On Wed, Nov 28, 2012 at 12:15:35PM +1300, Andrew McGregor wrote:
>
> On 28/11/2012, at 11:54 AM, "Paul E. McKenney" <[email protected]>
> wrote:
>
> > On Tue, Nov 27, 2012 at 02:31:53PM -0800, David Lang wrote:
> >> On Tue, 27 Nov 2012, Jim Gettys wrote:
> >>
> >>> 2) "fairness" is not necessarily what we ultimately want at all; you'd
> >>> really like to penalize those who induce congestion the most. But we
> >>> don't
> >>> currently have a solution (though Bob Briscoe at BT thinks he does, and is
> >>> seeing if he can get it out from under a BT patent), so the current
> >>> fq_codel round robins ultimately until/unless we can do something like
> >>> Bob's idea. This is a local information only subset of the ideas he's
> >>> been
> >>> working on in the congestion exposure (conex) group at the IETF.
> >>
> >> Even more than this, we _know_ that we don't want to be fair in
> >> terms of the raw packet priority.
> >>
> >> For example, we know that we want to prioritize DNS traffic over TCP
> >> streams (due to the fact that the TCP traffic usually can't even
> >> start until DNS resolution finishes)
> >>
> >> We strongly suspect that we want to prioritize short-lived
> >> connections over long lived connections. We don't know a good way to
> >> do this, but one good starting point would be to prioritize syn
> >> packets so that the initialization of the connection happens as fast
> >> as possible.
> >>
> >> Ideally we'd probably like to prioritize the first couple of packets
> >> of a connection so that very short lived connections finish quickly
>
> fq_codel does all of this, although it isn't explicit about it so it is hard
> to see how it happens.
>
> >> it may make sense to prioritize fin packets so that connection
> >> teardown (and the resulting release of resources and connection
> >> tracking) happens as fast as possible
> >>
> >> all of these are horribly unfair when you are looking at the raw
> >> packet flow, but they significantly help the user's percieved
> >> response time without making much difference on the large download
> >> cases.
> >
> > In all cases, to Jim's point, as long as we avoid starvation. And there
> > will likely be more corner cases that show up under extreme overload.
> >
> > Thanx, Paul
> >
>
> So, fq_codel exhibits a new kind of fairness: it is jitter fair, or in other
> words, each flow gets the same bound on how much jitter it can induce in the
> whole ensemble of flows. Exceed that bound, and flows get deprioritised.
> This achieves thin-flow and DNS prioritisation, while allowing TCP flows to
> build more buffer if required. The sub-flow CoDel queues then allow short
> flows to use a reasonably large buffer, while draining standing buffers for
> long TCP flows.
>
> The really interesting part of the jitter-fair behaviour is that
> jitter-sensitive traffic is protected as much as it can be, provided its own
> sending rate control does something sensible. Good news for interactive
> video, in other words.
>
> The actual jitter bound is the transmission time of max(mtu, quantum) *
> n_thin_flows bytes, where a thin flow is one that has not exceeded its own
> jitter allowance since the last time its queue drained. While it is possible
> that there might instantaneously be a fairly large number of thin flows, in
> practice on a home network link there are normally only a very few of these
> at any one moment, and so the jitter experienced is pretty good.
I will have to think about this, but at first glance I kinda like the
idea of describing FQ-CoDel as jitter fair. ;-)
Thanx, Paul
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