[Cake] More on 'target' corner cases - rate/target/interval confusion?

Jonathan Morton chromatix99 at gmail.com
Mon Nov 16 07:41:43 EST 2015

> Video & Voice can gang together to claim 3/4 bandwidth
> still leaving 1/4ish to be battled between bulk & best effort…

That’s the intended behaviour of the current settings, since Voice borrows all the bandwidth it uses from Video’s allocation.

With Video reduced to a ½ allocation, Video + Voice should also be limited to ½ in total if there is competing saturating traffic.  It’s possible that you haven’t altered the “bandwidth balance” DRR weights to match the new allocations, which might cause such an artefact.

>> The tin-specific tuning of target and interval was introduced when Cake had a separate hard shaper per tin.  It was the obvious design at the time. Now that Cake uses soft shaping between tins (allowing any tin to use the full link bandwidth if uncontended), it’s possible that choosing identical target and interval for all tins might suffice.  Alternatively, we might choose an even more conservative strategy.
>> But which - and how do we decide?

> I've no idea!  I tried the following experiment:  set ingress & egress
> b/widths to 2mbit & 448kbit respectively (like my parents adsl...on a
> good day)  Ran a 'rrul8_splittin' and noted ping box_plot for each tin -
> EF was about 30ms higher (120ms) than CS3 (90ms)  Thinking I'd found a
> smoking gun and that 120ms was quite close to the EF target of 140ms I
> tweaked code to copy tin 0 target across all tins .....  it barely made any
> difference at all!

Since you’re using saturating traffic in all tins, Voice will be operating in “deprioritised” mode to penalise this abuse, so a higher latency is entirely expected.  If you delete the EF bulk flows but leave the latency measurements in place, you should see a big latency advantage.

As I tried to explain before, the Codel settings affect latency and throughput *within* flows rather than *between* them.  Since you are presently measuring using a between-flows methodology, it’s not surprising that you saw no difference.

 - Jonathan Morton

More information about the Cake mailing list