netdev - Re: On Linux rate limiting and the magic value of 34.64 Gbps...

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Date:	Fri, 25 Mar 2011 08:17:39 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Maciej Żenczykowski <zenczykowski@...il.com>
Cc:	Linux NetDev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: On Linux rate limiting and the magic value of 34.64 Gbps...

Le vendredi 25 mars 2011 à 00:14 -0700, Maciej Żenczykowski a écrit :
> Hey,
> 
> The Linux rate limiting code relies on the rate field of struct tc_ratespec.
> This field is a __u32 and measures rate in "bytes per second".
> 
> This basically means maximum representable rate is 4GB per second.
> This is equivalent to 34.36 Gbps and I just ran across that limit with
> 40 Gbps (which behaves like 5.64 Gbps because of overflow/truncation).
> Seeing as this structure is exposed to userspace for both read and
> write via various netlink paths (in cbq, htb, tbf, etc...) there
> doesn't seem to be a particularly clean way to increase the size of
> this field.  While there is a __reserved field that could
> theoretically be repurposed as some sort of rate bit shift register, I
> don't think we can rely on __reserved having been set to zero by
> userspace (by older programs), and we will definitely see problems
> with output by programs (tc) that don't expect to have to parse this
> field to output an understandable rate limit...
> 
> Anybody have any bright ideas?

Well, netlink is extensible, so we can easily add a new structure, with
64bit fields if necessary.

We did that for 64bit stats already.



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