madvise.c

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <inttypes.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <sys/time.h>
#include <fcntl.h>

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

        int opt;
        const char *fname = NULL;
        unsigned long npages = 0;
        bool sequential = false;

        while ((opt = getopt(argc, argv, "f:n:s")) != -1) {
                switch (opt) {
                        case 'f':
                                fname = optarg;
                                break;
                        case 'n':
                                npages = strtoul(optarg, NULL, 10);
                                break;
                        case 's':
                                sequential = true;
                }
        }

        if (fname == NULL || npages == 0) {
                fprintf(stderr, "Usage: %s -f <fname> -n npages [-s ]\n", argv[0]);
                exit(1);
        }

        int fd = open(fname, O_RDONLY);
        if (fd == -1) {
                perror(fname);
                exit(1);
        }

        size_t len = npages*4096;
        void *ptr = mmap(0, len, PROT_READ, MAP_SHARED, fd, 0);
        if (ptr == (void *)-1) {
                perror("mmap");
                exit(1);
        }

        if (sequential) {
                int ret = madvise(ptr, len, MADV_SEQUENTIAL);
                if (ret == -1) {
                        perror("mmap");
                        exit(1);
                }
        }

        struct timeval tv_start, tv_end;

        gettimeofday(&tv_start, NULL);
        unsigned long sum = 0;
        unsigned long *elems = ptr;
        for (unsigned long i=0; i < len / sizeof(*elems); i++) {
                sum += elems[i];
        }
        gettimeofday(&tv_end, NULL);

        size_t usecs = (tv_end.tv_sec - tv_start.tv_sec) * 1000000;
        usecs += tv_end.tv_usec;
        usecs -= tv_start.tv_usec;
        double secs = usecs / 1000000.0;
        double mib = (double)len / (1024*1024.0);
        printf("MADV_SEQUENTIAL=%u time:%zuus tput:%.1lf MiB/s (%.1lf MiB) sum=%lu\n", sequential, usecs, mib / secs, mib, sum);
}