2048 worker_connections are not enough while connecting to upstream

John Moore grails at jmsd.co.uk
Tue May 10 14:32:29 MSD 2011

On 10/05/11 11:19, Maxim Dounin wrote:
> Hello!
> On Tue, May 10, 2011 at 10:31:06AM +0100, John Moore wrote:
>> I'm using nginx as a remote proxy server in front of some Tomcat
>> instances, with pretty high traffic. Suddenly yesterday afternoon we
>> started getting the error in the message subject, one I'd never seen
>> before, and it's occurred tens of thousands of times since. Now,
>> I've upped the worker_connections count to a much higher figure and
>> the problem has gone away for now. But I'm concerned that it might
>> be indicative of something else - some kind of DOS attack, maybe?
>> What does this signify and why might we suddenly start getting it?
>> Thanks.
> Without proper history graphs (use stub_status, Luke!) it's hard
> to say if you are under attack or just your normal load reached
> the bar (or your backend response time degraded and this resulted
> in more connections used, or your network uplink has problems and
> this caused more connections...).
I'll look into these.
> On the other hand, 2048 is pretty low, and easily reached even
> without keepalive.  Usual production values for "high traffic"
> sites is over 9000.
OK, I may bump it up a bit higher than the 8192 I set it as today.
> Please also note that using high worker_connections may also
> require tuning of your OS to allow appropriate number of file
> descriptors to be used.
I made this change a while back, so we should be OK with that.

Thanks for your extremely lucid help!


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