[Codel] [Cake] Today's cake

[Dave Taht]

Just when I thought it was safe to flash a router. Honestly. Your
email landed not less than 5 seconds after I finished a build.

On Tue, Jun 16, 2015 at 10:49 PM, Jonathan Morton <chromatix99 at gmail.com> wrote:
> I just put in three quite small tweaks to cake:
>
> 1) Codel now has a saturating increment for count, rather than a wrapping one.
>
> 2) The active flow counter is now an atomic-access variable.  This is really just an abundance of caution.

It is not clear to me if this is needed, either.

> 3) The overload drop routine should now be somewhat more efficient in most cases, since it iterates over the active flows rather than the entire array.  I haven’t measured how much of a difference it makes in practice, but it should be positive.

I was thinking that tracking the fattest 3 or so flows would suffice,
there would be constant overhead in keeping that statistic handy and
correct but constant overhead on figuring out where to drop on
overload, also.

>
> Point 1 turns out to matter rather a lot under unresponsive loads, given that count is only 16 bits.
>
> The “50 up 1 down asymmetric” test now produces the desired results for the download stream *without* the need to prioritise the acks; Codel now reaches a steady-ish state where it drops enough of them to keep everything moving, and this leaves more bandwidth in the up direction for the 50 streams.
>
> The version of that test with a prioritised download stream reveals that quite a lot of drops (as well as marks) are still occurring in the 50 streams.  This, to my mind, is a sign of too much aggression in the switch from marking to dropping, but I need to work out the numbers to be more confident about adjusting that.

k. What bandwidth?

I still disagree with you on count/2 because you are trying to hit the
next harmonic, and agree with you that count-1 is unstable, but I will
try to get some data instead.

>
>  - Jonathan Morton
>
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-- 
Dave Täht
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