netdev - Re: [RFC PATCH v2] tcp: TCP Small Queues

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Message-ID: <CABrhC0mONb3Ki5YYM1rfiBZnCKd2tYFvdYpuur-u3gDYRii1tw@mail.gmail.com>
Date:	Thu, 12 Jul 2012 12:44:55 -0400
From:	John Heffner <johnwheffner@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, ycheng@...gle.com,
	dave.taht@...il.com, netdev@...r.kernel.org,
	codel@...ts.bufferbloat.net, therbert@...gle.com,
	mattmathis@...gle.com, nanditad@...gle.com, ncardwell@...gle.com,
	andrewmcgr@...il.com
Subject: Re: [RFC PATCH v2] tcp: TCP Small Queues

On Thu, Jul 12, 2012 at 9:46 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2012-07-12 at 09:33 -0400, John Heffner wrote:
>> One general question: why a per-connection limit?  I haven't been
>> following the bufferbloat conversation closely so I may have missed
>> some of the conversation.  But it seems that multiple connections will
>> still cause longer queue times.
>
> We already have a per-device limit, in qdisc.
>
> If you want to monitor several tcp sessions, I urge you use a controller
> for that. Like codel or fq_codel.
>
> Experiments show that limiting to two TSO packets in qdisc per tcp flow
> is enough to stop insane qdisc queueing, without impact on throughput
> for people wanting fast tcp sessions.
>
> Thats not solving the more general problem of having 1000 competing
> flows.

Right, AQM (and probably some modifications to the congestion control)
is the more general solution.

I guess I'm just trying to justify in my mind that the case of a small
number of local connections is worth handling in this special way.  It
seems like a generally reasonable thing, but it's definitely not a
general solution to minimizing latency.  One thing worth noting: on a
system routing traffic, local connections may be at a disadvantage
relative to connections being forwarded, sharing the same interface
queue, if that queue is the bottleneck.

Architecturally, the inconsistency between a local queue and a queue
one hop away bothers me a bit, but it's something I can learn to live
with if it really does improve a common case significantly. ;-)

Thanks,
  -John
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