Executor

Concept of Executor

The executor is a long-standing concept that represents an abstract entity capable of executing logical code. An executor can be a thread pool, a coroutine/fiber, a CPU, a GPU, or even a remote server. The simplest code we usually write also has a default executor: the main thread.

Generally, an executor will have an interface similar to this:

void Execute(std::function<void()>);

This interface indicates that a task closure similar to std::function<void()> can be submitted to a specified executor for execution. When and where this task is executed depends on the specific implementation of the executor. The std::thread in the C++ standard library is a typical executor; its constructor accepts a std::function<void()> task closure and runs that task in a new thread.## Overview of the Basic Executor Interface

In AimRT, modules can obtain an aimrt::configurator::ExecutorManagerRef handle by calling the GetExecutorManager() interface of the CoreRef handle, which provides a simple interface for acquiring an Executor:

namespace aimrt::executor {

class ExecutorManagerRef {
 public:
  ExecutorRef GetExecutor(std::string_view executor_name) const;
};

}  // namespace aimrt::executor

Users can call the GetExecutor method of the ExecutorManagerRef type to obtain an aimrt::configurator::ExecutorRef handle with a specified name, in order to invoke executor-related functionality. The core interface of ExecutorRef is as follows:

namespace aimrt::executor {

class ExecutorRef {
 public:
  std::string_view Type() const;

  std::string_view Name() const;

  bool ThreadSafe() const;

  bool IsInCurrentExecutor() const;

  bool SupportTimerSchedule() const;

  void Execute(Task&& task) const;

  std::chrono::system_clock::time_point Now() const;

  void ExecuteAt(std::chrono::system_clock::time_point tp, Task&& task) const;

  void ExecuteAfter(std::chrono::nanoseconds dt, Task&& task) const;
};

}  // namespace aimrt::executor

Executors in AimRT have some inherent attributes, most of which are related to the executor type and do not change during runtime. These inherent attributes include:

Executor Type: A string field that identifies the type of the executor at runtime.
- In a single AimRT instance, there can be multiple types of executors. AimRT officially provides several executors, and plugins can also provide new executor types.
- For details on specific executor types and their characteristics, please refer to the executor configuration chapter in the deployment section.
- During logical development, you should not focus too much on the actual runtime executor type; instead, implement business logic based on the abstract executor interface.
Executor Name: A string field that identifies the name of the executor at runtime.
- Within an AimRT process, the name uniquely identifies an executor.
- All executor instance names are determined at runtime through configuration. For details, please refer to the executor configuration chapter in the deployment section.
- You can obtain an executor with a specified name via the GetExecutor method of ExecutorManagerRef.
Thread Safety: A boolean value indicating whether this executor is thread-safe.
- Usually related to the executor type.
- A thread-safe executor can guarantee that tasks posted to it will not run concurrently; otherwise, no such guarantee exists.
Support for Time-based Scheduling: A boolean value indicating whether this executor supports time-based scheduling interfaces, namely the ExecuteAt and ExecuteAfter interfaces.
- If the executor does not support time-based scheduling, calling the ExecuteAt or ExecuteAfter interfaces will throw an exception.

Detailed usage instructions for the ExecutorRef interface are as follows:

