netdev - Re: Why are network counters only updated once a second?

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Date:	Mon, 14 Jan 2008 16:00:27 -0500
From:	Mark Seger <Mark.Seger@...com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: Why are network counters only updated once a second?



Eric Dumazet wrote:
> Mark Seger a écrit :
>> I had mentioned this in my previous post but perhaps it might get 
>> more attention all by itself.  I can't say for sure when this 
>> changed, but for the longest time network counters were only updated 
>> once every 0.9765 seconds and unless you used a tools like collectl 
>> that could monitor at fractional intervals, your traffic was 
>> under-reported AND you'd get periodic spikes of double the actual 
>> rate.  See http://collectl.sourceforge.net/NetworkStats.html for a 
>> more complete explanation.
>>
>> Eventually the frequency became better aligned at a 1 second interval 
>> because now the number look better, but the problem I see is that 
>> when the sampling interval is very close to the monitoring interval 
>> you still get periodic incorrect data.  Furthermore, you now need to 
>> know which way the counters are updated before you pick a sampling 
>> interval!  But the real point is if anyone ever wants to do finer 
>> grained monitoring, say every 1/2 or even tenth of a second, they 
>> can't because the counters won't change between samples.  Has this 
>> ever been discussed before?
>>
>
> Yes it was discussed before. Some devices perform counters updates 
> directly at the NIC level, and one in a while a transfert of counters 
> is done to the host.
>
> This is supposed to be better, especially on SMP.
>
> Maybe you need to setup accounting rules with iptables, so that you 
> can perform counter sampling at whatever rate you want ?
Maybe I wasn't clear enough.  I'm grabbing the counters from 
/proc/net/dev and whatever mechanism is being used only ports them with 
a granularity of about once a second.  This means any of the standard 
tools that use /proc to get their data will all have the same problem.
-mark

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