Linux – Default Unix Socket Buffer Size Values

I believe that the idea of the socket being unavailable to a program is to allow any TCP data segments still in transit to arrive, and get discarded by the kernel. That is, it's possible for an application to call close(2) on a socket, but routing delays or mishaps to control packets or what have you can allow the other side of a TCP connection to send data for a while. The application has indicated it no longer wants to deal with TCP data segments, so the kernel should just discard them as they come in.

I hacked out a little program in C that you can compile and use to see how long the timeout is:

#include <stdio.h>        /* fprintf() */
#include <string.h>       /* strerror() */
#include <errno.h>        /* errno */
#include <stdlib.h>       /* strtol() */
#include <signal.h>       /* signal() */
#include <sys/time.h>     /* struct timeval */
#include <unistd.h>       /* read(), write(), close(), gettimeofday() */
#include <sys/types.h>    /* socket() */
#include <sys/socket.h>   /* socket-related stuff */
#include <netinet/in.h>
#include <arpa/inet.h>    /* inet_ntoa() */
float elapsed_time(struct timeval before, struct timeval after);
int
main(int ac, char **av)
{
        int opt;
        int listen_fd = -1;
        unsigned short port = 0;
        struct sockaddr_in  serv_addr;
        struct timeval before_bind;
        struct timeval after_bind;

        while (-1 != (opt = getopt(ac, av, "p:"))) {
                switch (opt) {
                case 'p':
                        port = (unsigned short)atoi(optarg);
                        break;
                }
        }

        if (0 == port) {
                fprintf(stderr, "Need a port to listen on\n");
                return 2;
        }

        if (0 > (listen_fd = socket(AF_INET, SOCK_STREAM, 0))) {
                fprintf(stderr, "Opening socket: %s\n", strerror(errno));
                return 1;
        }

        memset(&serv_addr, '\0', sizeof(serv_addr));
        serv_addr.sin_family      = AF_INET;
        serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
        serv_addr.sin_port        = htons(port);

        gettimeofday(&before_bind, NULL);
        while (0 > bind(listen_fd, (struct sockaddr *)&serv_addr, sizeof(serv_addr))) {
                fprintf(stderr, "binding socket to port %d: %s\n",
                        ntohs(serv_addr.sin_port),
                        strerror(errno));

                sleep(1);
        }
        gettimeofday(&after_bind, NULL);
        printf("bind took %.5f seconds\n", elapsed_time(before_bind, after_bind));

        printf("# Listening on port %d\n", ntohs(serv_addr.sin_port));
        if (0 > listen(listen_fd, 100)) {
                fprintf(stderr, "listen() on fd %d: %s\n",
                        listen_fd,
                        strerror(errno));
                return 1;
        }

        {
                struct sockaddr_in  cli_addr;
                struct timeval before;
                int newfd;
                socklen_t clilen;

                clilen = sizeof(cli_addr);

                if (0 > (newfd = accept(listen_fd, (struct sockaddr *)&cli_addr, &clilen))) {
                        fprintf(stderr, "accept() on fd %d: %s\n", listen_fd, strerror(errno));
                        exit(2);
                }
                gettimeofday(&before, NULL);
                printf("At %ld.%06ld\tconnected to: %s\n",
                        before.tv_sec, before.tv_usec,
                        inet_ntoa(cli_addr.sin_addr)
                );
                fflush(stdout);

                while (close(newfd) == EINTR) ;
        }

        if (0 > close(listen_fd))
                fprintf(stderr, "Closing socket: %s\n", strerror(errno));

        return 0;
}
float
elapsed_time(struct timeval before, struct timeval after)
{
        float r = 0.0;

        if (before.tv_usec > after.tv_usec) {
                after.tv_usec += 1000000;
                --after.tv_sec;
        }

        r = (float)(after.tv_sec - before.tv_sec)
                + (1.0E-6)*(float)(after.tv_usec - before.tv_usec);

        return r;
}

I tried this program on 3 different machines, and I get a variable time, between 55 and 59 seconds, when the kernel refuses to allow a non-root user to reopen a socket. I compiled the above code to an executable named "opener", and ran it like this:

./opener -p 7896; ./opener -p 7896

I opened another window and did this:

telnet otherhost 7896

That causes the first instance of "opener" to accept a connection, then close it. The second instance of "opener" tries to bind(2) to the TCP port 7896 every second. "opener" reports 55 to 59 seconds of delay.

Googling around, I find that people recommend doing this:

echo 30 > /proc/sys/net/ipv4/tcp_fin_timeout

to reduce that interval. It didn't work for me. Of the 4 linux machines I had access to, two had 30 and two had 60. I also set that value as low as 10. No difference to the "opener" program.

Doing this:

echo 1 > /proc/sys/net/ipv4/tcp_tw_recycle

did change things. The second "opener" only took about 3 seconds to get its new socket.

The problem ended up being using rate-control with the default configured sockets was creating a situation where the internal TCP buffer size was automatically-adjusting to larger and larger size due to the slow read out times. (the default max size is like 6MB) When the size was automatically growing, the TCP compact process would start to churn like crazy and thus eating into all the softirq. The way to fix this is to set an explicit TCP buffer size when using rate control to prevent this aberrant behavior.