Question

我看到一些奇怪的事情，其中一对AF_UNIX套接字是通过以下呼叫创建的：

 socketpair(AF_UNIX, SOCK_STREAM, 0, 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正好是122K（124928字节），而不是/ proc / sys / net中的任何内容（例如设置为128K的wmem_default）。有谁知道这个奇怪的缓冲区大小的原因？

其次，通过套接字写入小消息（8字节）。我只能在写块之前写入423个，这只是8 * 423 = 3384个字节，另一个是奇数。这些消息就好像它们每个占用了295 +一个小字节。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开销成本：

#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, SOCK_STREAM, 0, sfd) == -1) {
        perror("error");
    }


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

    printf("Data Size: %zd\n", 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:     %zd\n", wrote);

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

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

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

int main() {
    int sfd[2];
    socketpair(AF_UNIX, SOCK_STREAM, 0, sfd);

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

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

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

给出了：

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

使用管道确实有很多开销：

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

Answer 1

查看socket（7）手册页。有一节的内容如下：

SO_SNDBUF 以字节为单位设置或获取最大套接字发送缓冲区。内核将此值加倍（当使用setsockopt（2）设置时，允许空间进行簿记开销） getsockopt（2）返回doubled值。默认值由。设置 / proc / sys / net / core / wmem_default文件和最大允许值由 / proc / sys / net / core / wmem_max文件。此选项的最小（加倍）值为 2048。

所以看来，开销只是为了保存内核的簿记信息。

Answer 2

您是否查看了net.unix.max_dgram_qlen sysctl的价值？

内核对飞行中AF_UNIX数据报的最大数量施加限制。在我的系统上，限制实际上非常低：只有10。

AF_UNIX套接字开销？

2 个答案: