netdev - Re: [PATCH net-next] netem: refine early skb orphaning

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Date:	Mon, 16 Jul 2012 23:08:55 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...tta.com
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org, hagen@...u.net,
	msg@...gle.com, aterzis@...gle.com, ycheng@...gle.com
Subject: Re: [PATCH net-next] netem: refine early skb orphaning

From: Stephen Hemminger <shemminger@...tta.com>
Date: Sat, 14 Jul 2012 14:53:33 -0700

> On Sat, 14 Jul 2012 15:16:27 +0200
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
>> From: Eric Dumazet <edumazet@...gle.com>
>> 
>> netem does an early orphaning of skbs. Doing so breaks TCP Small Queue
>> or any mechanism relying on socket sk_wmem_alloc feedback.
>> 
>> Ideally, we should perform this orphaning after the rate module and
>> before the delay module, to mimic what happens on a real link :
>> 
>> skb orphaning is indeed normally done at TX completion, before the
>> transit on the link.
>> 
>> +-------+   +--------+  +---------------+  +-----------------+
>> + Qdisc +---> Device +--> TX completion +--> links / hops    +->
>> +       +   +  xmit  +  + skb orphaning +  + propagation     +
>> +-------+   +--------+  +---------------+  +-----------------+
>>       < rate limiting >                  < delay, drops, reorders >
>> 
>> If netem is used without delay feature (drops, reorders, rate
>> limiting), then we should avoid early skb orphaning, to keep pressure
>> on sockets as long as packets are still in qdisc queue.
>> 
>> Ideally, netem should be refactored to implement delay module
>> as the last stage. Current algorithm merges the two phases
>> (rate limiting + delay) so its not correct.
>> 
>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
 ...
> Acked-by: Stephen Hemminger <shemminger@...tta.com>

Applied, thanks.
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