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

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Date:	Sun, 20 Mar 2011 09:30:17 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hagen Paul Pfeifer <hagen@...u.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] socket: increase default maximum listen queue length

Le dimanche 20 mars 2011 à 02:50 +0100, Hagen Paul Pfeifer a écrit :
> sysctl_somaxconn (SOMAXCONN: 128) specifies the maximum number of
> sockets in state SYN_RECV per listen socket queue. At listen(2) time the
> backlog is adjusted to this limit if bigger then that.
> 
> Afterwards in reqsk_queue_alloc() the backlog value is checked again
> (nr_table_entries == backlog):
> 
>     nr_table_entries = min_t(u32, nr_table_entries, sysctl_max_syn_backlog);
>     nr_table_entries = max_t(u32, nr_table_entries, 8);
>     nr_table_entries = roundup_pow_of_two(nr_table_entries + 1);
> 
> sysctl_max_syn_backlog on the other hand is dynamically adjusted,
> depending on the memory characteristic of the system. Default is 256,
> 128 for small systems and up to 1024 for bigger systems.
> 
> For real server work the defacto sysctl_somaxconn limit seems inadequate:
> 
>     Experiments with real servers show, that it is absolutely not enough
>     even at 100conn/sec. 256 cures most of problems.
> 
> Increase default sysctl_somaxconn from 128 to 256 to meet todays condition by
> simultaneously limit nr_table_entries by sysctl_max_syn_backlog which is
> based on memory condition (max(128, (tcp_hashinfo.ehash_mask + 1 / 256)).
> 
> Signed_off-by: Hagen Paul Pfeifer <hagen@...u.net>
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> ---
>  include/linux/socket.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index edbb1d0..bf35ce2 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -237,7 +237,7 @@ struct ucred {
>  #define PF_MAX		AF_MAX
>  
>  /* Maximum queue length specifiable by listen.  */
> -#define SOMAXCONN	128
> +#define SOMAXCONN	256
>  
>  /* Flags we can use with send/ and recv. 
>     Added those for 1003.1g not all are supported yet


Hmm, real problem is not the 'maximum queue value', but the minimum one.

If application says : listen(fd, 10), you are stuck.

128 or 256 is way too small on some servers, where admin can tune
in /etc/sysctl.conf :

net.core.somaxconn = 8192
net.ipv4.tcp_max_syn_backlog = 8192

But application also needs to use : listen(fd, 8192)



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