> On 7 Jul, 2016, at 15:34, Michael Menth <[email protected]> wrote:
>
>> Based on our evaluations, with pure CoDel (without FQ-CoDel),
>> "reentering" is actually a common case. I think Dave and Toke should
>> have more experimental results to answer this question. (I included
>> Dave in CC)
>
> We also studied this issue about one or two years ago. The algorithm
> controlling the reentering of the dropping mode may have a significant
> impact on control variables, drop rates, drop pattern depending on the
> traffic. It can also impact utilization. Most interesting is probably
> Fig. 6 on page 7 in
> https://atlas.informatik.uni-tuebingen.de/~menth/papers/Menth16e.pdf
That’s a very interesting paper for Codel enthusiasts. IMHO, Codel-ACT
performs much better on the important metrics than plain Codel, especially when
faced with multiplexed flows.
[CK] I am familiar with CoDel and FQ-CoDel, but what is CoDel-ACT?
I had also noticed the potential for Codel’s count to increase unboundedly
while working on Cake; in fact when implemented as a 16-bit counter, it would
wrap around on certain workloads, causing apparently chaotic behaviour. I
therefore modified the variant of Codel that Cake used to *halve* the count on
re-entry, a much more aggressive reduction than Codel-ACT uses, as well as
using a saturating increment to prevent wraparound. However, I kept the basic
behaviour of waiting a full interval before re-entry.
More recently, I have changed Cake over to use COBALT, which is a combination
of Codel and BLUE; Codel is intended to handle responsive flows which behave in
a TCP-like manner, and BLUE to take over if the flow proves to be unresponsive
(by running into the queue length limit).
COBALT uses a re-implemented version of Codel with a much simplified main loop.
It also has a novel re-entry behaviour: the count is reduced while the queue
length is below target by running the algorithm in reverse, without marking or
dropping. This means that very short non-dropping periods result in a slight
reduction in count, and longer periods result in a larger reduction; the
reduction also occurs faster when count is high.
With your paper in mind, I may also change it to use the shorter re-entry delay
of Codel-ACT, as you explain very clearly how it may be beneficial, and this
shows in the graphs as a lower peak delay and a reduction in loss
synchronisation between flows. I also think it will allow Codel to adapt more
cleanly to RTTs significantly shorter than it is configured for - this is
important where the defaults are set for internet-scale traffic, and people
unthinkingly rely on those defaults in a LAN environment.
Here is the relevant part of COBALT’s Codel implementation, showing the
re-entry count reduction strategy:
if(next_due && vars->dropping) {
/* Use ECN mark if possible, otherwise drop */
drop = !(vars->ecn_marked = INET_ECN_set_ce(skb));
vars->count++;
if(!vars->count)
vars->count--;
cobalt_invsqrt(vars);
vars->drop_next = cobalt_control_law(vars->drop_next, p->interval,
vars->rec_inv_sqrt); } else {
while(next_due) {
vars->count--;
cobalt_invsqrt(vars);
vars->drop_next = cobalt_control_law(vars->drop_next,
p->interval, vars->rec_inv_sqrt);
schedule = now - vars->drop_next;
next_due = vars->count && schedule >= 0;
}
}
The full code can be found here:
https://github.com/dtaht/sch_cake/blob/master/cobalt.c
- Jonathan Morton
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