netdev - Re: Something hitting my total number of connections to the server

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Date:   Wed, 16 Aug 2017 03:34:21 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Akshat Kakkar <akshat.1984@...il.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: Something hitting my total number of connections to the server

On Wed, 2017-08-16 at 10:18 +0530, Akshat Kakkar wrote:
> On Mon, Aug 14, 2017 at 2:37 PM, Akshat Kakkar <akshat.1984@...il.com> wrote:
> > I have centos 7.3 (Kernel 3.10) running on a server with 128GB RAM and
> > 2 x 10 Core Xeon Processor.
> > I have hosted a webserver on it and enabled ssh for remote maintenance.
> > Previously it was running on Centos 6.3.
> > After upgrading to CentOS 7.3, occasionally (probably when number of
> > hits are more on the server), I am not able to create new connections
> > (neither on web nor on ssh). Existing connections keeps on running
> > fine.
> >
> > I did packet capturing using tcpdump to understand if its some
> > intermediate network issue.
> > What I found was the server is not replying for new SYN requests.
> >
> > So it's clear that its not at all application issue. Also, there are
> > no logs in applications logs for any connections dropped, if any.
> >
> > I check my firewall rules if there is some rate limiting imposed.
> > There is nothing in there.
> >
> > I check tc, if by mistake some rate limiting is imposed. There is
> > nothing in there too.
> >
> > I have increased noOfFiles to 1000000 and other sysctl parameters, but
> > the issue is still there.
> >
> > Has anybody experienced the same?
> >
> > How to go about? Anybody ... Please Help!!!
> 
> Its getting lonely out here. Anybody there ???

We wont help you unless you use a recent kernel.

3.10 misses all recent improvements in TCP stack (4 years of hard work)