sysctl kern.ipc.somaxconn limit 65535 why?

Dan The Man dan at
Wed Jan 4 21:15:11 UTC 2012

On Wed, 4 Jan 2012, Dan The Man wrote:

> On Wed, 4 Jan 2012, Chuck Swiger wrote:
>> On Jan 4, 2012, at 12:44 PM, Dan The Man wrote:
>>>> Even a backlog of a 1000 is large compared to the default listen queue 
>>>> size of around 50 or 128.  And if you can drain 1000 connections per 
>>>> second, a 65K backlog is big enough that plenty of clients (I'm thinking 
>>>> web-browsers here in particular) will have given up and maybe retried 
>>>> rather than waiting for 60+ seconds just to exchange data.
>>> For web browsers makes sense, but if your coding your own server 
>>> application its only a matter of increasing the read and write timeouts
>>> to fill queue that high and still process them.
>> Sure, agreed.
>>> Of course wouldn't need anything that high, but for benchmarking how much 
>>> can toss in that listen queue then write something to socket on each one 
>>> after connection established to see how fast application can finish them 
>>> all, I think its relevant.
>>> This linux box I have no issues:
>>> cappy:~# /sbin/sysctl -w net.core.somaxconn=200000
>>> net.core.somaxconn = 200000
>>> cappy:~# sysctl -w net.ipv4.tcp_max_syn_backlog=20000
>>> net.ipv4.tcp_max_syn_backlog = 200000
>>> cappy:~#
>> However, I'm not convinced that it is useful to do this.  At some point, 
>> you are better off timing out and retrying via exponential backoff than you 
>> are queuing hundreds of thousands of connections in the hopes that they 
>> will eventually be serviced by something sometime considerably later.
> I agree completely, in practical application this makes sense, but why should 
> the OS dictate not being able to temporarily set that setting higher in order 
> to fully benchmark the application at 100k+ in the listen queue if the 
> developer so chooses? I think that alone should be a good reason, to make 
> freebsd developer friendly.

Anyways its not a big deal I can live with a 60k or so benchmark, I just 
really wanted to see how freebsd's kqueue would perform processing that 
many connections right out of the listen queue.


Dan The Man
CTO/ Senior System Administrator
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