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POAF_UNIX socket overhead?
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copied!<p>I'm seeing a couple strange things with a pair of AF_UNIX sockets created by a call such as:</p> <pre><code> socketpair(AF_UNIX, SOCK_STREAM, 0, sfd); </code></pre> <p>Where sfd is an int[2] array for the file descriptors.</p> <p>First, the default buffer size seems to be exactly 122K (124928 bytes), rather than anything from /proc/sys/net (such as wmem_default which is set to 128K). Does anyone know the cause of this strange buffer size?</p> <p>Second, when writing small messages through the socket (8 bytes). I can only write 423 of them before the write blocks, which is only 8*423 = 3384 bytes, another odd size. The messages are acting as though they're taking up 295 + a little bytes each. What's the source of this overhead?</p> <p>Running on RHEL6 (2.6.32, 64-bit)</p> <p>I wrote a program to try different sizes of data to compare overhead costs:</p> <pre><code>#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); } } </code></pre> <p>Which gives:</p> <pre><code>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 </code></pre> <p>Versus using a pipe there's definitely a lot of overhead:</p> <pre><code>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 </code></pre>

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