netdev - Re: SOMAXCONN = 128, but max defaults to 2048

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Date:	Sat, 08 Oct 2011 14:35:38 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Olaf van der Spek <ml@...pek.org>
Cc:	netdev@...r.kernel.org
Subject: Re: SOMAXCONN = 128, but max defaults to 2048

Le samedi 08 octobre 2011 à 13:51 +0200, Olaf van der Spek a écrit :
> Hi,
> 
> SOMAXCONN is defined as 128, but the max
> ("/proc/sys/net/ipv4/tcp_max_syn_backlog") appears to be 2048 by
> default.

It depends on memory size and TCP hash table size :

dmesg | grep "TCP established hash table entries"

-> TCP established hash table entries: 524288 (order: 11, 8388608 bytes)

sysctl_max_syn_backlog = max(128, cnt / 256);
cnt = 524288
cnt/256 -> 2048

> Is the name of SOMAXCONN wrong and is it actually defining the default?
> Would there be a disadvantage to defining SOMAXCONN as INT_MAX and
> letting the kernel control the actual max?
> 

You mean : remove somaxconn tunable ?

accept()/listen() is not bound to TCP only.

But yes, 128 default is a bit old today, given that Apache uses a
listen(fd, 511) default value itself...

I routinely set net.core.somaxconn to 1024 on my servers.

We had a recent discussion on the matter lately, but Hagen Paul Pfeifer
did not polish his patches enough :

http://lists.openwall.net/netdev/2011/03/20/3

FreeBSD has a separate mechanism for not yet established sockets, called
syncache, that allows a low somaxconn per listener.

The socket queue holds only fully established (but not yet accept()ed)
sockets, while syncache holds all the SYN_RECV ones.

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