The tutorials are a collection of step-by-step instructions meant to steadily build skills in ROS 2. Thinking out loud, I wonder why combining polymorphism and type tagging, as opposed to just using polymorphism (i.e. Note: Another execution semantics is ALWAYS, which means, that the subscription callback is always executed when the spin-method of the executor is called. @wjwwood @clalancette @ivanpauno this PR is open since almost 2 years. Finally, you can run the executor with rclc_executor_spin(): This function runs forever without coming back. In the implementation that we proposed (see the other PRs) that's a raw pointer to an rclcpp subscription itself. The "output buffer" could be a fixed size buffer, to avoid dynamic allocations which would damage performance. That is, a new message will overwrite the older message if it has not been processed by the subscription. Example 2: Triggered execution example, demonstrating the capability of synchronizing the execution of callbacks based on the availability of new messages, how to use pre-defined trigger conditions, how to write custom-defined trigger conditions, how to setup quality-of-service parameters for a subscription. Perhaps a queue per clock type? I think this proposal is valid also in the case of the current wait set based executors. Executor is a component that is responsible for scheduling and executing tasks in the ROS2 . #include That is, only if both subscriptions have received a new message, then the executor shall start processing the callbacks. @adamdbrw Yes, but slowly. Is this something that needs to be brought up in the middleware WG? That's a good point. This should be a parameterized option with the defaults for the current behavior of a single single-threaded executor. That's the point in which we decided that if multiple-layer code modifications were to be done, not only the CPU improvement should be noticeable but also we should fix some of the issues of the current waitset based executors. Large enough circular buffers would probably be OK for most such cases (whatever large enough means for any given application). In this example, we are using two executors, one to schedule the publishers, and one to schedule the subscriptions: The executor executor_pub is first created with rclc_executor_get_zero_initialized_executor() and has two handles (aka 2 timers). The best way to approach the tutorials is to walk through them for the first time in order, as they build off of each other and are not meant to be comprehensive documentation. The reason for having the same paths as rclcpp was to simplify the transition, i.e. Add a node, complete all immediately available work, and remove the node. In the worst case scenario you need to use this function twice, which is going through all the waitset. To review, open the file in an editor that reveals hidden Unicode characters. Contributions and feedbacks are highly appreciated! template. The work queue looses the QoS depth, the event queue can fulfil it with the consequence of outdated NOP events as discussed above. I'm fully conscious that the ownership of entities is a weak point of the current implementation, but IMO is something not related to the EventsExecutor design in particular, but rather to its implementation or to the executors in general. This maybe goes in the direction of the out of tree proposal @ivanpauno had. The steps described here as Clear wait set, Add each entity to the rcl wait set, Add each entity to the rmw wait set won't be needed anymore. We're walking through all rmw entities again, when we only need to walk through the conditions that the DDS wait set marked as ready. 'Cause it wouldn't have made any difference, If you loved me. He can give you more details about that. So I am confused about the difference between direct-access and non-direct-access to dds wait_set? ros2/rclcpp#1416) on purpose does not modify any of the existing code, but just adds new APIs. void rclcpp::executor::Executor::spin_node_some, virtual void rclcpp::executor::Executor::spin_some, The maximum amount of time to spend executing work, or 0 for no limit. I'm not completely sure what do you mean. Example packages for ROS 2 C++ 509 272 . Therefore the trigger parameter rclc_executor_trigger_any can be used: Finally, the executors spin-some functions can be started. There are some differences in the rmw implementations. A different solution that we evaluated consisted in having additional event types to denote that we want to remove an entity from the executor. I agree with that, but it would be a quite big change for ROS as currently the waitable and the subscriptionBase, serviceBase, etc are very distinguished from each other. Depending on whether a DDS wait set or just a condition variable is used, the number of "walk-throughs" vary or are at different places. Your design is pretty solid in that sense. Thanks for contributing an answer to Stack Overflow! You can find here a Github repository containing an implementation of the executor for ROS 2 Humble: https://github.com/irobot-ros/events-executor. rmw_fastrtps ros2/rmw_fastrtps#468. But unless I'm missing something from your rclcpp based prototype (which I might), if any entity associated with the event executor is destroyed within one of the callbacks that consume events while execution_mutex_ is being held, on entity