[Oisf-users] number of alerts versus performance

[Peter Manev]

On Thu, 2016-06-30 at 15:54 +0000, Yasha Zislin wrote:
> Peter,
> 
> 
> I found one alert that was causing high alert count. After I've
> disabled it, count went down but packet loss is still around 20%.
> 
> 
> my stats.log does not contain anything useful such as flow emergency
> mode, or ssn memcap drop. The only thing that is off is kernel drops,
> and tcp reassembly gaps. 
> From my understanding kernel drops have nothing to do with Suricata
> and point to OS problems.
> 
> 
> I do see one of the CPUs peak at 100% when packet loss increases. One
> thing to note. Two other CPUs are working on capturing traffic with
> high IRQs. My guess would be flow manager or detection engine.
> 


You can see if you get more info from:
top -H -p `pidof suricata`
and
perf top -c cpu_number_here
example: perf top -c 0

> I dunno.
> 
> 
> Thanks
> 
> 
> 
> 
> ______________________________________________________________________
> From: Peter Manev <petermanev at gmail.com>
> Sent: Thursday, June 30, 2016 3:00 PM
> To: Yasha Zislin
> Cc: oisf-users at lists.openinfosecfoundation.org
> Subject: Re: [Oisf-users] number of alerts versus performance 
>  
> On Thu, 2016-06-30 at 14:41 +0000, Yasha Zislin wrote:
> > I have been trying to figure out a packet loss on one of my sensors
> > and I am puzzled.
> > 
> > It is has 16 gigs of RAM, one quad core AMD CPU, and nic sees about
> 3
> > million packets per minute. Nothing special in my mind. I am using
> > PFRING 6.5.0 and Suricata 3.1.
> > 
> > I get about 20% to 40% packet loss.  I have another identical server
> > which sees the same amount of traffic and maybe some of the same
> > traffic as well.
> > 
> > I've been messing around with NIC settings, IRQs, PFRING settings,
> > Suricata settings trying to figure out why such a high packet loss.
> > 
> > 
> > I have just realized one big difference in these two sensors.
> > Problematic one gets 2k to 4k of alerts per minute which sounds
> huge.
> > 
> 
> Any particular sig that is alerting in excess ?
> 
> > Second one gets like 80 alerts per minute. Both have the same
> > rulesets.
> > 
> > 
> > The difference of course is the home_net variable.
> > 
> > 
> > Can the fact that Suricata processes more rules due to HOME_NET
> > definition cause high performance strain on the server? 
> > 
> 
> Yes HOME_NET size has effect on performance as well (among other
> things). For example - 
> HOME_NET: "any"
> EXTERNAL_NET: "any"
> will certainly degrade your performance.
> 
> > 
> > If the packet does not match per HOME_NET, it will be discarded
> before
> > being processed in rules. Correct?
> > 
> > Versus if packet passes HOME_NET check, it would have to go through
> > all of the rules, hence cause higher CPU utilization.
> > 
> > 
> > Thank you for the clarification.
> > 
> > 
> > _______________________________________________
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> oisf-users at openinfosecfoundation.org
> > Site: http://suricata-ids.org | Support:
> http://suricata-ids.org/support/ 
> 
> 
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> 
> 
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> 
> 

-- 
Regards,
Peter Manev