Re: [PATCH net-next 2/2] fq_codel: implement L4S style ce_threshold_ect1 marking

Eric, (thanks Neal)

On 15/10/2021 16:49, Neal Cardwell wrote:

On Fri, Oct 15, 2021 at 10:08 AM Eric Dumazet [off-list ref] wrote:

quoted

On Fri, Oct 15, 2021 at 5:59 AM Bob Briscoe [off-list ref] wrote:

quoted

Eric,

Because the threshold is in time units, I suggest the condition for
exceeding it needs to be AND'd with (*backlog > mtu), otherwise you can
get 100% solid marking at low link rates.

When ce_threshold is for DCs, low link rates are unlikely.
However, given ce_threshold_ect1 is mainly for the Internet, during
testing with 1ms threshold we encountered solid marking at low link
rates, so we had to add a 1 packet floor:
https://bobbriscoe.net/projects/latency/dctth_journal_draft20190726.pdf

Sorry to chime in after your patch went to net-next.

What you describe about a minimal backlog was already there with
ce_threshold handling ?

For my education, do you have a pointer to where the ce_threshold
marking logic has a minimum backlog size requirement in packets or
bytes? AFAICT the ce_threshold marking in include/net/codel_impl.h
happens regardless of the current size of the backlog.

[BB] When I checked before my original posting, the only check for 
single packet backlog was within should_drop() here:
https://elixir.bootlin.com/linux/latest/source/include/net/codel_impl.h#L125
However, whether or not that causes should_drop() to return false, 
codel_dequeue() still always falls through to the ce_threshold marking 
after end:
https://elixir.bootlin.com/linux/latest/source/include/net/codel_impl.h#L249

quoted

Or is it something exclusive to L4S ?

I don't think it's exclusive to L4S. I think Bob is raising a general
issue about improving ECN marking based on ce_threshold. My
interpretation of Bob's point is that there is sort of a quantization
issue at very low link speeds, where the serialization delay for a
packet is at or above the ce_threshold delay. In such cases it seems
there can be behavior where the bottleneck marks every packet CE all
the time, causing any ECN-based algorithm (even DCTCP) to suffer poor
utilization.

I suppose with a fixed-speed link the operator could adjust the
ce_threshold based on the serialization delays implied by the link
speed, but perhaps in general this is infeasible due to variable-speed
(e.g., radio) links.

I guess perhaps this could be reproduced/tested with DCTCP (using
ECT(0)), a ce_threshold of 1ms (for ECT(0)), and an emulated
bottleneck link speed with a serialization delay well above 1ms (so a
link speed well below 12Mbps).

[BB] Yes.

quoted

This deserves a separate patch, if anything :)

Agreed, in the Linux development model this would make sense as a
separate patch, since it is conceptually separate and there do not
need to be any dependencies between the two changes. :-)

[BB] Sure. We'll see to it.

The (loose/indirect) dependency I saw was just that ce_threshold_ect1 
opens up the possibility of using the ce_threshold on the public 
Internet not just in DCs. So low rate links become a certainty, rather 
than a mere theoretical possibility.



Bob

neal

-- 
________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/