removal it will attempt to lock the execution_mutex_ again. Processing an event results in different operations depending on the entity that generated it. However anything that will allow the executor to get what is needed to call execute_subscription() will do the work: for example the executor may have a map of weak pointers to subscriptions and the event may include a key for that map. here is my code: When a condition variable has been triggered check predicate, so repeat step 2 to. Learn more. While the executor for the subscriptions shall only execute if both messages have arrived. I don't believe this is substantially different from the current approach, because it still doesn't let you remove items from within a callback. However, again I'm not sure what the use-case for that could be. Invalid ordering corner case), Thanks @alsora. See SingleThreadedExecutor and MultiThreadedExecutor for examples of execution paradigms. In the current implementation, mutex prevents entity destruction during event processing: In order to implement the EventsExecutor, we had to only add some boilerplate code in rmw/rcl layers. Connect and share knowledge within a single location that is structured and easy to search. There should be one default executor, of course. However, from our analysis, both these solutions still cause a considerable CPU overhead with respect to running your pub/sub application without ROS and directly using your middleware of choice. https://github.com/irobot-ros/rclcpp/blob/irobot/add-events-executor/rclcpp/include/rclcpp/executors/events_executor.hpp#L225, In the current implementation, mutex prevents entity destruction during event processing. :), @timonegk this PR ros2/rmw_fastrtps#625 fixes the issue you got about an invalid qos type falsely triggered. And even if run my code with std::launch::async, I still can't stop this thread as I want. That's a pragmatic and efficient solution for a guaranteed temporal order. In the my_timer_string_callback, the message pub_msg is created and filled with the string Hello World plus an integer, which is incremented by one, each time the timer callback is called. The my_string_subscriber callback prints out the string of the message msg->data.data: The integer callback prints out the received integer msg->data: To publish messages with different frequencies, we setup two timers. run () void rclcpp::executors::MultiThreadedExecutor::run ( size_t this_thread_number ) protected The documentation for this class was generated from the following file: If the executor is blocked at the rmw layer while waiting for work and it is notified that a new node was added, it will wake up. hidmic left review comments, mauropasse @mauropasse did a proof of concept where he took the StaticSingleThreadedExecutor and directly accessed the dds wait set from the rclcpp layer, thus removing all the maintenance. It will only be interrupted by a CTRL-C (managed by the global signal handler). The performance definitely improved, but not as much as we expected. You must change the existing code in this line in order to create a valid suggestion. In that case, the latest data of high-frequency topic is used. How long to wait for work to become available. We really tried to provide a design that allows to implement the new executor while not disrupting the current ROS system. // timeout specified in ns (here: 1s). the number of ROS 2 entities is fixed and known at startup. The current implementation addresses this problem by providing an additional function pointer to each associated entity. I agree with you that we're digging a bit too much into implementation details here, but how so? I'm not sure how much more I can get done tonight, but I'll at least run CI on it. @wjwwood @alsora What is currently blocking the progress of merging the EventsExecutor, especially to rclcpp? wait_for_ready_callbacks returns a tuple now ( #194 ) ros2/rclpy#159 changed wait_for_ready_callbacks to manage the generator internally and return just a tuple. There are definitely other solutions to handle this situation. It won't trigger an event, sure, but that's a non-issue. https://discourse.ros.org/t/ros2-middleware-change-proposal/15863/20 Union structure that can hold any executable type (timer, subscription, service, client). I also think that rmw/rcl API should be flexible enough so you can implement an event queue/work queue/wait set based executor out of tree, and I think that would be possible. Convenience function which takes Node and forwards NodeBaseInterface. In AsyncSpinner I can use start() and stop() methods. For example, On a single-core CPU machine, concurrency can be achieved through the use of threads. It seems that ROS2 is sometimes picking up things from the "other" python. ros2 Public. When you call spin() without directly creating an executor, what executor is used inside that is an important choice we must make if we have multiple available. The memory strategy: an interface for handling user-defined memory allocation strategies. Alberto Soragna. Timers can be created and added to the executor, which will call the timer callback periodically once it is spinning. Well occasionally send you account related emails. This allows the most determinism as the executor then would provide a plan that is guaranteed to be executed. Yes, exactly. Invocation of Polski Package