Continuing issues with limit_conn

[Maxim Dounin]

Hello!

On Mon, Apr 28, 2008 at 11:29:08PM -0400, Calomel wrote:

>Building Nginx 0.6.29, we are also _unable_ to get limit_zone/limit_conn to
>work as expected.
>
>As a test we setup the relative lines in the http and server sections. Nginx
>should only accept ONE concurrent connection for any single ip address.  
>
>http{
>  limit_zone gulag $binary_remote_addr 1m;
>    server{
>      limit_conn gulag 1;
>    }
>}
>
>When I run "ab -c 50 -n 10000 http://testbox/" the server answers all
>requests with response code 200. As you mentioned, this is _not_ the
>expected behavior.
>
>Perhaps we are missing something? The code could be at fault or perhaps
>something has been omitted from the Wiki and the documentation. If there is
>a proper solution I will make sure to document it.

It looks like there is some misunderstanding regarding to what 
limit_conn actually limits.  It limits concurrent connections 
*processed* by nginx (not keep-alive ones), and only after header 
has been received (and thus configuration for request has been
determined).

Since nginx is event-based, with one worker process you shouldn't 
expect requests to hit limit_conn unless they block at some stage 
(i.e. responses bigger than socket buffers if sendfile off, 
replies bigger than sendfile_max_chunk if sendfile on, proxy_pass 
...).  With many workers limit_conn may be hit without blocking, 
but this generally requires _very_ high concurrency for small 
requests.

Maxim Dounin