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

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Date:	Mon, 14 Jan 2008 14:31:49 -0800
From:	"Michael Chan" <mchan@...adcom.com>
To:	"Mark Seger" <Mark.Seger@...com>
cc:	"netdev" <netdev@...r.kernel.org>
Subject: Re: Why are network counters only updated once a second?

On Mon, 2008-01-14 at 15:30 -0500, Mark Seger wrote:

> 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?

On most Broadcom NICs, the statistics counters are periodically DMA'ed
from the chip and the default interval is roughly 1 second.  On most of
these chips, you can override this interval using ethtool -C eth0.

As I mentioned earlier, the 5706/5708 has a bug in the statistics DMA
engine that can corrupt counters from time to time.  The workaround is
effective, but you now lose the ability to control the DMA interval.

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