On Tue, 2013-01-22 at 17:10 +0100, Leandro Lucarella wrote: > Hi, I'm having some problems with missing SYNs in a server with a high > rate of incoming connections and, even when far from understanding the > kernel, I ended up looking at the kernel's source to try to understand > better what's going on, because some stuff doesn't make a lot of sense > to me. > > The path I followed is this (line numbers for Linux 3.7): > net/socket.c[3] > SYSCALL_DEFINE2(listen, int, fd, int, backlog) > backlog is truncated to sysctl_somaxconn and > sock->ops->listen(sock, backlog) is called, which I guess it > calls to inet_listen(). > > net/ipv4/af_inet.c[4] > int inet_listen(struct socket *sock, int backlog) > the backlog is assigned to sk->sk_max_ack_backlog and > inet_csk_listen_start(sk, backlog) is called (if the socket > wans't already in TCP_LISTEN state) > > net/ipv4/inet_connection_sock.c[5] > int inet_csk_listen_start(struct sock *sk, const int nr_table_entries) > reqsk_queue_alloc(&icsk->icsk_accept_queue, nr_table_entries) is > called, which I guess it creates the actual queue > > net/core/request_sock.c[6] > int reqsk_queue_alloc(struct request_sock_queue *queue, > unsigned int nr_table_entries) > nr_table_entries is first adjusted to satisfy: > 8 <= nr_table_entries <= sysctl_max_syn_backlog > and then incremented by one and rounded up to the next power of > 2. > > So here are a couple of questions: > > 1. What's the relation between the socket backlog and the queue created > by reqsk_queue_alloc()? Because the backlog is only adjusted not to > be grater than sysctl_somaxconn, but the queue size can be quite > different. > 2. The comment just above the definition of reqsk_queue_alloc() about > sysctl_max_syn_backlog says "Maximum number of SYN_RECV sockets in > queue per LISTEN socket.". But then nr_table_entries is not only > rounded up to the next power of 2, is incremented by one before that, > so a backlog of, for example, 128, would end up with 256 table > entries even if sysctl_max_syn_backlog is 128. > 3. Why is there a nr_table_entries + 1 at all in there? Looking at the > commit that introduced this[1] I can't find any explanation and I've > read some big projects are using backlogs of 511 because of this[2]. > (which BTW, ff the queue is really a hash table, looks like an awful > idea). > 4. I found some places sk->sk_ack_backlog is checked against > sk->sk_max_ack_backlog to see if new requests should be dropped, but > I also saw checks like inet_csk_reqsk_queue_young(sk) > 1 or > inet_csk_reqsk_queue_is_full(sk), so I guess the queue is used too. > > > Thanks a lot. > > [1] > http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=commitdiff;h=72a3effaf633bcae9034b7e176bdbd78d64a71db > [2] > http://blog.dubbelboer.com/2012/04/09/syn-cookies.html#a_reasonably_backlog_size > [3] > http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob;f=net/socket.c;h=2ca51c719ef984cdadef749008456cf7bd5e1ae4;hb=HEAD#l1544 > [4] > http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob;f=net/ipv4/af_inet.c;h=24b384b7903ea7a59a11e7a4cbf06db996498924;hb=HEAD#l192 > [5] > http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob;f=net/ipv4/inet_connection_sock.c;h=d0670f00d5243f95bec4536f60edf32fa2ded850;hb=HEAD#l729 > [6] > http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob;f=net/core/request_sock.c;h=c31d9e8668c30346894adbf3be55eed4beeb1258;hb=HEAD#l23 >
What particular problem do you have ? A serious rewrite of LISTEN code is needed, because the current implementation doesn't scale : The SYNACK retransmits are done by a single timer wheel, holding the socket lock for too long. So increasing the backlog to 2^16 or 2^17 is not really an option. Hash table are nice, but if we have to scan them, holding a single lock, they are not so nice. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
