6.828/2019 Exercise 1.6.1: pingpong

Recently I’ve started following MIT 6.828 Operating System Engineering. Without further ado, this is my solution to its first exercise, 1.6.1:

Write a program that uses UNIX system calls to “ping-pong” a byte between two processes over a pair of pipes, one for each direction. Measure the program’s performance, in exchanges per second (19).

Russ Cox, Frans Kaashoek, Robert Morris. xv6: a simple, Unix-like teaching operating system. October 27, 2019

//ping-pong.c
#include<sys/types.h>
#include<sys/wait.h>
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<time.h>

int main() {
    int p1[2], p2[2];
    pipe(p1);
    pipe(p2);

    if (fork() != 0) {
        //parent
        close(p1[0]);
        close(p2[1]);
        char ping[1] = "B";

        clock_t start, end;
        double rtt;
	
        start = clock();
        for (int i=0; i<100000; i++) {
            write(p1[1], ping, 1);
            read(p2[0], ping, 1);
        }
        end = clock();

        wait(0);
        close(p1[1]);
        close(p2[0]);

        rtt = (((double)(end - start)) / CLOCKS_PER_SEC) / 100000;
        printf("average RTT: %f ms\n", (rtt * 1000));
        printf("exchanges per second: %ld times\n", (unsigned long)(1 / rtt));
    }
    else {
        //child
        close(p1[1]);
        close(p2[0]);
        char pong[1];

        for (int j=0; j<100000; j++) {
            read(p1[0], pong, 1);
            write(p2[1], pong, 1);
        }

        close(p1[0]);
        close(p2[1]);
        exit(0);
    }

    return 0;
}

Well, printf() is not a syscall, but, ugh…:-)

Anyway, let’s run it:

peilin@PWN:~/6.828/1.6.1-ping_pong$ gcc -o ping_pong ping_pong.c
peilin@PWN:~/6.828/1.6.1-ping_pong$ ./ping_pong
average RTT: 0.001964 ms
exchanges per second: 509053 times

More than 500,000 times on my sluggish VM. Pretty impressive to me.

Another trivia: When initializing pong, char *pong; doesn’t work, since this declaration is not actually allocating any memory for the buffer, and I can’t read() to an nonexistent buffer. One may want to do char pong[1]; instead. Maybe I’ve written too much Python. Ugh.

So, that’s it! Looking forward to learn a lot from this course.