Re: [RFC PATCH v6 00/25] Hierarchical Constant Bandwidth Server

[luca abeni]

Hi Juri,

very interesting demo, thanks!

On Tue, 23 Jun 2026 12:15:08 +0200
Juri Lelli <juri.lelli@xxxxxxxxxx> wrote:
[...]
>  - At 1ms task periods, the dl-server period is the critical tuning
>    parameter, less the bandwidth. A 10ms dl-server with 60% bandwidth
>    caused ~10% miss rates because the worst-case throttle gap (4ms)
>    spanned multiple 1ms deadlines. Switching to a 2ms dl-server period
>    at just 30% bandwidth eliminated all misses.
> 
>  - A simple Rule of thumb might be to set the dl-server period to at
>    most 2x the shortest task period in the cgroup (e.g., 2ms dl-server
>    for 1ms tasks, 10ms for 10ms tasks). Would you (and Luca?) agree or
>    would you suggest something different?

With one single RT task in the cgroup (or with multiple synchronized RT
tasks having the same period), I agree... Technically, the cgroup period
P should be such that P - Q = T - WCET (where "Q" is the cgroup's
runtime and "T" is the period of the task), but to see missed deadlines
you need a relevant competing deadline (or HCBS) workload.

So, yes, I agree with your findings above.

If we consider multi-core analysis, or multiple RT threads with
different, non synchronized, periods, then analysis tool by Yuri
(leveraging CSF analysis from real-time literature) is needed... But
that is pretty pessimistic. The rule you suggest above is a better
starting point in practical situations.



				Luca