A message queue is a software construct that allows multiple processes or threads to exchange data in a safe and predictable way. It acts as a buffer that holds messages and allows processes or threads to send and receive messages without blocking. Message queues are commonly used in real-time operating systems (RTOS) and other multi-threaded systems to implement inter-process communication and synchronization. They provide a simple and efficient mechanism for tasks to communicate with each other and exchange data.T he use of message queues can help to improve the performance and scalability of a system by decoupling the sender and receiver tasks, allowing them to run independently.
What are message queues in FreeRTOS?
In FreeRTOS queue is a buffer that holds messages that are sent by tasks and received by other tasks. A message queue is a way for different tasks to communicate with each other and exchange data. The tasks can send messages to the queue and receive messages from the queue.
Message queues are a way of implementing inter-task communication and synchronization in a real-time operating system. They allow tasks to send and receive data in a safe and predictable way, without the need for complex locking mechanisms.
FreeRTOS queues are implemented as a software construct and are thread-safe and can be used from different tasks and interrupts. This makes it easy to implement multi-threaded and concurrent systems.
FreeRTOS message queues are implemented as a software construct and are similar to FIFO(first in first out) queue in data structure. They can hold a fixed number of items and each item has a fixed size. Once the queue is full, new items cannot be added to the queue until some items are removed. Once the queue is empty, items cannot be removed until some items are added.
FreeRTOS message queues provide several functions to create, send, and receive messages, and other functions to check the status of the queue. These functions make it easy to use message queues in your application, and they are fully integrated with the FreeRTOS scheduler.
How message queues work?
Message queues work by creating a buffer that holds messages that are sent by tasks and received by other tasks. The buffer is implemented as a data structure, such as an array or a linked list, and it has a fixed size.
When a task wants to send a message to a message queue, it calls a function, such as xQueueSend() in FreeRTOS, and passes the message and the queue handle as arguments. The function then adds the message to the buffer, typically at the end of the buffer (FIFO - first in, first out) and increments a counter that keeps track of the number of messages in the buffer.
When a task wants to receive a message from a message queue, it calls a function, such as xQueueReceive() in FreeRTOS, and passes the queue handle and a pointer to a variable as arguments. The function then removes the oldest message from the buffer, typically from the beginning of the buffer, copies the data to the variable, and decrements the counter.
If the buffer is full, the xQueueSend() function will block or return an error, depending on the implementation, until there is space available in the buffer. Similarly, if the buffer is empty, the xQueueReceive() function will block or return an error, until there is a message in the buffer.
When a message queue is used in a real-time operating system like FreeRTOS, the message queue is integrated with the scheduler and the messages are passed between tasks in a safe and predictable way. Tasks can be blocked or unblocked depending on the state of the message queue. This allows for efficient use of the system resources and ensures that the tasks can communicate without interfering with each other.
It's worth noting that message queues can be configured as FIFO or priority-based queue, depending on the requirements of the application. This allows the developer to choose the best option for their system based on their requirements.
Advantages of using queues in FreeRTOS
There are several advantages to using message queues in FreeRTOS:- Inter-task communication: Message queues provide a simple and efficient mechanism for tasks to communicate with each other and exchange data. This allows tasks to work together and coordinate their activities.
- Synchronization: Message queues can be used to synchronize tasks and to ensure that tasks are executed in the correct order. For example, one task can send a message to a queue, and another task can receive the message and perform an action based on the data in the message.
- Thread safety: Message queues are thread-safe, which means that they can be used from different tasks and interrupts without the need for complex locking mechanisms. This makes it easy to implement multi-threaded and concurrent systems.
- Buffering: Message queues provide a buffer that holds messages and allows tasks to send and receive messages without blocking. This can improve the performance of the system by decoupling the sender and receiver tasks.
- Error handling: Message queues can be used to handle errors and to ensure that data is not lost in case of a failure. For example, if a task is unable to process a message, the message can be sent to a queue and handled by another task.
- Priority-based: FreeRTOS message queues can be configured as FIFO or priority-based queue, depending on the requirements of the application. This allows the developer to choose the best option for their system based on their requirements.
- Flexibility: Message queues can be used for a wide range of applications, from simple data transfer to complex communication protocols. They are also highly configurable, allowing the developer to optimize the queue for their specific use case.
- Integration: FreeRTOS message queues are fully integrated with the FreeRTOS scheduler, which allows for efficient use of system resources and ensures that tasks can communicate without interfering with each other.
Drawbacks of message Queues
While message queues have many advantages, there are also some drawbacks to using them in FreeRTOS or other real-time operating systems:
- Limited buffer size: The buffer size of message queues is fixed, which means that the maximum number of messages that can be stored in the queue is limited. This can be a problem if the queue is used for high-frequency or high-volume data transfer.
