AF_UNIX套接字开销？
AF_UNIX套接字开销？
提问于 2012-06-05 23:04:07
通过调用创建的一对AF_UNIX套接字，我看到了一些奇怪的事情，例如：
socketpair(AF_UNIX, SOCK_STREAM, 0, sfd); 其中sfd是文件描述符的int2数组。
首先，默认的缓冲区大小似乎恰好是122K (124928字节)，而不是/proc/sys/net中的任何值(比如设置为128K的wmem_default )。有人知道这种奇怪的缓冲区大小的原因吗？
其次，当通过套接字(8字节)写入小消息时。我只能在写块之前写入其中的423个，也就是8*423 = 3384个字节，这是另一个奇数。这些消息表现得好像每条消息占用了295 +一点字节。这种开销的来源是什么？
在RHEL6 (2.6.32，64位)上运行
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);
}
}这就给出了：
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与使用管道相比，这肯定有很大的开销：
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回答 1
Stack Overflow用户
发布于 2012-06-07 00:03:25
您看过net.unix.max_dgram_qlen sysctl的价值了吗？
内核对正在传输的AF_UNIX数据报的最大数量施加了限制。在我的系统上，这个限制实际上非常低:只有10个。
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/10899814
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