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

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Date:	Thu, 28 Jun 2012 07:00:54 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	Ben Greear <greearb@...delatech.com>,
	Stephen Hemminger <shemminger@...tta.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, 2012-06-27 at 16:01 -0700, Rick Jones wrote:

> Today, sure, generalizing to packet counters in general, that bloat is 
> likely on its way.  At 100 Gbit/s Ethernet, that is upwards of 147 
> million packets per second each way.  At 1 GbE it is 125 million octets 
> per second.  So, if 32 bit octet counters were insufficient for 1 GbE, 
> 32 bit packet counters likely will be insufficient for 100GbE.
> 
> Or, I suppose, 3 or more bonded 40 GbEs or 10 or more bonded 10 GbEs 
> (unlikely though that last one may be) assuming there is stats 
> aggregation in the bond interface.

Note that I am all for 64bit counters on 64bit kernels because they are
almost[1] free, since they fit in a machine word (unsigned long).

tx_dropped is the count of dropped _packets_.

If more than 32bits are needed, and someone must run this 100GbE on a
32bit machine of the last century, he really has a big problem.

[1] : LLTX drivers case 
   since ndo_start_xmit() can be run concurrently by many cpus, safely
updating an "unsigned long" requires additional hassle :

   1) Use of a spinlock to protect the update.
   2) Use atomic_long_t instead of "unsigned long"
   3) Use percpu data

3) is overkill for devices with light traffic, because it consumes lot
of RAM on machines with 2048 possible cpus, _and_ the reader must fold
the data of all possible values.

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