std::string_view Type(): Gets the type of the executor.
std::string_view Name(): Gets the name of the executor.
bool ThreadSafe(): Returns whether this executor is thread-safe.
bool IsInCurrentExecutor(): Determines whether the current environment is within this executor when this function is called.
- Note: If it returns true, the current environment is definitely within this executor; if it returns false, the current environment may or may not be within this executor.
bool SupportTimerSchedule(): Returns whether this executor supports time-based scheduling interfaces, namely the ExecuteAt and ExecuteAfter interfaces.
void Execute(Task&& task): Posts a task to this executor for immediate execution upon scheduling.
- The parameter Task can simply be viewed as a task closure that satisfies the std::function<void()> signature.
- This interface can be called during the Initialize/Start phase, but the executor only guarantees to start execution after the Start phase. Therefore, calling this interface before the Start phase may only enqueue the task into the executor’s task queue without immediate execution, and the task will start executing only after the Start phase begins.
std::chrono::system_clock::time_point Now(): Gets the time in this executor’s time system.
- For general executors, this returns the result of std::chrono::system_clock::now().
- Some special executors with time-scaling functionality may return processed time here.
void ExecuteAt(std::chrono::system_clock::time_point tp, Task&& task): Executes a task at a specific time point.
- The first parameter—time point—is based on this executor’s time system.
- The second parameter Task can simply be viewed as a task closure that satisfies the std::function<void()> signature.
- If this executor does not support time-based scheduling, calling this interface will throw an exception.
- This interface can be called during the Initialize/Start phase, but the executor only guarantees to start execution after the Start phase. Therefore, calling this interface before the Start phase may only enqueue the task into the executor’s task queue without immediate execution, and the task will start executing only after the Start phase begins.
void ExecuteAfter(std::chrono::nanoseconds dt, Task&& task): Executes a task after a certain duration.
- The first parameter—duration—is based on this executor’s time system.
- The second parameter Task can simply be viewed as a task closure that satisfies the std::function<void()> signature.
- If this executor does not support time-based scheduling, calling this interface will throw an exception.
- This interface can be called during the Initialize/Start phase, but the executor only guarantees to start execution after the Start phase. Therefore, calling this interface before the Start phase may only enqueue the task into the executor’s task queue without immediate execution, and the task will start executing only after the Start phase begins.## Basic Executor Interface Usage Example

The following is a simple usage example demonstrating how to obtain an executor handle and dispatch a simple task to that executor for execution:

#include "aimrt_module_cpp_interface/module_base.h"

class HelloWorldModule : public aimrt::ModuleBase {
 public:
  bool Initialize(aimrt::CoreRef core) override {
    core_ = core;

    return true;
  }

  bool Start() override {
    // Get an executor handle named 'work_executor'
    auto work_executor = core_.GetExecutorManager().GetExecutor("work_executor");

    // Check
    AIMRT_CHECK_ERROR_THROW(work_executor, "Can not get work_executor");

    // Post a task to this executor
    work_executor.Execute([this]() {
      AIMRT_INFO("This is a simple task");
    });
  }

  // ...
 private:
  aimrt::CoreRef core_;
};

If it is a thread-safe executor, then tasks dispatched to it do not need to be locked to ensure thread safety, as shown in the example below:

#include "aimrt_module_cpp_interface/module_base.h"

class HelloWorldModule : public aimrt::ModuleBase {
 public:
  bool Initialize(aimrt::CoreRef core) override {
    core_ = core;

    return true;
  }

  bool Start() override {
    // Get an executor handle named 'thread_safe_executor'
    auto thread_safe_executor = core_.GetExecutorManager().GetExecutor("thread_safe_executor");

    // Check
    AIMRT_CHECK_ERROR_THROW(thread_safe_executor && thread_safe_executor.ThreadSafe(),
                            "Can not get thread_safe_executor");

    // Post some tasks to this executor
    uint32_t n = 0;
    for (uint32_t ii = 0; ii < 10000; ++ii) {
      thread_safe_executor_.Execute([&n]() {
        n++;
      });
    }

    std::this_thread::sleep_for(std::chrono::seconds(5));

    AIMRT_INFO("Value of n is {}", n);
  }

  // ...
 private:
  aimrt::CoreRef core_;
};

The following example demonstrates how to use the Time Schedule interface to implement periodic scheduling:

#include "aimrt_module_cpp_interface/module_base.h"

class HelloWorldModule : public aimrt::ModuleBase {
 public:
  bool Initialize(aimrt::CoreRef core) override {
    core_ = core;

    // Get an executor handle named 'time_schedule_executor'
    auto time_schedule_executor_ = core_.GetExecutorManager().GetExecutor("time_schedule_executor");

    // Check
    AIMRT_CHECK_ERROR_THROW(time_schedule_executor_ && time_schedule_executor_.SupportTimerSchedule(),
                            "Can not get time_schedule_executor");

    return true;
  }

  // Task
  void ExecutorModule::TimeScheduleDemo() {
    // Check shutdown
    if (!run_flag_) return;

    AIMRT_INFO("Loop count : {}", loop_count_++);

    // Execute itself
    time_schedule_executor_.ExecuteAfter(
        std::chrono::seconds(1),
        std::bind(&ExecutorModule::TimeScheduleDemo, this));
  }

  bool Start() override {
    TimeScheduleDemo();
  }

  void ExecutorModule::Shutdown() {
    run_flag_ = false;

    std::this_thread::sleep_for(std::chrono::seconds(1));
  }

  // ...

