Juliano

/*
 * Demonstrate madvice() usage on Linux.
 * See: http://stackoverflow.com/questions/3788977/linux-memory-mapped-files-reserve-lots-of-physical-memory
 *
 * Usage: madvtest FILENAME y/n
 * Use a very big file as first argument, and pass 'y' as second argument
 * to enable the call madvice(..., MADV_SEQUENTIAL);
 */

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/resource.h>

#define CHUNK 1048576
#define MIN(a, b) ((a) < (b) ? (a) : (b))

int main(int argc, char **argv) {
    int fd, i;
    char *data, *end, *p, buf[CHUNK];
    struct stat stbuf;
    struct rusage usage;

    assert(argc == 3);
    assert((fd = open(argv[1], O_RDONLY)) > 0);
    assert(fstat(fd, &stbuf) == 0);
    assert((data = mmap(NULL, stbuf.st_size, PROT_READ, MAP_SHARED, fd, 0)) != MAP_FAILED);
    end = data + stbuf.st_size;

    if (argv[2][0] == 'y')
        assert(madvise(data, stbuf.st_size, MADV_SEQUENTIAL) == 0);

    for (i = 0, p = data; p < end; p += CHUNK, i++) {
        memcpy(buf, p, MIN(CHUNK, end - p));    // read 1 MB from mmapped file
        if (i % 200 == 0) {
            assert(getrusage(RUSAGE_SELF, &usage) == 0);
            printf("%6d : %5ld MB\n", i, usage.ru_maxrss / 1024);
            fflush(stdout);
        }
    }

    munmap(data, stbuf.st_size);
    close(fd);
    return 0;
}