串口收发性能测试

对于本框架来说，有一个问题经常会被问到："LibXR对底层驱动封装之后，性能是否会很差？"

这个问题的答案是：完全不必担心。虽然相比与直接调用厂家SDK(例如hal库，ESP-IDF)提供的API来自行管理DMA等资源，性能会有一定的损失，但是这种损失是很小的，可以忽略不计。下面会附上一些性能测试结果，大家可以自行分析。

测试环境

串口驱动是整个LibXR里面最复杂的部分，性能测试的环境如下：

• STM32F103C8，Cortex-M3 @ 72Mhz
• CH32V307VC, Risc-V @ 144Mhz
• STM32F407IG，Cortex-M4 @ 168Mhz
• Debug模式只有用户代码是-Og优化，HAL库，FreeRTOS，USB等库是-O2优化。Release模式都为-O3优化。
• 使用杜邦线将USART1的TX RX连接起来，数据位为8，停止位为1，无流控，无奇偶校验
• 收到数据后对整包进行CRC校验，每秒统计发送成功、发送失败和总共校验失败的次数

测试代码

同步方式收发各使用一个线程，虽然上下文切换会带来一些性能损失，但是影响不大。追求极限性能可以使用异步方式收发。

  constexpr size_t BUFFER_SIZE = 32;
  constexpr size_t BAUDRATE = 2000000;

  static uint8_t read_buffer[BUFFER_SIZE], write_buffer[BUFFER_SIZE];
  static uint32_t count_read = 0, count_write = 0, count_error = 0;

  for (uint32_t i = 0; i < sizeof(write_buffer); i++) {
    write_buffer[i] = i;
  }

  STDIO::write_ = uart_cdc.write_port_;

  void (*fun)(void *) = [](void *) {
    LibXR::STDIO::Printf("read count: %d, write count: %d, error count: %d\r\n",
                         count_read, count_write, count_error);
    LibXR::STDIO::Printf("speed: %d BAUD\r\n",
                         count_read * 10 * sizeof(write_buffer));
    count_read = 0;
    count_write = 0;
  };

  auto print_task =
      LibXR::Timer::CreateTask(fun, reinterpret_cast<void *>(0), 1000);
  LibXR::Timer::Add(print_task);
  LibXR::Timer::Start(print_task);

  void (*thread_read)(LibXR::UART *) = [](LibXR::UART *uart) {
    LibXR::Semaphore sem(0);
    LibXR::ReadOperation op(sem);
    while (true) {
      uart->Read(read_buffer, op);
      if (LibXR::CRC8::Verify(read_buffer, sizeof(read_buffer))) {
        count_read++;
      } else {
        count_error++;
      }
    }
  };

  void (*thread_write)(LibXR::UART *) = [](LibXR::UART *uart) {
    LibXR::Semaphore sem(1);
    LibXR::WriteOperation op(sem);

    uart->SetConfig({BAUDRATE, LibXR::UART::Parity::NO_PARITY, 8, 1});

    while (true) {
      write_buffer[0]++;
      write_buffer[sizeof(write_buffer) - 1] = LibXR::CRC8::Calculate(
          write_buffer, sizeof(write_buffer) - sizeof(uint8_t));

      uart->Write(write_buffer, op);
      count_write++;
    }
  };

  LibXR::Thread read_thread, write_thread;

  read_thread.Create(reinterpret_cast<LibXR::UART *>(&usart1), thread_read,
                     "read_thread", 2048,
                     static_cast<LibXR::Thread::Priority>(4));

  write_thread.Create(reinterpret_cast<LibXR::UART *>(&usart1), thread_write,
                      "write_thread", 2048,
                      static_cast<LibXR::Thread::Priority>(3));

  while (true) {
    LibXR::Thread::Sleep(UINT32_MAX);
  }

速率测试

STM32F1 -0g 32字节 2M波特率

32字节接近实际的串口数据包大小，在2M波特率下能够逼近串口理论速率。

read count: 6000, write count: 5999, error count: 0
speed: 1920000 BAUD
read count: 6000, write count: 6000, error count: 0
speed: 1920000 BAUD
read count: 5999, write count: 6000, error count: 0
speed: 1919680 BAUD
read count: 6000, write count: 5999, error count: 0
speed: 1920000 BAUD
read count: 6000, write count: 6000, error count: 0
speed: 1920000 BAUD
read count: 5999, write count: 6000, error count: 0
speed: 1919680 BAUD

STM32F1 -03 32字节 2M波特率

O3优化下没有明显区别

read count: 6000, write count: 5999, error count: 0
speed: 1920000 BAUD
read count: 5999, write count: 6000, error count: 0
speed: 1919680 BAUD
read count: 6000, write count: 6000, error count: 0
speed: 1920000 BAUD
read count: 6000, write count: 5999, error count: 0
speed: 1920000 BAUD
read count: 5999, write count: 6000, error count: 0
speed: 1919680 BAUD
read count: 6000, write count: 6000, error count: 0
speed: 1920000 BAUD

STM32F1 -0g 128字节 4M波特率

更大的数据包，更高的波特率

read count: 3061, write count: 3061, error count: 0
speed: 3918080 BAUD
read count: 3062, write count: 3062, error count: 0
speed: 3919360 BAUD
read count: 3062, write count: 3062, error count: 0
speed: 3919360 BAUD
read count: 3061, write count: 3061, error count: 0
speed: 3918080 BAUD
read count: 3062, write count: 3062, error count: 0
speed: 3919360 BAUD
read count: 3063, write count: 3062, error count: 0
speed: 3920640 BAUD

CH32V307 -Og 64字节 9M波特率

9M波特率下一样能够达到理论速率

read count: 14000, write count: 14000, error count: 0
speed: 8960000 BAUD
read count: 14000, write count: 14000, error count: 0
speed: 8960000 BAUD
read count: 14000, write count: 14000, error count: 0
speed: 8960000 BAUD
read count: 14000, write count: 14000, error count: 0
speed: 8960000 BAUD
read count: 14000, write count: 14000, error count: 0
speed: 8960000 BAUD
read count: 14000, write count: 14000, error count: 0
speed: 8960000 BAUD

系统调用分析

再测速率已经没有意义了，这里使用STM32F4+SystemView展示收发过程的系统调用情况。本身收发过程完全无锁，所有的系统调用都为线程本身的唤醒和同步操作所需的信号量。

较低波特率

低波特率下多个包被连起来，无法触发IDLE中断。写线程周期性被唤醒，读线程等到半满/全满中断时被唤醒。

较高波特率

高波特率下，一收一发两个线程轮流唤醒。

总结

LibXR 框架在对底层串口驱动进行封装后，在绝大多数情况下不会造成明显的性能损失。在在 STM32F103（72MHz）平台实测中，2~4 Mbps 波特率下数据收发稳定无误，最高实际吞吐接近物理上限，说明即便在低性能平台也具备高效性能表现，完全可用于对实时性与带宽有要求的应用。