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

[Dave Taht]

Pete Heist <peteheist at gmail.com> writes:

>> 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?

Only if you want unlimited mode, and birthday problems, and don't have
cpu to burn.

>
> 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...
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