我试着编写一个小应用程序来熟悉用户空间中的写时复制的概念.我已经阅读了 answer by MSalters,并认为只有当我开始使用mmap的文件来存储我的数据时它才会起作用.因为我不需要基于文件的持久性,所以我尝试用共享内存做同样的事情.首先我mmap’ed并初始化了一个shm fd,然后我用MAP_PRIVATE映射了第二个副本并再次从中读取.然而,只需从中读取就会导致内核复制整个内容,花费更多时间并占用内存的两倍.为什么不做COW？

这是我提出的用于说明行为的程序：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <assert.h>

static const size_t ARRAYSIZE = 1UL<<30;

void init(int* A)
{
    for (size_t i = 0; i < ARRAYSIZE; ++i)
        A[i] = i;
}

size_t agg(const int* A)
{
    size_t sum = 0;
    for (size_t i = 0; i < ARRAYSIZE; ++i)
        sum += A[i];
    return sum;
}

int main()
{
    assert(sizeof(int) == 4);
    shm_unlink("/cowtest");
    printf("ARRAYSIZE: %lu\n",ARRAYSIZE);
    int fd = shm_open("/cowtest",O_RDWR | O_CREAT | O_TRUNC,0);
    if (fd == -1)
    {
        perror("Error allocating fd\n");
        return 1;
    }
    if (ftruncate(fd,sizeof(int) * ARRAYSIZE) == -1)
    {
        perror("Error ftruncate\n");
        return 1;
    }
    /* Open shm */
    int* A= (int*)mmap(NULL,sizeof(int) * ARRAYSIZE,PROT_READ | PROT_WRITE,MAP_SHARED,fd,0);
    if (A == (int*)-1)
    {
        perror("Error mapping A to memory\n");
        return 1;
    }
    init(A);

    /* Create cow copy */
    int* Acopy = (int*)mmap(NULL,PROT_READ,MAP_PRIVATE,0);
    if (Acopy == (int*)-1)
    {
        printf("Error mapping copy from file\n");
        return 1;
    }

    /* Aggregate over A */
    size_t sumA = agg(A);
    size_t expected = (ARRAYSIZE * (ARRAYSIZE - 1)) >> 1;
    assert(expected == sumA);

    /* Aggregate over Acopy */
    size_t sumCopy = agg(Acopy);
    assert(expected == sumCopy);


    shm_unlink("/cowtest");
    printf("Enter to exit\n");
    getchar();
    return 0;
}

我用g -O3 -mtune = native -march = native -o shm-min shm-min.cpp -lrt编译它.

它创建的数组包含4GB的整数值.在终止程序之前,然后分配8GB的共享内存,在/ proc /< pid> / smaps中,您可以看到它在只读操作期间实际上执行了完整拷贝.我不知道为什么会那样做.这是内核错误吗？或者我错过了什么？

非常感谢任何见解.拉尔斯

编辑
这是Ubuntu 14.04(3.13.0-24)上/ proc /< pid> / smaps的相关内容：

7f3b9b4ae000-7f3c9b4ae000 r--p 00000000 00:14 168154                     /run/shm/cowtest (deleted)
Size:            4194304 kB
RSS:             4194304 kB
Pss:             2097152 kB
Shared_Clean:          0 kB
Shared_Dirty:    4194304 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:      4194304 kB
Anonymous:             0 kB
AnonHugePages:         0 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Locked:                0 kB
VmFlags: rd mr mw me sd
7f3c9b4ae000-7f3d9b4ae000 rw-s 00000000 00:14 168154                     /run/shm/cowtest (deleted)
Size:            4194304 kB
RSS:             4194304 kB
Pss:             2097152 kB
Shared_Clean:          0 kB
Shared_Dirty:    4194304 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:      4194304 kB
Anonymous:             0 kB
AnonHugePages:         0 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Locked:                0 kB
VmFlags: rd wr sh mr mw me ms sd

Size:            4194304 kB
RSS:             4194304 kB
Pss:             2097152 kB