the standard executor because only 1 item is processed in every iteration and the waitables are managed by the memory strategy.

Unit-tests for static executor

Hi,

I created a bunch of "unit-tests" for the static executor.
They test the following scenarios:

1. add 1 pub node and 1 sub node to the executor and spin
2. add 1 pub node to the executor and spin, then add 1 sub node while spinning
3. add 1 pub node and 1 node with no entities to the executor and spin, then add 1 sub to the second node while spinning
4. add 1 pub node and 1 sub node to the executor and spin, then while spinning remove the sub node from the executor

You can find them here
https://github.com/alsora/ros2-code-examples/blob/master/simple_static_executor/static_executor_test.cpp

These tests are meant to be used with your rclcpp version (nobleo/feature/static_executor), I opened the ticket here since it was not possible in that repo.

To build colcon build --packages-select simple_static_executor
To run ./install/simple_static_executor/lib/simple_static_executor/static_executor_test 2
Where the number at the end denotes the id of the test you want to run (1, 2, 3, 4)

The results are unfortunately not good.

Test 2 throws an exception

[ERROR] [1584978899.809709877] [rclcpp]: Couldn't add guard_condition to wait set: guard_conditions set is full, at /root/ros2_ws/src/ros2/rcl/rcl/src/rcl/wait.c:456
terminate called after throwing an instance of 'std::runtime_error'
  what():  Couldn't fill wait set
Aborted (core dumped)

In test 4 the subscription keeps receiving messages after having been removed from the executor

Could you have a look at the test?
Maybe I'm not using the executor as it is supposed to be used.

FYI the standard SingleThreadExecutor allows to pass all the tests successfully.

@MartinCornelis2 @mauropasse