Subject: Re: Doubts about listen backlog and tcp_max_syn_backlog

From: Leandro Lucarella <[email-masked]>
Date: Tue, 22 Jan 2013 17:59:29 +0100

On Tue, Jan 22, 2013 at 08:45:42AM -0800, Eric Dumazet wrote:
> 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.
[snip]
> > 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.
[snip]
> 
> What particular problem do you have ?

What I'm seeing are clients taking either useconds to connect, or 3
seconds, which suggest SYNs are getting lost, but the network doesn't
seem to be the problem. I'm still investigating this, so unfortunately
I'm not really sure.

> 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.

So, the queue is really a hash table, then? So using any (2^n)-1 would
be a bad idea because when the backlog is next to full, the hash table
will be really slow? Is that why the + 1 is there? Is assuming everyone
will use a power of 2 an thus having a load factor of 0.5 at most?

-- 
Leandro Lucarella
Senior R&D Developer
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