Linux：UDP接收消息的速度比发送消息的速度慢

Question

I am using a arm-cpu based board, running a uclinux with kernel Linux version 2.6.12.3-a9-17 to receive data from a PC by eithernet, using UDP protocol.

My problem is, on the board, although I can receive all the messages (no lost), the time interval between incoming messages is limited to 20ms at most. That is, if I send out messages every 30ms, everything is OK. However, if I send out message every 10ms, the board shows incoming UDP package every 20ms ( So it will take twice the time that PC use to receive all the messages).

I am using these code to measure the time interval between two incoming message:

struct timeval tnew, told, dt;
gettimeofday(&tnew,0);
told = tnew;

while (1) {
    memset(buf, 100, 0);
    int recvlen = recvfrom(fd, buf, 100 , 0,
                           (struct sockaddr *)&addr_recv, &addr_len);
    if (recvlen>0) {
        gettimeofday(&tnew ,0);
        timersub(&tnew,&told,&dt);
        told = tnew;
        printf("UDP intput: %fms, string:%s\n", (float)dt.tv_usec / 1000.0,buf);
    }

Here is the result:

on PC:

UDP output: 10.019000ms, string: 353:ABCDEFG
UDP output: 10.067000ms, string: 354:ABCDEFG
UDP output: 10.068000ms, string: 355:ABCDEFG
UDP output: 10.068000ms, string: 356:ABCDEFG
UDP output: 10.004000ms, string: 357:ABCDEFG
UDP output: 10.120000ms, string: 358:ABCDEFG

on board:

UDP intput: 20.000000ms, string: 353:ABCDEFG, 
UDP intput: 20.002000ms, string: 354:ABCDEFG, 
UDP intput: 19.998000ms, string: 355:ABCDEFG, 
UDP intput: 20.051000ms, string: 356:ABCDEFG, 
UDP intput: 19.953000ms, string: 357:ABCDEFG, 
UDP intput: 19.996000ms, string: 358:ABCDEFG, 

I think this interval should be defined by something in the linux system. How can I fix it?

Btw, iptables of the board shows (EXT = eth0):

 iptables -A INPUT -i $EXT -p udp -d 0/0 --dport 1024:65535 -j ACCEPT

Thank you very much.

Answer 1

thank you for your kind replies. I found out the problem is due to a usleep(10) and the end of while. On the board, usleep(10) takes exactly 20ms, while on another PC it takes 0.1ms.