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MEDIUM: dns: implement synchronous send

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In dns_send_query(), there's no point in first waking up the FD, to get
called back by the poller to send the request and sleep. Instead let's
simply send the request as soon as it's known and only subscribe to the
poller when the socket buffers are full and it's required to poll (i.e.
almost never).

This significantly reduces the number of calls to the poller. A large
config sees the number of epoll_ctl() calls reduced from 577 to 7 over
10 seconds, the number of recvfrom() from 1533 to 582 and the number of
sendto() from 369 to 162.

It also has the extra benefit of building each requests only once per
resolution and sending it to multiple resolvers instead of rebuilding
it for each and every resolver.

This will reduce the risk of seeing situations similar to bug #416 in
the future.
This commit is contained in:
Willy Tarreau 2019-12-20 11:18:54 +01:00
parent e5891ca6c1
commit 0eae6323bf
1 changed files with 40 additions and 7 deletions
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47
src/dns.c
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@ -272,22 +272,49 @@ static int dns_send_query(struct dns_resolution *resolution)
{
struct dns_resolvers *resolvers = resolution->resolvers;
struct dns_nameserver *ns;
int len;
/* Update resolution */
resolution->nb_queries = 0;
resolution->nb_responses = 0;
resolution->last_query = now_ms;
len = dns_build_query(resolution->query_id, resolution->query_type,
resolvers->accepted_payload_size,
resolution->hostname_dn, resolution->hostname_dn_len,
trash.area, trash.size);
list_for_each_entry(ns, &resolvers->nameservers, list) {
int fd = ns->dgram->t.sock.fd;
int ret;
if (fd == -1) {
if (dns_connect_namesaver(ns) == -1)
continue;
fd = ns->dgram->t.sock.fd;
resolvers->nb_nameservers++;
}
fd_want_send(fd);
}
/* Update resolution */
resolution->nb_queries = 0;
resolution->nb_responses = 0;
resolution->last_query = now_ms;
if (len < 0)
goto snd_error;
ret = send(fd, trash.area, len, 0);
if (ret == len) {
ns->counters.sent++;
resolution->nb_queries++;
continue;
}
if (ret == -1 && errno == EAGAIN) {
/* retry once the socket is ready */
fd_cant_send(fd);
continue;
}
snd_error:
ns->counters.snd_error++;
resolution->nb_queries++;
}
/* Push the resolution at the end of the active list */
LIST_DEL(&resolution->list);
@ -1751,8 +1778,14 @@ static void dns_resolve_send(struct dgram_conn *dgram)
goto snd_error;
ret = send(fd, trash.area, len, 0);
if (ret != len)
if (ret != len) {
if (ret == -1 && errno == EAGAIN) {
/* retry once the socket is ready */
fd_cant_send(fd);
continue;
}
goto snd_error;
}
ns->counters.sent++;
res->nb_queries++;