netdev - Re: [PATCH] Rate should be u64 to avoid integer overflow at high speeds (>= ~35Gbit)

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Message-Id: <20130313020156.c9dd9841.billfink@mindspring.com>
Date:	Wed, 13 Mar 2013 02:01:56 -0400
From:	Bill Fink <billfink@...dspring.com>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	Chris Friesen <chris.friesen@...band.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vimal <j.vimal@...il.com>, netdev@...r.kernel.org,
	shemminger <shemminger@...tta.com>
Subject: Re: [PATCH] Rate should be u64 to avoid integer overflow at high
 speeds (>= ~35Gbit)

On Tue, 12 Mar 2013, Thomas Graf wrote:

> On 03/12/13 at 08:29am, Chris Friesen wrote:
> > The only problem I see is that you can't set the multiplier with a
> > new tool and then query the rate with old tools.
> > 
> > But you're going to run into that problem with the old tools no
> > matter what you do--and not doing anything is a crappy option as
> > well.
> > 
> > Some kind of multiplier or shift makes as much sense as anything
> > else. With old tools you get current behaviour, with new tools you
> > can specify a multiplying factor to trade off resolution vs
> > precision.
> 
> The introduction of a shift operator or multiplier introduces
> inprecision. I'd much rather see new 64bit Netlink attributes
> that, if present, replace the old rate values and statistics.
> 
> You will need to add a new Netlink attribute anyway and we might
> as well transfer the actual rate instead of a multiplier. Just
> like we did with IFLA_STATS64.

The last time this was discussed appears to be (on 2011-03-28):

	http://marc.info/?l=linux-netdev&m=130128741907282&w=2

where Maciej Żenczykowski argued that creating a new 64-bit
Netlink attribute for this would be much more complex than for
the IFLA_STATS64 support.  There was no reply.

Providing a new multiplier/shift parameter would be a simple
way to extend support for higher rates, and would not break
existing user space that doesn't require the higher rates.
I imagine the user would not explicitly specify the multiplier/
shift parameter, but would just normally specify the desired
rate, and a newer tc would figure out what multiplier/shift
to use if a high enough rate demanded it.  To maintain user
space compatibility, the kernel should report back the same
rate and multiplier/shift it was given, and the newer tc would
convert it back to the user's originally specified rate.  Older
user space that was fine with the ~34 Gbps rate limitation would
always have the default multiplier of 1 or shift of 0 bits, and
would see the exact same unmultiplied/unshifted rate it always
did.

I also believe 32 bits of precision is significant enough
at these higher data rates.

					-Bill
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