Re: [aqm] CoDel on high-speed links

[Steven Blake <[e-mail]>]

On Tue, 2015-06-09 at 13:52 -0700, David Lang wrote:
> On Tue, 9 Jun 2015, Steven Blake wrote:
> 
> >>
> >> Except that tcp's drop their rates by (typically) half on a drop, and
> >> a matter of debate as to when on CE.
> >
> > Ex/ 10 GE link, ~10K flows (average).  During a congestion epoch, CoDel
> > with interval = 100 msec starts dropping 257 packets/sec after 5 secs.
> > How many flows is that effectively managing?
> 
> how fast are the flows ramping back up to the prior speed?
> 
> if you have 10K flows and ~250 drops/sec, over 40 seconds each could end up with 
> one drop. If that keeps the link uncongested, it's doing it's job.

According to my calculations, with RTT = 25 msec and MTU = 1500 bytes,
you need to be going around 29 Mbps average (oscillating between 2/3 and
4/3 of this) to need to see a drop every 40 seconds.  For 1 Mbps average
you need ~1.4 secs between drops.    For 10K 1 Mbps flows you then need
to drop ~7000 packets/sec (~0.8% drop frequency for MTU-sized packets on
a 10 GE link).

Of course this all assumes uniform stationary elephants which is never
the case in real life, but you see how CoDel's drop frequency (for
interval = 100 msec) is not even in the right ballpark.

> 
> Unfortunantly, when there is a drop, the affected flow slows down a LOT, so if 
> you are near the edge of being uncongested, you may not need to slow that many 
> flows down to be uncongested. Then as the flows ramp back up, the link becomes 
> congested again and some flow needs to be slowed down. Hopefully CoDel is going 
> to slow down a different flow the next time.
> 
> With the extisting feedback that can be provided, it's not possible to slow all 
> flows down 5%, all you could do is to slow 10% of the flows by 50% to reduce 
> overall load by 5% 

The more flows you have, the more flows you need to nuke to slow things
down even a little bit.  The more flows you need to nuke, the faster you
need to drop packets.


Regards,

// Steve