錯誤消息： Connection to database "" failed: could not connect to server: Resource temporarily unavailable Is the server running locally and accepting connections on Unix domain socket "/tmp/.s.PGSQL.5432"?
[code] From:Andres Freund To:pgsql-hackers(at)postgresql(dot)orgSubject:PQConnectPoll, connect(2), EWOULDBLOCK and somaxconnDate:2013-06-17 14:16:22Message-ID:20130617141622.GH5875@alap2.anarazel.de (view raw, whole thread or download thread mbox)Thread: 2013-06-17 14:16:22 from Andres Freund 2013-06-26 11:22:58 from Andres Freund 2013-06-26 16:07:54 from Tom Lane 2013-06-26 18:12:00 from Andres Freund 2013-06-27 00:07:40 from Tom Lane 2013-06-27 06:17:57 from Andres Freund 2013-06-27 13:48:25 from Tom Lane 2013-06-27 16:42:47 from Tom Lane Lists:pgsql-hackersHi,
When postgres on linux receives connection on a high rate client connections sometimes error out with: could not send data to server: Transport endpoint is not connected could not send startup packet: Transport endpoint is not connected
To reproduce start something like on a server with sufficiently high max_connections: pgbench -h /tmp -p 5440 -T 10 -c 400 -j 400 -n -f /tmp/simplequery.sql
Now that's strange since that error should happen at connect(2) time, not when sending the startup packet. Some investigation led me to fe-secure.c's PQConnectPoll:
if (connect(conn->sock, addr_cur->ai_addr, addr_cur->ai_addrlen) < 0) { if (SOCK_ERRNO == EINPROGRESS || SOCK_ERRNO == EWOULDBLOCK || SOCK_ERRNO == EINTR || SOCK_ERRNO == 0) { /* * This is fine - we're in non-blocking mode, and * the connection is in progress. Tell caller to * wait for write-ready on socket. */ conn->status = CONNECTION_STARTED; return PGRES_POLLING_WRITING; } /* otherwise, trouble */ }
So, we're accepting EWOULDBLOCK as a valid return value for connect(2). Which it isn't. EAGAIN in contrast is on some BSDs and on linux. Unfortunately POSIX allows those two to share the same value...
My manpage tells me: EAGAIN No more free local ports or insufficient entries in the routing cache. For AF_INET see the description of /proc/sys/net/ipv4/ip_local_port_range ip(7) for information on how to increase the number of local ports.
So, the problem is that we took a failed connection as having been initially successfull but in progress.
Not accepting EWOULDBLOCK in the above if() results in: could not connect to server: Resource temporarily unavailable Is the server running locally and accepting connections on Unix domain socket "/tmp/.s.PGSQL.5440"?
which makes more sense.
Trivial patch attached.
Now, the question is why we cannot complete connections on unix sockets? Some code reading reading shows net/unix/af_unix.c:unix_stream_connect() shows: if (unix_recvq_full(other)) { err = -EAGAIN; if (!timeo) goto out_unlock; So, if we're in nonblocking mode - which we are - and the receive queue is full we return EAGAIN. The receive queue for unix sockets is defined as static inline int unix_recvq_full(struct sock const *sk) { return skb_queue_len(&sk->sk_receive_queue) > sk->sk_max_ack_backlog; } Where sk_max_ack_backlog is whatever has been passed to the listen(backlog) on the listening side.
Question: But postgres does listen(fd, MaxBackends * 2), how can that be a problem? Answer: If the backlog argument is greater than the value in /proc/sys/net/core/somaxconn, then it is silently truncated to that value; the default value in this file is 128. In kernels before 2.4.25, this limit was a hard coded value, SOMAXCONN, with the value 128.
Setting somaxconn to something higher indeed makes the problem go away.
I'd guess that pretty much the same holds true for tcp connections, although I didn't verify that which would explain some previous reports on the lists.
TLDR: Increase /proc/sys/net/core/somaxconn
Greetings,
Andres Freund
-- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
