[Cerowrt-devel] [PATCH 2/2] codel: reduce count after exiting dropping state after one maxpacket

[Dave Taht]

On Thu, Aug 23, 2012 at 12:17 PM,  <dpreed at reed.com> wrote:
> I would be cautious about cutting down the information rate to TCP.
> Remember the collection of TCP pairs are already seeking this information.
> You risk driving TCP's control loop crazy, especially if you end up
> "resonating" with it with positive feedback.

We have been wrestling what we call s3 (re-entering dropping state)
for a very long time. From k&v's current test ns2 code:

https://github.com/dtaht/ns2/blob/master/queue/codel.cc

...

        r = dodeque();
        dropping_ = 1; // mdt: thought was to to change this to r.ok
                                // and do something mildly different below

        // If min went above target close to when it last went below,
        // assume that the drop rate that controlled the queue on the
        // last cycle is a good starting point to control it now.
	// Unfortunately, this is not easy to get at. "n*interval" is
	// used to indicate "close to" and values from 2 - 16 have been
	// used. A value of 8 has worked well. The count value doesn't
	// decay well enough with this control law. Until a better one
	// is devised, the below is a hack that appears to improve things.

	if(count_ > 2 && now- drop_next_ < 8*interval_) {
		count_ = count_ - 2;
		// kmn decay tests // mdt - floating point is not an option!
		if(count_ > 126) count_ = 0.9844 * (count_ + 2);

	} else
		count_ = 1;
        drop_next_ = control_law(now);
    }
    return (r.p);



>
>
>
> Rather than focus on "precision" or "stability" at the contended resource,
> remember the point is to minimize latency on an end-to-end basis so that all
> the TCP's can have tight control loops.  Thus there is no "goal" to making
> the contended queue maximized in capacity.  The goal is that it should drain
> quickly, which means keeping it small!   The goal is (as the codel paper
> says) to use the *minimum* latency as the estimator, not the "average" or
> weighted (delayed) average.
>
>
>
> Keep focused on the control theory principles, NOT on throughput
> maximization.  If you push toward maximizing throughput at any cost, even
> with codel, you will end up with *large* excursions in latency that stick
> for a long time.  I.e. bufferbloat.

I might argue from a control theory perspective that we're not banging
on or observing all the right things.

I was dubious about the value of the 2nd patch, given the more
aggressive decline in drop probability I already had vs a vs the ns2
code, (which was admittedly far less aggressive than the original
linux code). Might retain
the idea, flip it around some more, certainly plan to scratch head a
lot... was glad to figure out newton didn't run in reverse that well,
tho.

Anyway, in my limited testing today the 3rd version of the patches
worked fine at short RTTs and at a range of bandwidths from 1Mbit to
100Mbit, but that's the easy part and plenty more fiddling remains
with more streams and longer RTTs.

http://snapon.lab.bufferbloat.net/~d/fq_codel/rtts.png

(It's also somewhat weird getting used to nearly perfect
bi-directional utilization all the time in fq_codel - no matter what
underlying version
of codel is there)

Incidentally there is this 10 second periodicity to my captures that bothers me

http://snapon.lab.bufferbloat.net/~d/fq_codel/alwaysthoughtthese10secondintervalswereaflawofthecapturetoolbut.png

Anyway



>
> -----Original Message-----
> From: "Dave Täht" <dave.taht at bufferbloat.net>
> Sent: Thursday, August 23, 2012 9:58am
> To: cerowrt-devel at lists.bufferbloat.net
> Subject: [Cerowrt-devel] [PATCH 2/2] codel: reduce count after exiting
> dropping state after one maxpacket
>
> From: Dave Taht <dave.taht at bufferbloat.net>
>
> At a knife's edge, where we are rapidly entering and existing
> a dropping state, seek lower to find the optimimum.
> ---
> include/net/codel.h | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/include/net/codel.h b/include/net/codel.h
> index dbfccb7..5e85632 100644
> --- a/include/net/codel.h
> +++ b/include/net/codel.h
> @@ -342,6 +342,9 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
> vars->drop_next = codel_control_law(vars->drop_next,
> params->interval,
> vars->rec_inv_sqrt);
> + } else { /* we dropped out of the dropping state in 1 pkt */
> + vars->count = vars->count > 1 ? vars->count - 1 : 1;
> + codel_Newton_step(vars);
> }
> }
> end:
> --
> 1.7.9.5
>
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-- 
Dave Täht
http://www.bufferbloat.net/projects/cerowrt/wiki - "3.3.8-17 is out
with fq_codel!"