[Cake] [RFC PATCH 4/5] q_netem: support delivering packets in delayed time slots

[Pete Heist]

> On Nov 18, 2017, at 8:02 PM, Dave Taht <dave.taht at gmail.com> wrote:
> 
> On Sat, Nov 18, 2017 at 5:18 AM, Pete Heist <peteheist at gmail.com> wrote:
>> 
>> On Nov 17, 2017, at 11:55 PM,dave.taht at gmail.com wrote:
>> 
>> Slotting is a crude approximation of the behaviors of shared media such
>> as cable, wifi, and LTE, which gather up a bunch of packets within a
>> varying delay window and deliver them, relative to that, nearly all at
>> once.
>> 
>> Nice…
> 
> Meh. It really is "crude", and I keeping kicking about ways to somehow
> emulate half (or less) duplex, variable rates around a mean, mcast,
> etc.
> 
> it IS very nice to have a rate limiter that actually behaves a bit
> more like wifi, and I hope to also add the new ack fitering stuff to
> it.

I guess there may never be a way to make this perfect, only to try to reproduce the behavior that matters enough to make it usable for testing.

>> One of the things I also notice in my LAN tests is latencies for different
>> flows staying at more or less fixed (and different) positions relative to
>> the mean in flent results. Those positions, and the mean, can change with
>> each test run. Do you think this could result from the hashing to different
>> hardware queues (four in my case) changing between test runs?
> 
> yes if you are using bql probably. Is it sch_mq on top?

Yep with bql. I hadn’t thought about mq before. What my qos setup scripts are doing though is replacing the root qdisc (which now I see defaults to mq) with a single cake instance. With bql, should rather be leaving mq and putting four cake instances underneath it?

>> And is it
>> worth trying to simulate this effect, or not really?
> 
> Dunno. There are a couple ways to turn it off.

Fair enough...