netdev - Re: [PATCH v2] l2tp: use per-cpu variables for u64

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Date:	Wed, 27 Jun 2012 13:50:34 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Rick Jones <rick.jones2@...com>, Tom Parkin <tparkin@...alix.com>,
	netdev@...r.kernel.org, David.Laight@...LAB.COM,
	James Chapman <jchapman@...alix.com>
Subject: Re: [PATCH v2] l2tp: use per-cpu variables for u64_stats updates

On Wed, 27 Jun 2012 22:39:01 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> All sane SNMP applications are ready to cope with 32bits counters
> wrapping.

Actually that statement depends on the data rate. SNMP daemons work
by polling at periodic intervals. The limit for detecting roll over depends
on the rate and the interval. I believe the ubiquitous net-snmp code uses
something a 30 second polling interval for lots of it's caches. This means
it rolls over too fast at 10G. Polling faster can help but net-snmp is
a pig about updates.

I just realized the whole x32 (running 32 bit apps on 64 bit kernel) is broken
for things like /proc/net/dev where 64 bit kernel will give 64 bit values and
the 32 bit app (like net-snmp) is expecting unsigned long (32 bits).

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