1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
#include <fcntl.h>
#include <string.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <time.h>
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>

// gcc -Wall -std=gnu99 -march=native -O2 this.c -ggdb3

int try_mmap(int fd, int count, int chunks) {
    srand(count);
    int running_total = 0;
    char *map = mmap(NULL, 10000000, PROT_READ, MAP_PRIVATE, fd, 0);
    if (map == MAP_FAILED) perror("mmap"), exit(1);
    for (int i = 0; i < count; i++) {
        unsigned off = (random() % 10000000) & 0xffff0000;
        unsigned buf[1024];
        memcpy(&buf, map + off, chunks * sizeof(unsigned));
        for(int j = 0; j < chunks; j++) {
            running_total += buf[j];
        }        
    }
    munmap(map, 10000000);
    return running_total;
}

int64_t try_read(int fd, int count, int chunks) {
    srand(count);
    int running_total = 0;
    for (int i = 0; i < count; i++) {
        unsigned off = (random() % 10000000) & 0xffff0000;
        unsigned buf[1024];
        lseek(fd, off, SEEK_SET);
        read(fd, buf, chunks * sizeof(unsigned));
        for(int j = 0; j < chunks; j++) {
            running_total += buf[j];
        }        
    }
    return running_total;
}

int main(int argc, char **argv) {
    int fd = open("test.raw", O_RDONLY); 
    if (fd < 0) perror("open"), exit(1);

    int count = 1000000;
    int mmap_total = try_mmap(fd, count, 5);
    int read_total = try_read(fd, count, 5);

    printf("mmap_total: %u\n", mmap_total);
    printf("read_total: %u\n", read_total);

    close(fd);

    return 0;
}