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

Fri Aug 24 19:14:40 EDT 2012

All excellent points.

On 8/24/12 8:29 AM, dpreed at reed.com wrote:
> One other thing that worries me is this:  if you have 100 TCP
> connections all trying to get through a congested link, each "drop"
> signals only one of those connections that there is a need to cut the
> receive window size (by a factor of 2).   So if each of the 100
> connections has a window size that corresponds to the
> bitrate-delay-product of the channel, you need on the order of 100
> log2(100) ~ 650 drops to "fairly" slow all of the 100 connections down
> to each have a bitrate-delay-product equal to the 1/100 fair share, at
> minimum.  Because of this: you have to signal all 100 independent
> end-to-end control loops enough times to slow their rate.
> 
>  
> 
> Avoiding "drops" means that it takes much longer to get to the right,
> relatively fair, operating point.   And even if you are near the right
> operating point already, being selective on drops is pretty "unfair" in
> a statistical sense.
> 
>  
> 
> Also, remember that "fair" should not try to give capacity to TCP
> connections that are already constrained by other bottleneck links.  
> You can't give them capacity, usefully - doing so creates buffer
> "inflation" pressures on those links, which causes unnecessary
> oscillation in the other links.
> 
>  
> 
> Remember that the Internet is not at a "stable equilibrium" ever -
> "waves" flow through it, and the goal is to not "amplify" the waves.  
> The end-to-end control loops are the damping functions, while the link
> queue management (with drops and ECN) merely provide signals and keep
> latency short.  They can't "damp" traffic and may amplify if they try
> misguidedly to damp out waves.
> 
>  
> 
> -----Original Message-----
> From: "Dave Taht" <dave.taht at gmail.com>
> Sent: Friday, August 24, 2012 4:08am
> To: dpreed at reed.com
> Cc: "Dave Täht" <dave.taht at bufferbloat.net>,
> cerowrt-devel at lists.bufferbloat.net, codel at lists.bufferbloat.net
> Subject: Re: [Cerowrt-devel] [PATCH 2/2] codel: reduce count after
> exiting dropping state after one maxpacket
> 
> 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!"
> 
> 
> 
> This body part will be downloaded on demand.
> 