- Overhead: Message queues add some overhead to the system, as they need to be created, initialized, and managed. This can affect the performance of the system, especially in resource-constrained systems.
- Complexity: Message queues can add complexity to the system, especially if they are used for complex communication protocols. This can make it more difficult to debug and test the system.
- Limited functionality: Message queues are a simple mechanism for inter-task communication, but they may not be suitable for all types of applications. For example, if the system needs to support real-time data transfer or low-latency communication, message queues may not be the best option.
- Deadlock: Deadlock can occur if two tasks are waiting for each other to send/receive a message from a queue. This can happen if the tasks are not designed to handle this situation.
- Memory consumption: Creating and managing message queues require memory. This can be an issue for systems with limited memory resources.
- Blocking: Tasks can be blocked if the queue is full, or empty. This can cause the system to stop or pause and can make the system less responsive.
It's worth noting that the drawbacks of message queues may depend on the specific requirements of the application, and they may be outweighed by the advantages of using message queues in some cases.
Code example to implement queues in FreeRTOS
Here is an example of how to use message queues in FreeRTOS:
#if CONFIG_FREERTOS_UNICORE
#define ARDUINO_RUNNING_CORE 0
#else
#define ARDUINO_RUNNING_CORE 1
#endif
QueueHandle_t xQueue;
void senderTask(void *param)
{
(void) param;
int data = 100;
while (1)
{
xQueueSend(xQueue, &data, portMAX_DELAY);
data++;
vTaskDelay(1000 / portTICK_PERIOD_MS); // delay for 1 second
}
}
void receiverTask(void *param)
{
(void) param;
int data;
while (1)
{
xQueueReceive(xQueue, &data, portMAX_DELAY);
Serial.println ("Received data: ");
Serial.println (data);
}
}
void setup()
{
Serial.begin(115200);
xQueue = xQueueCreate(10, sizeof(int)); // create a queue with 10 elements of size int
xTaskCreatePinnedToCore(senderTask, "Sender", configMINIMAL_STACK_SIZE, NULL, 1, NULL,ARDUINO_RUNNING_CORE);
xTaskCreatePinnedToCore(receiverTask, "Receiver", configMINIMAL_STACK_SIZE, NULL, 1, NULL,ARDUINO_RUNNING_CORE);
// vTaskStartScheduler();
}
void loop()
{
}
This example creates a message queue with 10 elements of size int using the xQueueCreate() function. The queue handle is stored in the xQueue variable.
Two tasks are created, senderTask and receiverTask, using the xTaskCreate() function. The senderTask sends integers to the queue using the xQueueSend() function, and the receiverTask receives integers from the queue using the xQueueReceive() function and prints the received data.
The xQueueSend() function takes three arguments: the queue handle, a pointer to the data, and the time to wait for space in the queue. The xQueueReceive() function also takes three arguments: the queue handle, a pointer to the data, and the time to wait for a message in the queue.
Both functions can be configured to block or not block the task if the queue is full or empty. This is controlled by the time to wait, for example, portMAX_DELAY means block until space is available.
Few more things to consider when implementing message queues in FreeRTOS:
Queue size: The size of the queue is an important factor to consider when creating a message queue. A queue that is too small can cause data loss or blocking, while a queue that is too large can consume unnecessary resources.
Timeout: The timeout value is an important factor to consider when sending or receiving messages. A timeout value of 0 means non-blocking and returns immediately if the queue is full or empty. A timeout value of portMAX_DELAY means blocking and waits indefinitely for a message or space in the queue.
Priority: In FreeRTOS message queues can be configured as FIFO or priority-based queue. This allows the developer to choose the best option for their system based on their requirements.
Interrupts: Message queues can also be used from interrupt service routines (ISRs). FreeRTOS provides functions such as xQueueSendFromISR() and xQueueReceiveFromISR() to send and receive messages from ISRs.
Error handling: You should also consider error handling and return values when using message queues. For example, if a message queue is full, the xQueueSend() function will return pdFALSE.
Other functions: FreeRTOS provides many other functions to work with message queues, such as xQueuePeek(), xQueueIsQueueEmpty(), xQueueIsQueueFull() etc. You can use these functions to check the status of the queue, or to access the oldest message without removing it from the queue.
Deletion: When you're done with a queue, you should use the vQueueDelete() function to delete it, this release the memory that was allocated for the queue.
Conclusion
FreeRTOS queues are a powerful mechanism for inter-task communication, but they may not be the best option for all types of applications. Therefore, it's important to consider the specific requirements of the application when choosing a communication mechanism.