 private:
  aimrt::CoreRef core_;

  bool run_flag_ = true;
  uint32_t loop_count_ = 0;
  aimrt::executor::ExecutorRef time_schedule_executor_;
};

Executor Coroutine Interface Overview

In AimRT, a coroutine-style interface is encapsulated for the executor based on C++20 coroutines and the libunifex library, providing a relatively important class: aimrt::co::AimRTScheduler, which can be constructed from an aimrt::executor::ExecutorRef handle. This class wraps the native AimRT executor handle into a coroutine form, with its core interfaces as follows:

namespace aimrt::co {

// Corresponding to ExecutorRef
class AimRTScheduler {
 public:
  explicit AimRTScheduler(executor::ExecutorRef executor_ref = {}) noexcept;
};

// Corresponding to ExecutorManagerRef
class AimRTContext {
 public:
  explicit AimRTContext(executor::ExecutorManagerRef executor_manager_ref = {}) noexcept;

  AimRTScheduler GetScheduler(std::string_view executor_name) const;
};

}  // namespace aimrt::co

Executor Coroutine Interface Usage Example

With the AimRTScheduler handle, you can use a series of coroutine tools under the aimrt::co namespace. The following is a simple usage example demonstrating how to start a coroutine and schedule tasks to a specified executor within the coroutine:

#include "aimrt_module_cpp_interface/co/async_scope.h"
#include "aimrt_module_cpp_interface/co/task.h"
#include "aimrt_module_cpp_interface/co/inline_scheduler.h"
#include "aimrt_module_cpp_interface/co/on.h"
#include "aimrt_module_cpp_interface/co/schedule.h"
#include "aimrt_module_cpp_interface/module_base.h"

class HelloWorldModule : public aimrt::ModuleBase {
 public:
  bool Initialize(aimrt::CoreRef core) override {
    core_ = core;

    // Get an executor handle named 'work_executor_1' and check
    work_executor_1_ = core_.GetExecutorManager().GetExecutor("work_executor_1");
    AIMRT_CHECK_ERROR_THROW(work_executor_1_, "Can not get work_executor_1");

    // Get an executor handle named 'work_executor_2' and check
    work_executor_2_ = core_.GetExecutorManager().GetExecutor("work_executor_2");
    AIMRT_CHECK_ERROR_THROW(work_executor_2_, "Can not get work_executor_2");

    return true;
  }

  bool Start() override {
    // Start a coroutine and use the current executor (main thread) to execute the coroutine
    scope_.spawn(co::On(co::InlineScheduler(), MyTask()));

    return true;
  }

  aimrt::co::Task<void> MyTask() {
    AIMRT_INFO("Now run in init executor");

    // Encapsulate the executor handle as the scheduler handle
    auto work_executor_1_scheduler = co::AimRTScheduler(work_executor_1_);

    // Schedule to work_executor_1_
    co_await aimrt::co::Schedule(work_executor_1_scheduler);

    AIMRT_INFO("Now run in work_executor_1_");

    // Encapsulate the executor handle as the scheduler handle
    auto work_executor_2_scheduler = co::AimRTScheduler(work_executor_2_);

    // Schedule to work_executor_2_
    co_await aimrt::co::Schedule(work_executor_2_scheduler);

    AIMRT_INFO("Now run in work_executor_2_");

    co_return;
  }

  void ExecutorCoModule::Shutdown() {
    // Blocked waiting for all coroutines in the scope to complete execution
    co::SyncWait(scope_.complete());

    AIMRT_INFO("Shutdown succeeded.");
  }

 private:
  aimrt::CoreRef core_;
  aimrt::co::AsyncScope scope_;

  aimrt::executor::ExecutorRef work_executor_1_;
  aimrt::executor::ExecutorRef work_executor_2_;
};

The following example demonstrates how to use the Time Schedule interface to implement a timed loop based on coroutines:

