Not stable serial port reading from raspberry pi - c++

Question

I am trying to read serial data from an arduino serial port with my raspberry pi with a 33 bytes package in 800 Hz. I tried to time the duration, it turned to be very unstable.
The code I use to receive data is as follow, basically it just calculate how low does the "read" take. It will be called in 800 Hz.
std::chrono::system_clock::time_point startGet= std::chrono::system_clock::now();
int readBytes = read(serialPort, this->serialBuf+alreadyRead, this->dataNeedRead);
std::chrono::system_clock::time_point endGet= std::chrono::system_clock::now();
microsecs_t get_time(std::chrono::duration_cast<microsecs_t>(endGet-startGet));
cout<<"spend "<<get_time.count()<<endl;
Since the above script will be called in 800 Hz, ideally, I should see the port received 33 bytes in a short period of time.Yet it turned out the output is:
read: 33
spend 3817
read: 33
spend 5
read: 33
spend 1301
read: 33
spend 40
read: 33
spend 1333
read: 33
spend 31
I can see the port does read 33 bytes, but the processing time is too long( 800Hz need to be ran within 1250 us, but some of them is more than 3000 us.
And the way I setup the serialport is as follow
int Sensor::serialPortConnect(char *portName)
{
// Open the serial port. Change device path as needed (currently set to an standard FTDI USB-UART cable type device)
int serial_port = open(portName, O_RDWR);
// Create new termios struc, we call it 'tty' for convention
struct termios tty;
memset(&tty, 0, sizeof tty);
// Read in existing settings, and handle any error
if (tcgetattr(serial_port, &tty) != 0)
{
printf("Error %i from tcgetattr: %s\n", errno, strerror(errno));
}
tty.c_cflag &= ~PARENB; // set parity bit, disabling parity (most common)
tty.c_cflag &= ~CSTOPB; // Clear stop field, only one stop bit used in communication (most common)
tty.c_cflag |= CS8; // 8 bits per byte (most common)
tty.c_cflag &= ~CRTSCTS; // Disable RTS/CTS hardware flow control (most common)
tty.c_cflag |= CREAD | CLOCAL; // Turn on READ & ignore ctrl lines (CLOCAL = 1)
tty.c_lflag &= ~ICANON;
tty.c_lflag &= ~ECHO; // Disable echo
tty.c_lflag &= ~ECHOE; // Disable erasure
tty.c_lflag &= ~ECHONL; // Disable new-line echo
tty.c_lflag &= ~ISIG; // Disable interpretation of INTR, QUIT and SUSP
tty.c_iflag &= ~(IXON | IXOFF | IXANY); // Turn off s/w flow ctrl
tty.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL); // Disable any special handling of received bytes
tty.c_oflag &= ~OPOST; // Prevent special interpretation of output bytes (e.g. newline chars)
tty.c_oflag &= ~ONLCR; // Prevent conversion of newline to carriage return/line feed
// tty.c_oflag &= ~OXTABS; // Prevent conversion of tabs to spaces (NOT PRESENT ON LINUX)
// tty.c_oflag &= ~ONOEOT; // Prevent removal of C-d chars (0x004) in output (NOT PRESENT ON LINUX)
tty.c_cc[VTIME] = 0;
tty.c_cc[VMIN] =33;
// Set in/out baud rate to be 1000000
cfsetispeed(&tty, B1000000);
cfsetospeed(&tty, B1000000);
// Save tty settings, also checking for error
if (tcsetattr(serial_port, TCSANOW, &tty) != 0)
{
printf("Error %i from tcsetattr: %s\n", errno, strerror(errno));
}
//Allocate buffer for read buffer
//tcflush(serial_port, TCIFLUSH);
return serial_port;
}
I am not sure why this happen. The RPi is installed with RT-PREEMPT kernel and the application I am running has the highest priority (RT). I thought it shouldn't be this unstable.