AF_UNIX socket开销？

socketpair(AF_UNIX,SOCK_STREAM,sfd);

其中sfd是文件描述符的int [2]数组。

首先，默认缓冲区大小似乎是122K(124928字节)，而不是/ proc / sys / net(如wmem_default，设置为128K)的任何东西。有没有人知道这个奇怪的缓冲区大小的原因？

第二，当通过套接字写入小消息(8字节)时。我只能在写入块之前写入其中的423个，这只有8 * 423 = 3384字节，另一个奇数的大小。这些消息的作用就好像它们每个占用了一点点字节一样。这个开销的来源是什么？

在RHEL6上运行(2.6.32,64位)

我写了一个程序来尝试不同大小的数据来比较间接成本：

#include <errno.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

#define DATA_SIZE 4

void run(size_t size) {
    int sfd[2];
    if (socketpair(AF_UNIX,sfd) == -1) {
        perror("error");
    }


    int sndbuf,sbsize = sizeof(sndbuf);
    getsockopt(sfd[0],SOL_SOCKET,SO_SNDBUF,&sndbuf,(socklen_t*)&sbsize);

    printf("Data Size: %zdn",size);
    char buff[size];   
    size_t wrote=0;
    for (size_t ii=0; ii < 32768; ii++) {
        if ((send(sfd[0],buff,size,MSG_DONTWAIT) == -1) && (errno == EAGAIN)) {
            wrote = ii;
            break;
        }
    }

    printf("Wrote:     %zdn",wrote);

    if (wrote != 0) { 
        int bpm = sndbuf/wrote;
        int oh  = bpm - size;

        printf("Bytes/msg: %in",bpm);
        printf("Overhead:  %in",oh);
        printf("n");
    }

    close(sfd[0]); close(sfd[1]);
}

int main() {
    int sfd[2];
    socketpair(AF_UNIX,sfd);

    int sndbuf,(socklen_t*)&sbsize);

    printf("Buffer Size: %inn",sndbuf);
    close(sfd[0]); close(sfd[1]);

    for (size_t ii=4; ii <= 4096; ii *= 2) {
        run(ii);
    }
}

这使：

Buffer Size: 124928

Data Size: 4
Wrote:     423
Bytes/msg: 295
Overhead:  291

Data Size: 8
Wrote:     423
Bytes/msg: 295
Overhead:  287

Data Size: 16
Wrote:     423
Bytes/msg: 295
Overhead:  279

Data Size: 32
Wrote:     423
Bytes/msg: 295
Overhead:  263

Data Size: 64
Wrote:     423
Bytes/msg: 295
Overhead:  231

Data Size: 128
Wrote:     348
Bytes/msg: 358
Overhead:  230

Data Size: 256
Wrote:     256
Bytes/msg: 488
Overhead:  232

Data Size: 512
Wrote:     168
Bytes/msg: 743
Overhead:  231

Data Size: 1024
Wrote:     100
Bytes/msg: 1249
Overhead:  225

Data Size: 2048
Wrote:     55
Bytes/msg: 2271
Overhead:  223

Data Size: 4096
Wrote:     29
Bytes/msg: 4307
Overhead:  211

与管道相比，绝对有很多开销：

Data Size: 4
Wrote:     16384
Bytes/msg: 4
Overhead:  0

Data Size: 8
Wrote:     8192
Bytes/msg: 8
Overhead:  0

Data Size: 16
Wrote:     4096
Bytes/msg: 16
Overhead:  0

Data Size: 32
Wrote:     2048
Bytes/msg: 32
Overhead:  0

Data Size: 64
Wrote:     1024
Bytes/msg: 64
Overhead:  0

Data Size: 128
Wrote:     512
Bytes/msg: 128
Overhead:  0

Data Size: 256
Wrote:     256
Bytes/msg: 256
Overhead:  0

Data Size: 512
Wrote:     128
Bytes/msg: 512
Overhead:  0

Data Size: 1024
Wrote:     64
Bytes/msg: 1024
Overhead:  0

Data Size: 2048
Wrote:     32
Bytes/msg: 2048
Overhead:  0

Data Size: 4096
Wrote:     16
Bytes/msg: 4096
Overhead:  0