#include "aimrt_module_cpp_interface/co/async_scope.h"
#include "aimrt_module_cpp_interface/co/task.h"
#include "aimrt_module_cpp_interface/co/inline_scheduler.h"
#include "aimrt_module_cpp_interface/co/on.h"
#include "aimrt_module_cpp_interface/co/schedule.h"
#include "aimrt_module_cpp_interface/module_base.h"

class HelloWorldModule : public aimrt::ModuleBase {
 public:
  bool Initialize(aimrt::CoreRef core) override {
    core_ = core;

    // Get an executor handle named 'time_schedule_executor' and check
    time_schedule_executor_ = core_.GetExecutorManager().GetExecutor("time_schedule_executor");
    AIMRT_CHECK_ERROR_THROW(time_schedule_executor_ && time_schedule_executor_.SupportTimerSchedule(),
                            "Can not get time_schedule_executor");

    return true;
  }

  bool Start() override {
    // Start a coroutine and use the current executor (main thread) to execute the coroutine
    scope_.spawn(co::On(co::InlineScheduler(), MainLoop()));

    return true;
  }

  aimrt::co::Task<void> MainLoop() {
    auto time_scheduler = co::AimRTScheduler(time_schedule_executor_);

    // Schedule to time_schedule_executor
    co_await co::Schedule(time_scheduler);

    uint32_t count = 0;
    while (run_flag_) {
      count++;
      AIMRT_INFO("Loop count : {} -------------------------", count);

      // Schedule to time_schedule_executor after some time. Equivalent to non blocking sleep
      co_await co::ScheduleAfter(time_scheduler, std::chrono::seconds(1));
    }

    AIMRT_INFO("Exit loop.");

    co_return;
  }

  void ExecutorCoModule::Shutdown() {
    run_flag_ = false;    // Blocked waiting for all coroutines in the scope to complete execution
    co::SyncWait(scope_.complete());

    AIMRT_INFO("Shutdown succeeded.");
  }

 private:
  aimrt::CoreRef core_;
  aimrt::co::AsyncScope scope_;
  std::atomic_bool run_flag_ = true;
  aimrt::executor::ExecutorRef time_schedule_executor_;
};## Timer Based on Executor

### Timer Interface

Code files:
- {{ '[aimrt_module_cpp_interface/executor/timer.h]({}/src/interface/aimrt_module_cpp_interface/executor/timer.h)'.format(code_site_root_path_url) }}

Reference example:
- {{ '[timer_module.cc]({}/src/examples/cpp/executor/module/timer_module/timer_module.cc)'.format(code_site_root_path_url) }}

### Timer Concept

A timer is a tool provided by the executor for scheduling tasks at fixed intervals. You can create a timer based on an executor and specify the execution period.

