[Codel] The next slice of cake

[Dave Taht]

On Wed, Mar 18, 2015 at 8:20 AM, Jonathan Morton <chromatix99 at gmail.com>
wrote:

>
> > On 18 Mar, 2015, at 17:10, Kathleen Nichols <nichols at pollere.com> wrote:
> >
> > How are you relating target delay to bandwidth?
>
> Essentially, I use 5ms as a minimum, and increase it if necessary to
> accommodate a couple of MTU-sized packets at the shaping rate.  This keeps
> things nicely under control at low bandwidths, and I find that cake remains
> useful and usable even at 64Kbps (without making even the usual adjustments
> to host or link configuration for such low speeds).
>

In the cake2 (or maybe it was the unpublished cake3) version, I had a
lighter weight version of the codel algorithm, that did not have a target
parameter at all. Instead it just took the interval parameter and shifted
it
right 4 (yielding a target of 6.xms from an interval of 100ms)

This saves on a memory access (and storage per queue!) , and I felt that
any differences in behavior would be unnoticeable. And they were. This is
also above the bound for cable-modem media access that greg white (rightly
or wrongly) believed existed. So I have no problem in eliminating "target"
entirely.

Cake (without bandwidth shaping engaged) uses more cpu than fq_codel did
and this was one of many optimizations I'd attempted (or successfully
added). Cake with shaping is a bit less cpu than sqm-scripts htb + fq_codel
+ filters.

It also looked like cake could be poured into gates, with a bit more
research, and testing.


> I can do this in cake because the shaping rate is known, whereas the pure
> codel and fq_codel qdiscs do not have reliable link-speed information.


As for this bit, we seemed to need to account for a MTU's worth of data at
the lower speeds, and I did not explore what fiddling with the interval and
auto-calc-ing the target did at these speeds, as yet.


> - Jonathan Morton
>
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-- 
Dave Täht
Let's make wifi fast, less jittery and reliable again!

https://plus.google.com/u/0/107942175615993706558/posts/TVX3o84jjmb
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