netdev - Re: [PATCH 1/2] socket: increase default maximum listen queue length

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Message-ID: <20110320235253.GD3038@nuttenaction>
Date:	Mon, 21 Mar 2011 00:52:53 +0100
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH 1/2] socket: increase default maximum listen queue
 length

* David Miller | 2011-03-20 16:09:06 [-0700]:

>Absolutely no context is provided for this number.
>
>What's the RTT?  How fast are the cpus?  etc.
>
>You must tell the whole story in order to justify these changes
>properly.

(you can skip the first paragraphs and read the last one ;)

The number is somewhat magically - like many other values. I greped
tglx/history.git but the comment (at that time tcp_ipv4.c) seems pre 2002 era.

Providing context is a little bit artificial: I can construct an scenario with
a RTT of 200ms and 1000 connection request per second and the table will
overflow. This can happen, sure. On the other hand there are scenarios with a
RTT of 20ms and 10 connection requests per second - no problem there.

Increasing the number _has_ one essential advantage: it is aligned on
sysctl_max_syn_backlog which in turn is determined by memory characteristics.

Without patch (sysctl not modified, BUT sysctl_max_syn_backlog depending on memory characteristic):

listen-queue-length = max(8, min(userspace_backlog, min(128, sysctl_max_syn_backlog))

Wit patch (sysctl not modified, BUT sysctl_max_syn_backlog depending on memory characteristic):

listen-queue-length = max(8, min(userspace_backlog, min(256, sysctl_max_syn_backlog))

The point is now: sysctl_max_syn_backlog is per default 256, 128 for small
systems and up to 1024 for larger systems. But sysctl_somaxconn (128) will
_always_ restrict the queue length to 128 and make therefore
sysctl_max_syn_backlog defacto unfeasible - it will always restrict the value
to 128. IMHO sysctl_somaxconn should be removed, the overhead of the
listen-queue size per listening socket is insignificant. Especially because
sysctl_max_syn_backlog already consider the memory characteristic. There are a
bunch more connected sockets as these <10 listening sockets, but performance
lack because of will always be noticeable:

netstat -s | grep overflowed
    2621 times the listen queue of a socket overflowed

Hagen
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