### Timer Interface

Use the `aimrt::executor::CreateTimer` interface to create a timer and specify its execution period and task. The function declaration is as follows:


```cpp
namespace aimrt::executor {

template <typename TaskType>
std::shared_ptr<TimerBase> CreateTimer(ExecutorRef executor, std::chrono::nanoseconds period,
                                       TaskType&& task, bool auto_start = true);

}  // namespace aimrt::executor

Where ExecutorRef is the executor handle, TaskType is the task type, period is the timer execution period, and auto_start indicates whether to start the timer automatically, defaulting to true.

The ExecutorRef used by the timer must support timer scheduling, i.e., SupportTimerSchedule() returns true. Refer to the Executor Configuration chapter to check whether the executor supports timer scheduling.

TaskType is the task type, accepting a callable object. You can use std::function, std::bind, lambda expressions, etc., as long as its function signature meets one of the following requirements:

void()
void(TimerBase&)
void(const TimerBase&)

In the function signature, TimerBase& is the timer object itself, and const TimerBase& is a const reference to the timer object.

TimerBase is the base class of the timer object, and Timer is a derived class of the timer object, mainly encapsulating the execution of the user-specified timer task. We generally use the smart pointer type of TimerBase: std::shared_ptr<TimerBase>.

The core interfaces of TimerBase are as follows:

class TimerBase {
 public:
  virtual void Reset() = 0;
  virtual void Cancel() = 0;
  virtual void ExecuteTask() = 0;

  [[nodiscard]] bool IsCancelled() const;
  [[nodiscard]] std::chrono::nanoseconds Period() const;
  [[nodiscard]] std::chrono::system_clock::time_point NextCallTime() const;
  [[nodiscard]] std::chrono::nanoseconds TimeUntilNextCall() const;
  [[nodiscard]] ExecutorRef Executor() const;
};

Detailed usage instructions for the interfaces in the TimerBase class are as follows:

void Cancel(): Cancels the timer and sets the cancel state.
void Reset(): Resets the timer, cancels the cancel state, and resets the next execution time. The next execution time is calculated based on the current time plus the period.
void ExecuteTask(): Executes the timer task.
bool IsCancelled(): Returns whether the timer has been cancelled.
std::chrono::nanoseconds Period(): Returns the timer execution period.
std::chrono::system_clock::time_point NextCallTime(): Returns the next execution time of the timer.
std::chrono::nanoseconds TimeUntilNextCall(): Returns the time difference between the next execution time and the current time.
ExecutorRef Executor(): Returns the executor to which the timer belongs.

Timer Behavior Overview

The behavior of the timer is as follows:

After the timer is created, it is automatically started by default, equivalent to automatically calling the Reset() interface. If you do not want it to start automatically, you can set auto_start to false, in which case the timer will be in the cancel state.
Regardless of whether the timer is started or not, calling the Cancel() interface will cancel the timer and set the cancel state.
Regardless of whether the timer is started or not, calling the Reset() interface will reset the timer, cancel the cancel state, and reset the next execution time. The next execution time is calculated based on the current time plus the period.
The Reset() interface can override the original timer task. After calling the Reset() interface, calling the Reset() interface again will reschedule the task according to the new period, and the original timer task will be overridden by the new task.
If the task execution time is too long or there are blocking operations in the executor used by the timer, causing some timer periods to be missed, the timer will not make up for the missed executions. Instead, it will wait until the next execution time arrives to execute the task. For example:
- Suppose the timer period is 1000 ms, and the task is expected to execute at 0, 1000, 2000, 3000, 4000, … ms
- Suppose the task execution time is 1500 ms, then the task started at 0 ms will complete at 1500 ms, missing the execution at 1000 ms
- The timer will reset the next execution time to 2000 ms and execute the task at 2000 ms, without making up for the execution at 1000 ms
- The final task execution start times will be: 0, 2000, 4000, 6000, … ms### Timer Usage Example

Below is a simple usage example demonstrating how to create a timer and use it to execute a task:

bool TimerModule::Initialize(aimrt::CoreRef core) {
  core_ = core;

  timer_executor_ = core_.GetExecutorManager().GetExecutor("timer_executor");
  AIMRT_CHECK_ERROR_THROW(timer_executor_, "Can not get timer_executor");
  AIMRT_CHECK_ERROR_THROW(timer_executor_.SupportTimerSchedule(),
                          "timer_executor does not support timer schedule");

  return true;
}

bool TimerModule::Start() {
  using namespace std::chrono_literals;

  auto start_time = timer_executor_.Now();
  auto task = [logger = core_.GetLogger(), start_time](aimrt::executor::TimerBase& timer) {
    static int count = 0;

    auto now = timer.Executor().Now();
    auto timepoint = std::chrono::duration_cast<std::chrono::milliseconds>(now - start_time).count();
    AIMRT_HL_INFO(logger, "Executed {} times, execute timepoint: {} ms", ++count, timepoint);

    if (count >= 10) {
      timer.Cancel();
      AIMRT_HL_INFO(logger, "Timer cancelled at timepoint: {} ms", timepoint);
    }
  };

  timer_ = aimrt::executor::CreateTimer(timer_executor_, 100ms, std::move(task));
  AIMRT_INFO("Timer created at timepoint: 0 ms");

  timer_executor_.ExecuteAfter(350ms, [this, logger = core_.GetLogger()]() {
    timer_->Reset();
    AIMRT_HL_INFO(logger, "Timer reset at timepoint: 350 ms");
  });

  timer_executor_.ExecuteAfter(600ms, [this, logger = core_.GetLogger()]() {
    timer_->Reset();
    AIMRT_HL_INFO(logger, "Timer reset at timepoint: 600 ms");
  });

  return true;
}

void TimerModule::Shutdown() { timer_->Cancel(); }

Executor

Related Links

Basic Executor Interface

Coroutine Interface

Concept of Executor

Executor Coroutine Interface Overview

Executor Coroutine Interface Usage Example

Timer Behavior Overview