C - select() 似乎阻塞的时间比超时时间长

Question

我正在写一个数据采集程序，需要

• 使用 select() 等待串行

• 读取串行数据(RS232，波特率为 115200)，

• 为其添加时间戳 (clock_gettime())，

• 读取 SPI 上的 ADC，

• 解释它，

• 通过另一个 tty 设备发送新数据

• 循环和重复

ADC 目前无关紧要。

在循环结束时，我再次使用 select() 和 0 超时来轮询并查看数据是否已经可用，如果是则意味着我已经溢出，即我希望循环在更多数据之前结束，并且循环开始时的 select() 会阻塞并在数据到达时立即获取。

数据应每 5 毫秒到达一次，我的第一个 select() 超时计算为(5.5 毫秒 - 循环时间) - 应该约为 4 毫秒。

我没有超时，但有很多超限。

检查时间戳表明 select() 阻塞的时间比超时时间长(但仍返回 >0)。看起来 select() 在超时前获取数据后返回较晚。

这可能会在 1000 次重复中发生 20 次。可能是什么原因？我该如何解决？

编辑:这是代码的缩减版本(我做的错误检查比这多得多!)

#include <bcm2835.h> /* for bcm2835_init(), bcm2835_close() */

int main(int argc, char **argv){

    int err = 0;

    /* Set real time priority SCHED_FIFO */
    struct sched_param sp;
    sp.sched_priority = 30;
    if ( pthread_setschedparam(pthread_self(), SCHED_FIFO, &sp) ){
        perror("pthread_setschedparam():");
        err = 1;
    }

    /* 5ms between samples on /dev/ttyUSB0 */
    int interval = 5;

    /* Setup tty devices with termios, both totally uncooked, 8 bit, odd parity, 1 stop bit, 115200baud */
    int fd_wc=setup_serial("/dev/ttyAMA0");
    int fd_sc=setup_serial("/dev/ttyUSB0");

    /* Setup GPIO for SPI, SPI mode, clock is ~1MHz which equates to more than 50ksps */
    bcm2835_init();
    setup_mcp3201spi();

    int collecting = 1;

    struct timespec starttime;
    struct timespec time;
    struct timespec ftime;
    ftime.tv_nsec = 0;

    fd_set readfds;
    int countfd;
    struct timeval interval_timeout;
    struct timeval notime;

    uint16_t p1;
    float w1;

    uint8_t *datap = malloc(8);
    int data_size;
    char output[25];

    clock_gettime(CLOCK_MONOTONIC, &starttime);

    while ( !err && collecting ){   
        /* Set timeout to (5*1.2)ms - (looptime)ms, or 0 if looptime was longer than (5*1.2)ms */
        interval_timeout.tv_sec = 0;
        interval_timeout.tv_usec = interval * 1200 - ftime.tv_nsec / 1000;
        interval_timeout.tv_usec = (interval_timeout.tv_usec < 0)? 0 : interval_timeout.tv_usec;
        FD_ZERO(&readfds);
        FD_SET(fd_wc, &readfds);    
        FD_SET(0, &readfds); /* so that we can quit, code not included */   
        if ( (countfd=select(fd_wc+1, &readfds, NULL, NULL, &interval_timeout))<0 ){
            perror("select()");
            err = 1;
        } else if (countfd == 0){
            printf("Timeout on select()\n");
            fflush(stdout);
            err = 1;
        } else if (FD_ISSET(fd_wc, &readfds)){
            /* timestamp for when data is just available */
            clock_gettime(CLOCK_MONOTONIC, &time)
            if (starttime.tv_nsec > time.tv_nsec){
                time.tv_nsec = 1000000000 + time.tv_nsec - starttime.tv_nsec;
                time.tv_sec = time.tv_sec - starttime.tv_sec - 1;
            } else {
                time.tv_nsec = time.tv_nsec - starttime.tv_nsec;
                time.tv_sec = time.tv_sec - starttime.tv_sec;
            }

            /* get ADC value, which is sampled fast so corresponds to timestamp */
            p1 = getADCvalue();

