Speed up DNS by avoiding a system call if possible

diff --git a/ChangeLog b/ChangeLog
index e8d7809..fd46252 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2012-03-30  Ulrich Drepper  <drepper@gmail.com>
 
+       * resolv/res_send.c (send_dg): Use sendmmsg if we have to write two
+       requests to save a system call.
+
        * sysdeps/unix/sysv/linux/bits/socket.h (struct mmsghdr): Fix up
        comments for sendmmsg.
 

diff --git a/resolv/res_send.c b/resolv/res_send.c
index a001c1e..0b5bc91 100644 (file)
--- a/resolv/res_send.c
+++ b/resolv/res_send.c
@@ -1013,8 +1013,9 @@ send_dg(res_state statp,
                seconds /= statp->nscount;
        if (seconds <= 0)
                seconds = 1;
-       bool single_request = (statp->options & RES_SNGLKUP) != 0;
        bool single_request_reopen = (statp->options & RES_SNGLKUPREOP) != 0;
+       bool single_request = (((statp->options & RES_SNGLKUP) != 0)
+                              | single_request_reopen);
        int save_gotsomewhere = *gotsomewhere;
 
        int retval;
@@ -1100,24 +1101,89 @@ send_dg(res_state statp,
        }
        __set_errno (0);
        if (pfd[0].revents & POLLOUT) {
-               ssize_t sr;
-               if (nwritten != 0)
-                 sr = send (pfd[0].fd, buf2, buflen2, MSG_NOSIGNAL);
-               else
-                 sr = send (pfd[0].fd, buf, buflen, MSG_NOSIGNAL);
+#ifndef __ASSUME_SENDMMSG
+               static int have_sendmmsg;
+#else
+# define have_sendmmsg 1
+#endif
+               if (have_sendmmsg >= 0 && nwritten == 0 && buf2 != NULL
+                   && !single_request)
+                 {
+                   struct iovec iov[2];
+                   struct mmsghdr reqs[2];
+                   reqs[0].msg_hdr.msg_name = NULL;
+                   reqs[0].msg_hdr.msg_namelen = 0;
+                   reqs[0].msg_hdr.msg_iov = &iov[0];
+                   reqs[0].msg_hdr.msg_iovlen = 1;
+                   iov[0].iov_base = (void *) buf;
+                   iov[0].iov_len = buflen;
+                   reqs[0].msg_hdr.msg_control = NULL;
+                   reqs[0].msg_hdr.msg_controllen = 0;
+
+                   reqs[1].msg_hdr.msg_name = NULL;
+                   reqs[1].msg_hdr.msg_namelen = 0;
+                   reqs[1].msg_hdr.msg_iov = &iov[1];
+                   reqs[1].msg_hdr.msg_iovlen = 1;
+                   iov[1].iov_base = (void *) buf2;
+                   iov[1].iov_len = buflen2;
+                   reqs[1].msg_hdr.msg_control = NULL;
+                   reqs[1].msg_hdr.msg_controllen = 0;
+
+                   int ndg = sendmmsg (pfd[0].fd, reqs, 2, MSG_NOSIGNAL);
+                   if (__builtin_expect (ndg == 2, 1))
+                     {
+                       assert (reqs[0].msg_len == buflen);
+                       assert (reqs[1].msg_len == buflen2);
 
-               if (sr != buflen) {
-                       if (errno == EINTR || errno == EAGAIN)
-                               goto recompute_resend;
-                       Perror(statp, stderr, "send", errno);
+                       pfd[0].events = POLLIN;
+                       nwritten += 2;
+                     }
+                   else if (ndg == 1 && reqs[0].msg_len == buflen)
+                     goto just_one;
+                   else if (errno == EINTR || errno == EAGAIN)
+                     goto recompute_resend;
+                   else
+                     {
+#ifndef __ASSUME_SENDMMSG
+                       if (have_sendmmsg == 0)
+                         {
+                           if (errno == ENOSYS)
+                             {
+                               have_sendmmsg = -1;
+                               goto try_send;
+                             }
+                           have_sendmmsg = 1;
+                         }
+#endif
+
+                       Perror(statp, stderr, "sendmmsg", errno);
                        goto err_out;
-               }
-               if (nwritten != 0 || buf2 == NULL
-                   || single_request || single_request_reopen)
-                 pfd[0].events = POLLIN;
+                     }
+                 }
                else
-                 pfd[0].events = POLLIN | POLLOUT;
-               ++nwritten;
+                 {
+                   ssize_t sr;
+#ifndef __ASSUME_SENDMMSG
+                 try_send:
+#endif
+                   if (nwritten != 0)
+                     sr = send (pfd[0].fd, buf2, buflen2, MSG_NOSIGNAL);
+                   else
+                     sr = send (pfd[0].fd, buf, buflen, MSG_NOSIGNAL);
+
+                   if (sr != buflen) {
+                     if (errno == EINTR || errno == EAGAIN)
+                       goto recompute_resend;
+                     Perror(statp, stderr, "send", errno);
+                     goto err_out;
+                   }
+                 just_one:
+                   if (nwritten != 0 || buf2 == NULL || single_request)
+                     pfd[0].events = POLLIN;
+                   else
+                     pfd[0].events = POLLIN | POLLOUT;
+                   ++nwritten;
+                 }
                goto wait;
        } else if (pfd[0].revents & POLLIN) {
                int *thisanssizp;
@@ -1327,7 +1393,7 @@ send_dg(res_state statp,
                        recvresp2 = 1;
                /* Repeat waiting if we have a second answer to arrive.  */
                if ((recvresp1 & recvresp2) == 0) {
-                       if (single_request || single_request_reopen) {
+                       if (single_request) {
                                pfd[0].events = POLLOUT;
                                if (single_request_reopen) {
                                        __res_iclose (statp, false);