Sometimes Produces Strange Hyphenation. Lenny Story As the current state of the ROS2 executors is not really usable for high frequency systems, this would be a great improvement and I / we would be glad to help move this forward wherever we can. However I see the point for the potential confusion. There is also the possibility that a particular language may provide multiple ways to handle event queues and multi-threading, so one executor for each might make sense in that language's client library. For quick solutions to more specific questions, see the How-to Guides. that in some situations the order may be invalidated) is simply completely ignored by the wait set approach, where there are no guarantees at all about the order in which entities are processed. @alsora Thanks for sharing you and your teams amazing work! | privacy | imprint, #include What are the next steps for this? You signed in with another tab or window. Having the static executor as an optional package would prevent the bloating of ROS2. I see. The data variables used for the publisher messages in the timer callbacks need to be initialized first: The first subscription my_string_sub is created with the function rcl_subscription_init because we change the quality-of-service parameter to last-is-best. I don't know why that happened but will investigate. I feel that with the waitset approach there are at least these steps, As you said all the clearing, attaching and detaching with every wait can be optimized. //printf("Timer: time since last call %d\n", (int) last_call_time); "Error in my_timer_string_callback: publishing message %s, "Error in my_timer_string_callback: timer parameter is NULL, "Error in my_timer_int_callback: publishing message %d, "Error in my_timer_int_callback: timer parameter is NULL, // - publishes 'my_string_pub' every 'timer_timeout' ms. "Created timer 'my_string_timer' with timeout %d ms. // - publishes 'my_int_pub' every 'timer_int_timeout' ms, // qos: last is best = register semantics. After defining the callback functions, we present now the main() function. but that had even worse results and running ros2 from the prompt resulted in "failed to create process." Guard condition for signaling the rmw layer to wake up for special events. If CPU usage ends up being that main motivation of the change, I wouldn't like to be comparing a well optimized implementation with a badly optimized one. The basic code is structured as follows: The code compiles and appears to set up the correct publishers and subscribers (which are all abstracted into the "Health" object). Note that, virtual void rclcpp::executor::Executor::spin_once. This means that you would have a regular subscription that, depending on the content of the message it receives, could decide to destroy itself or maybe to destroy another entity? There are still some corner cases in an event based executor, as described at the end of the article and in this comment. I think that executor performance is something ROS 2 should really improve. I think that ROS does not really support this workflow as entities are usually handled with shared ownership. Off the top of my head, I think that if rmw API supported attaching listeners to entities, that would make this possible. If this is then optimized for performance, latency, determinism or however sorted is the job of the flexible part of the executor. The timers my_string_timer and my_int_timer for the publishing executor; and, likewise, the subscriptions my_string_sub and my_int_sub for the subscribing executor. To start with, we provide a very simple example for an rclc Executor with one timer and one subscription, so to say, a Hello world example. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The "Requirements" section summarizes what we expected from a new design. All nodes, callbackgroups, timers, subscriptions etc. An executor can have zero or more nodes which provide work during spin functions. Though it reaches beyond the scope of this PR, bear in mind that eventually the TimersManager class will have to deal with different timers associated with different clocks. So, if the ExecutableLlist was made of: the ouput buffer array was [1,3] , so we had to process subscription 1 and 3. This suggestion has been applied or marked resolved. This was mostly rclcpp, because most of the other stuff was already merged. It does not add a new Executor but leverages callback groups for refining the Executor API to callback-group-level granularity. Agenda Objectives behind Executor design Default scheduling semantics -and its issues Static Executor Callback-group-level Executor Determinism -and particularly FIFO ordering rclcExecutor (micro-ROS) Executor Design User code I think that the big advantage that the wait set based executor has is that it doesn't suffer for this two issues (that's already handled by the queue of each entity based on its QoS settings). In addition, I created packages for ubuntu 22.04. Suggestions cannot be applied from pending reviews. We are updating the rclcpp implementation to have weak ownership of entities. Likewise, you can add the timer my_timer with the function rclc_c_executor_add_timer: A key feature of the rclc Executor is that the order of these rclc-executor-add-*-functions matters. These sensors can have different frequencies, for example, a high frequency IMU sensor and