netdev - Re: [PATCH] sch_htb.c consume the classes's tokens bellow the HTB_CAN

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Date:	Wed, 4 Nov 2009 09:53:52 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Jamal Hadi Salim <hadi@...erus.ca>, devik@....cz,
	netdev@...r.kernel.org
Subject: Re: [PATCH] sch_htb.c consume the classes's tokens bellow the 
	HTB_CAN_SEND level

On Wed, Nov 4, 2009 at 7:00 AM, Jarek Poplawski <jarkao2@...il.com> wrote:
> On Tue, Nov 03, 2009 at 09:18:49PM +0800, Changli Gao wrote:
>> On Tue, Nov 3, 2009 at 6:05 PM, Jarek Poplawski <jarkao2@...il.com> wrote:
>> >
>> > The ceil specification is controlled only by ctokens, which are always
>> > updated, so no such risk.
>> >
>> Nevertheless, updating tokens is necessary too.
>
> If it's really necessary you should present some test case fixed by
> your patch, I guess.
>
> In the meantime let's consider what could be broken:
> class 1:1 (parent) rate 10 packets/sec
> class 1:2 rate 5 packets/sec ceil 10 packets/sec
> class 1:3 rate 5 packets/sec ceil 10 packets/sec
>
> class 1:2 doesn't use all its rate, and sends every other second
>        (in even seconds)
> class 1:3 sends 10 packets during the first second, so with your
>        patch it will use its tokens for 2 seconds
> class 1:2 uses its rate in the second second..., so class 1:1
>        can't lend anything
> class 1:3 can only borrow, so it won't be able to send during
>        this second anything
>
> So, the effect would be class 1:3 sending every odd second 10 packets
> while every even second - nothing...

class 1:3 can send, as its parent rate is 10, but class 1:2 only uses
half of it, and class 1:1 is still in HTB_CAN_SEND mode.

The result is, hasn't any difference with or without my patch :
class 1:1 sends 10 packets in odd seconds, and 5 packets in even seconds.
class 1:2 sends 5 packets in even seconds.
class 1:1 (parent) sends 10 packets in every second.

Let's think this case in another way: which class sends packets in
even seconds first, class 1:2 or class 1:3.
With my patch, as 1:3 in HTB_MAY_BORROW mode, and 1:2 in HTB_CAN_SEND
mode, so 1:2 sends all its 5 packets first.
Without my patch, as 1:2 and 1:3 are both in HTB_CAN_SEND mode, the
sequence is undetermined. In other word, 1:2 and 1:3 are treated
fairly, and it isn't fair for 1:2, because 1:2 sends nothing in odd
seconds, and has no deficit in rate as 1:3.

-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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