netdev - Re: Packetlost when "tc qdisc del dev eth0 root"

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Date:	Wed, 16 Jan 2008 06:02:00 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Jarek Poplawski <jarkao2@...il.com>
CC:	Badalian Vyacheslav <slavon@...telecom.ru>, netdev@...r.kernel.org
Subject: Re: Packetlost when "tc qdisc del dev eth0 root"

Jarek Poplawski wrote:
> Patrick McHardy wrote, On 01/15/2008 05:05 PM:
> 
>> Badalian Vyacheslav wrote:
> 
> ...
> 
>> Yes, packets in the old qdisc are lost.
>>
>>> Maybe if tc do changes - need create second queue (hash of rules or how 
>>> you named it?) and do changes at it. Then replace old queue rules by 
>>> created new.
>>> Logic -
>>> 1. Do snapshot
>>> 2. Do changes in shapshot
>>> 3. All new packets go to snapshot
>>> 4. If old queue not have packets - delete it.
>>> 5. Snapshot its default.
>>
>> That doesn't really work since qdiscs keep internal state that
>> in large parts depends on the packets queued. Take the qlen as
>> a simple example, the new qdisc doesn't know about the packets
>> in the old one and will exceed the limit.
> 
> But, some similar alternative to killing packets 'to death' could
> be imagined, I suppose (in the future, of course!). So, e.g. doing
> the switch automatically after last packet has been dequeued (maybe
> even with some 'special' function/mode for this). After all even
> with accuracy lost, it could be less visible for clients than
> current way?


This would need support from the qdiscs to do it properly. Looks
non-trivial for HTB/HFSC/CBQ, but the others shouldn't be that hard.

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