            /* receive_frame, receiving is slower so do it after getting ADC value. It is timestamped anyway */
            /* This function consists of a loop that gets data from serial 1 byte at a time until a 'frame' is collected. */
            /* it uses select() with a very short timeout (enough for 1 byte at baudrate) just to check comms are still going */
            /* It never times out and behaves well */
            /* The interval_timeout is passed because it is used as a timeout for responding an ACK to the device */
            /* That select also never times out */
            ireceive_frame(&datap, fd_wc, &data_size, interval_timeout.tv_sec, interval_timeout.tv_usec);

            /* do stuff with it */
            /* This takes most of the time in the loop, about 1.3ms at 115200 baud */
            snprintf(output, 24, "%d.%04d,%d,%.2f\n", time.tv_sec, time.tv_nsec/100000, pressure, w1);
            write(fd_sc, output, strnlen(output, 23)); 

            /* Check how long the loop took (minus the polling select() that follows */ 
            clock_gettime(CLOCK_MONOTONIC, &ftime);
            if ((time.tv_nsec+starttime.tv_nsec) > ftime.tv_nsec){
                ftime.tv_nsec = 1000000000 + ftime.tv_nsec - time.tv_nsec - starttime.tv_nsec;
                ftime.tv_sec = ftime.tv_sec - time.tv_sec - starttime.tv_sec - 1;
            } else {
                ftime.tv_nsec = ftime.tv_nsec - time.tv_nsec - starttime.tv_nsec;
                ftime.tv_sec = ftime.tv_sec - time.tv_sec - starttime.tv_sec; 
            }

            /* Poll with 0 timeout to check that data hasn't arrived before we're ready yet */
            FD_ZERO(&readfds);
            FD_SET(fd_wc, &readfds);
            notime.tv_sec = 0;  
            notime.tv_usec = 0; 
            if ( !err && ( (countfd=select(fd_wc+1, &readfds, NULL, NULL, &notime)) < 0 )){
                perror("select()");
                err = 1;
            } else if (countfd > 0){
                printf("OVERRUN!\n");
                snprintf(output, 25, ",,,%d.%04d\n\n", ftime.tv_sec, ftime.tv_nsec/100000);
                write(fd_sc, output, strnlen(output, 24)); 
            }

        }

    }


    return 0;

}

我在输出的串行流上看到的时间戳是相当规则的(偏差通常会被下一个循环捕获)。输出片段:

6.1810,0,225.25
6.1867,0,225.25
6.1922,0,225.25
6,2063,0,225.25
,,,0.0010

在这里，直到 6.1922s 一切正常。下一个样本是 6.2063 - 比上一个样本晚 14.1 毫秒，但它没有超时，之前的 6.1922-6.2063 循环也没有用轮询 select() 捕获溢出。我的结论是，最后一个循环在采样时间内，select 花费了 -10ms 太长的返回时间而没有超时。

,,,0.0010 表示之后循环的循环时间 (ftime) - 我真的应该检查出错时的循环时间。我明天试试。

Answer 1

传递给 select 的超时是一个粗略的下限 — select 允许延迟您的进程略多于此。特别是，如果您的进程被其他进程(上下文切换)或内核中的中断处理抢占，则会延迟。

这是 Linux 手册页关于这个主题的内容:

Note that the timeout interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that the blocking interval may overrun by a small amount.

这是 POSIX 标准:

Implementations may also place limitations on the granularity of timeout intervals. If the requested timeout interval requires a finer granularity than the implementation supports, the actual timeout interval shall be rounded up to the next supported value.

在通用系统上很难避免这种情况。通过将进程锁定在内存中 (mlockall) 并将进程设置为实时优先级(使用 sched_setscheduler)，您将获得合理的结果，尤其是在多核系统上使用 SCHED_FIFO，并记住要经常休眠以便让其他进程有机会运行)。

一种更困难的方法是使用专用于运行实时代码的实时微 Controller 。有人声称reliably sample at 20MHz on fairly cheap hardware使用该技术。