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Subject: Re: Packet time delays on multi-core systems
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Alexey Vlasov <renton@renton.name>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>
In-Reply-To: <20100930123048.GA4094@beaver.vrungel.ru>
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	 <1285796721.5211.156.camel@edumazet-laptop>
	 <20100930123048.GA4094@beaver.vrungel.ru>
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Date: Thu, 30 Sep 2010 14:46:52 +0200
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Le jeudi 30 septembre 2010 à 16:30 +0400, Alexey Vlasov a écrit :
> On Wed, Sep 29, 2010 at 11:45:21PM +0200, Eric Dumazet wrote:
> > But if you send SYN packets in the same time, (logged), this might
> > slow
> > down the reception (and answers) of ICMP frames. LOG target can be
> > quite
> > expensive...
> 
> Yes, it's clear that  some slow down can appear, but 100 ms is too much,
> and this happens at 200 SYN packets in 2 minutes just as in my example.
> On old servers where some tx/rx are missing in NIC card I don't see
> such a situation even at more then 1000 SYN-packets per sec.

Because all cpus were servicing interrupts, which was good for your
needs. Things apparently changed with 2.6.32. 

You have a multiqueue NIC, but using a single CPU to handle the
workload.

> 
> > Is using other rules gives same problem ?
> >
> > iptables -A OUTPUT -p tcp -m tcp --dport 80 --tcp-flags
> 
> No, only LOG gives such a scheme.
>