a low frequency laser scanner. Add a node to executor, execute the next available unit of work, and remove the node. https://discourse.ros.org/t/ros2-middleware-change-proposal/15863. The wait set and the event queue approach both indicate that there is work to do and the actual work is grabbed later. Like in the Hello-World example, the subscription callbacks just prints out the received message. Such a behavior cannot be defined currently with the rclcpp Executor and is useful to implement a deterministic execution semantics. This approach lead to a improvement in CPU, but the amount of changes in the code in multiple layers didn't justify the not-so-great CPU improvement, so we discarded the idea. If there is already work go to 4. after waiting check all the attached conditions (rmw entities) for work to do, two threads: one thread is used to process timers callbacks and another thread is used to process everything else, this document is now out-of-date (the events executor has changed), we're trying to move away from design docs on this site and towards REPs and design docs in the code repositories (maybe a version of this should go into rclcpp as we bring the events executor into rclcpp), there are lots of tasks competing for our limited resources and improvements to the events executor and moving it into rclcpp are higher priority than resolving this long-open pr. Multithreading behaviour with ROS AsyncSpinner, How to implement async/await syntax properly using Boost.Asio, Ros callback function parameter explained, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. making everything a Waitable). Spin (blocking) until the future is complete, it times out waiting, or rclcpp is interrupted. The future to wait on. Examples Sense-plan-act pipeline in robotics example Synchronization of multiple rates example High-priority processing path example Real-time embedded applications example ROS 2 Executor Workshop Reference System Future work Download Callback-group-level Executor API Changes Test Bench Related Work Fawkes Framework References Acknowledgments For quick solutions to more specific questions, see the How-to Guides. On the other hand, the same test with EventsExecutor results in 11% CPU usage. Ok, thank you all for the discussion. Convenience function which takes Node and forwards NodeBaseInterface. when you have Vim mapped to always print two? However, the package would also require a maintainer. You can have a look at the specific PRs The weak ownership model is not performant as the "fast path" require you to lock all the entities every time. See the default constructor for Executor. We can probably just change the top-level directory from rclcpp to irobot_rclcpp. with blocking callbacks will cause this function to block (which may have unintended consequences). Regarding your comment about profiling of the current executors, you can find a PDF executors benchmark.pdf here: As far as I know, there is no AsyncSpinner implementation for ROS2 Foxy. Why is Bb8 better than Bc7 in this position? Synchronous API for spin_once() implementation will probably suffer though. Add this suggestion to a batch that can be applied as a single commit. All nodes, callbackgroups, timers, subscriptions etc. Now, you can add a subscription with the function rclc_c_executor_add_subscription with the previously defined subscription my_sub, its message sub_msgand its callback my_subscriber_callback: The option ON_NEW_DATA selects the execution semantics of the spin-method. Then, each element of the handle list is checked for new data (or a timer is ready) by evaluating the field handles[i].data_available and its handle pointer is compared to the pointer of the communicatoin object. If at least one timer is ready, then the trigger condition returns true. That is the or-logic. The default implementation is suitable for a single-threaded model of execution. Attaching a handler to a condition could allow you to set the output buffer directly where the listener is notified about new data. We have explored the "output buffer" approach, using a fixed size buffer which allowed to not rebuild the waitsets at every iteration. There is no reason that a client library should be limited to one executor. In short: Then, on top of this, we added the direct access to DDS wait set and the performance improved to 20%. Did an AI-enabled drone attack the human operator in a simulation environment? We'll soon push this changes to the rclcpp PR. That are the consequences of having two queues that are processed at different times. And execution_mutex_ isn't recursive. In robotic applications often multiple sensors are used to improve localization precision. The waitset still has to be walked-through on rmw_wait though, to identify entities ready to work. First the publisher message is initialized with std_msgs__msg__String__init. I don't think that an approach with only a wait set can provide correct ordering of events at all. Ah I didn't see the pdf, thanks! Find the next available executable and do the work associated with it. The StaticSingleThreadedExecutor has shared ownership of entities. In a wait set, each entity can only signal if it has work to do or not, but not how many "events" have to be processed. How do the existing executors handle this situation? This is useful if the last node was removed from the executor