netdev - Re: HTB accuracy on 10GbE

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Date:	Wed, 4 Nov 2009 12:13:32 +0900
From:	Ryousei Takano <ryousei@...il.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Patrick McHardy <kaber@...sh.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	takano-ryousei@...t.go.jp
Subject: Re: HTB accuracy on 10GbE

Hi Patrick and Stephen,

Thanks for your comments.

I retried on the newer kernel and iproute2, and added the experimental result
on my page.  Please see 'Experimental result 2':
    http://code.google.com/p/pspacer/wiki/HTBon10GbE

The accuracy improves compared with the previous experiment.
The difference reduces from +810 Mbps to +430 Mbps.
It is because the timer resolution improves from 1 usec to 1/64 usec.
But it is not perfect.

Best regards,
Ryousei Takano


On Tue, Nov 3, 2009 at 5:53 AM, Stephen Hemminger <shemminger@...tta.com> wrote:
> On Mon, 02 Nov 2009 16:43:42 +0100
> Patrick McHardy <kaber@...sh.net> wrote:
>
>> Ryousei Takano wrote:
>> > Hi Stephen and all,
>> >
>> > I have observed a HTB accuracy problem on the Linux kernel 2.6.30 and
>> > the Myri-10G 10 GbE NIC.
>> > HTB can control the transmission rate at Gigabit speed, however it can
>> > not work well at 10 Gigabit speed.
>> >
>> > I asked Stephen this problem at Japan Linux Symposium.  He mentioned a
>> > HTB bug related to the timer granularity.
>> > I want to know what is happen, and what should be do for fixing it.
>> >
>> > Any comments and suggestions will be welcome.
>> >
>> > For more detail, please see the following page:
>> > http://code.google.com/p/pspacer/wiki/HTBon10GbE
>>
>> This is not an easy problem to fix. Userspace, the kernel and the
>> netlink API use 32 bit for timing related values, which is too small
>> to use more than microsecond resolution. All of them need to be
>> converted to use bigger types, additionally some kind of compatibility
>> handling to deal with old iproute versions still using microsecond
>> resolution is required.
>
> The existing API is a legacy mish-mash. The field is limited to 32 bits,
> but it might be possible to use a finer scale.
>
> Maybe if kernel advertised finer resolution through /proc/net/psched
> then table could be finer grained. This would maintain compatibility
> between kernel and user space. You would need to have new kernel and
> new iproute to get nanosecond resolution but older combinations would
> still work.
>
> The downside is that by using nanosecond resolution the rates are upper
> bounded at 4.2seconds / packet.
>
>
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