mmap：無法分配內存

Question

我有一個C程序，用C計算頁面錯誤服務時間。對於這個程序，我有2個大文件（每個小於3GB - 幾乎是RAM的大小）

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "rdstc.h"
#include "config.h"

#define KB 1024
#define MB 1024 * KB
#define GB 1024 * MB
#define SIZE_OF_MEMORY 1 * GB   // Main memory size

#define handle_error(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)

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

    int fd1, fd2;
    char *addr1, *addr2, c;
    int i, j;
    long long unsigned int s_t, e_t, t=0;

    if (argc != 3){
        printf("usage: a.out <file1> <file2> \n");
        exit(EXIT_FAILURE);
    }

    if ((fd1 = open(argv[1], O_RDONLY)) == -1){
        handle_error("open");
    }

    if ((fd2 = open(argv[2], O_RDONLY)) == -1){
        handle_error("open");
    }

    posix_fadvise(fd1, 0, 0, POSIX_FADV_RANDOM);
    posix_fadvise(fd2, 0, 0, POSIX_FADV_RANDOM);

    addr1 = (char *) mmap(0, SIZE_OF_MEMORY, PROT_READ, MAP_PRIVATE | MAP_POPULATE, fd1, 0);
    if (addr1 == MAP_FAILED){
        handle_error("mmap");
    }
    addr2 = (char *) mmap(0, SIZE_OF_MEMORY, PROT_READ, MAP_PRIVATE | MAP_POPULATE, fd2, 0);
    if (addr2 == MAP_FAILED){
        handle_error("mmap");
    }

    madvise(addr1, 0, MADV_RANDOM);
    madvise(addr2, 0, MADV_RANDOM);

    j = 32;     // default read ahead size if 256 blocks (assuming each block is of 512 bytes)
    for(i = 0; i < ITERATIONS; i++){
        s_t = rdtsc();
            c = addr1[i + j*4*KB];      // read at multiple of page size, so every read causes a page fault
            j *= 2;
        e_t = rdtsc();
        t += (e_t - s_t);
    }
    printf("Time required to service a page faut is %f \n", (t/ITERATIONS)/CPU_FREQ);

    munmap(addr1, SIZE_OF_MEMORY);
    munmap(addr2, SIZE_OF_MEMORY);

    return 0;
}

我得到以下編譯器警告：

lmelvix@Melvix:~/projects/mem$ gcc mem1_4.c -lm
mem1_4.c: In function ‘main’:
mem1_4.c:11:17: warning: integer overflow in expression [-Woverflow]
 #define MB 1024 * KB
                ^
mem1_4.c:12:19: note: in expansion of macro ‘MB’
 #define GB 1024 * MB
                ^
mem1_4.c:13:28: note: in expansion of macro ‘GB’
 #define SIZE_OF_MEMORY 2 * GB   // Main memory size
                            ^
mem1_4.c:40:30: note: in expansion of macro ‘SIZE_OF_MEMORY’
    addr1 = (char *) mmap(0, SIZE_OF_MEMORY, PROT_READ, MAP_PRIVATE | MAP_POPULATE, fd1, 0);
                            ^
mem1_4.c:11:17: warning: integer overflow in expression [-Woverflow]
 #define MB 1024 * KB
                ^
mem1_4.c:12:19: note: in expansion of macro ‘MB’
 #define GB 1024 * MB
                ^
mem1_4.c:13:28: note: in expansion of macro ‘GB’
 #define SIZE_OF_MEMORY 2 * GB   // Main memory size
                            ^
mem1_4.c:44:30: note: in expansion of macro ‘SIZE_OF_MEMORY’
    addr2 = (char *) mmap(0, SIZE_OF_MEMORY, PROT_READ, MAP_PRIVATE | MAP_POPULATE, fd2, 0);
                            ^
mem1_4.c:11:17: warning: integer overflow in expression [-Woverflow]
 #define MB 1024 * KB
                ^
mem1_4.c:12:19: note: in expansion of macro ‘MB’
 #define GB 1024 * MB
                ^
mem1_4.c:13:28: note: in expansion of macro ‘GB’
 #define SIZE_OF_MEMORY 2 * GB   // Main memory size
                            ^
mem1_4.c:62:19: note: in expansion of macro ‘SIZE_OF_MEMORY’
    munmap(addr1, SIZE_OF_MEMORY);
                ^
mem1_4.c:11:17: warning: integer overflow in expression [-Woverflow]
 #define MB 1024 * KB
                ^
mem1_4.c:12:19: note: in expansion of macro ‘MB’
 #define GB 1024 * MB
                ^
mem1_4.c:13:28: note: in expansion of macro ‘GB’
 #define SIZE_OF_MEMORY 2 * GB   // Main memory size
                            ^
mem1_4.c:63:19: note: in expansion of macro ‘SIZE_OF_MEMORY’
    munmap(addr2, SIZE_OF_MEMORY);
                ^

當我用命令運行它時，我得到了錯誤

./a.out file1.txt file2.txt
mmap: Cannot allocate memory

代碼做了什么？ 我們使用標志映射這兩個文件

MAP_PRIVATE（以便其他進程不應訪問此文件）和MAP_POPULATE（這樣

當我們調用mmap（）完成文件映射到內存中）以及PROT_READ保護標志。

首先我們映射file1，因為我們使用MAP_POPULATE完成RAM由對應於該文件的數據填充。 在此之后，我們使用相同的標志映射file2，因此現在我們將file2完全映射到RAM中。 因此，訪問file1的數據將導致頁面錯誤，因為file2占用了所有可用的RAM。 我們還調用帶有MADV_RANDOM標志的madvise（）系統調用，建議內核不要對這兩個文件進行頁面預讀。 所以現在一旦這個初始設置完成，file2占用了所有可用的RAM，我們隨機訪問對應於file1的數據（以避免由內核執行的預讀讀取優化的任何影響，並且還避免從L3緩存讀取）因為，RAM充滿了對應於file2的數據，每次訪問對應文件的數據都會導致頁面錯誤。 我們在循環中的映射區域上執行10個隨機讀數，並測量此操作所需的平均時間。

Answer 1

看看你得到的編譯器警告。 你在這里有一個整數溢出： #define SIZE_OF_MEMORY 2 * GB 。 等於2^31 == 0b1000 ... 0 ，對於signed int等於INT_MIN。 這就是mmap失敗的原因。

您應該在定義中使用無符號文字 ：

#define KB (1024u)
#define MB (1024u * KB)
#define GB (1024u * MB)
#define SIZE_OF_MEMORY (2u * GB)