while the executor was blocked waiting for work in another thread, because otherwise the executor would never be notified. Problem I have created a simple ROS2 application with a set of timers, publishers, and subscribers. Developed by iRobot These are the results including the implementation of the EventsExecutor having weak ownership of entities, and strong ownership when using them. This function can be overridden. The only needed step would be to "zero" the output buffer. The approach could have a bigger impact when not using rclcpp IPC. Spinning state, used to prevent multi threaded calls to spin and to cancel blocking spins. If it really is as decoupled as you say, then changing the executor implementation AND the sharing behavior is changing two things at once, and is generally not desired. To merge this, we should keep the existing ownership behavior, and therefore preserve the ability to "delete" (or start asynchronously deleting) entities that might be being used by the executor from within a callback. Hi Ivan. The data structure so includes the type of the entity that generated the event and a handle to its corresponding rclcpp object. Coordinate the order and timing of available communication tasks. When the the attaching and detaching to an underlying DDS wait set would be done in the context of calls like rcl_wait_set_add_subscription(), I'm wondering if rmw_wait() would no more need the rmw entities as parameter but only the rmw_wait_set, I'm wondering if rmw_wait() would no more need the rmw entities as parameter but only the rmw_wait_set. This is a repository that contains an example of a ROS 2 node that can have a service and a subscription callback running in separate threads. What's the purpose of a convex saw blade? Inheritance diagram for rclcpp::executor::Executor: Collaboration diagram for rclcpp::executor::Executor: template, template, template, rclcpp::executors::SingleThreadedExecutor, rclcpp::node_interfaces::NodeBaseInterface::SharedPtr, rclcpp::callback_group::CallbackGroup::SharedPtr, memory_strategy::MemoryStrategy::SharedPtr, virtual rclcpp::executor::Executor::~Executor, virtual void rclcpp::executor::Executor::spin, virtual void rclcpp::executor::Executor::add_node. Based on your experience, do you think it's possible to implement an event based executor out of tree? Now were are all set for the main() function: First rcl is initialized with the rclc_support_init using the default allocator. I don't think this is clear that that is the objective best solution, but it is a useful option to make . Then you need to allocate memory for pub_msg.data.data, set the maximum capacity pub_msg.data.capacity and set the length of the message pub_msg.data.size accordingly. I also improved the documentation and implementation for rcl. To learn more, see our tips on writing great answers. Is it possible to type a single quote/paren/etc. We also have a rclcpp::WaitSet that we don't use in any of the wait set based executors. You can create a timer my_timer with a period of one second, which executes the callback my_timer_callback like this: The string Hello World! You can find more details on the problem at the bottom of the design page (i.e. The wait timeout for checking for new messages at the DDS-queue or waiting timers to get ready is configured to be one second. rmw ros2/rmw#286 As i understand this Event Executor is a Single threaded executor. #include The second subscription my_int_sub is created with the rclc convenience function rclc_subscription_default and the message int_sub_msg is properly initialized. However this would require many more changes to ROS. @budrus it is possible to do that, but it requires additional APIs. are created before spin() is called, and modified only when an entity is added/removed to/from a node. I think that should be evaluated and addressed separately from this proposed change in the executor design. The best way to approach the tutorials is to walk through them for the first time in order, as they build off of each other and are not meant to be comprehensive documentation. static void rclcpp::executor::Executor::execute_subscription, static void rclcpp::executor::Executor::execute_intra_process_subscription, static void rclcpp::executor::Executor::execute_timer, static void rclcpp::executor::Executor::execute_service, static void rclcpp::executor::Executor::execute_client, void rclcpp::executor::Executor::wait_for_work, rclcpp::node_interfaces::NodeBaseInterface::SharedPtr rclcpp::executor::Executor::get_node_by_group, rclcpp::callback_group::CallbackGroup::SharedPtr rclcpp::executor::Executor::get_group_by_timer, void rclcpp::executor::Executor::get_next_timer, bool rclcpp::executor::Executor::get_next_ready_executable, bool rclcpp::executor::Executor::get_next_executable, std::atomic_bool rclcpp::executor::Executor::spinning, memory_strategy::MemoryStrategy::SharedPtr rclcpp::executor::Executor::memory_strategy_. Add a node, complete all immediately available work, and remove the node. void rclcpp::executors::StaticSingleThreadedExecutor::execute_ready_executables, Structure that can hold subscriptionbases, timerbases, etc. The EventsExecutor does not own entities mainly to align with the current ROS 2 design paradigm where there are no APIs for removing/destroying entities.
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