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

[Juri Lelli]

On 09/06/26 18:23, Yuri Andriaccio wrote:
> Hi Juri,
> 
> Thanks for looking into this.
> 
> > I started playing with the new interface and ended up with the following
> >
> > bash-5.3# cat cpu.rt.max  (root)
> > 10000 100000
> > bash-5.3# cat g1/cpu.rt.max
> > 10000 100000
> > bash-5.3# cat g1/cpu.rt.internal
> > 9999 100000
> >
> > which looks odd to me, as nothing is running on g1 yet and no children
> > groups either. Maybe a rounding error of some kind?
> 
> You are right. I should have mentioned that it is just a rounding error that
> occurs when converting from a bandwidth value to a runtime value. This
> happens because the tg_rt_internal_bandwidth() function truncates the value
> when transforming the runtime from nanoseconds to micros. Rounding could be
> used here to report a more accurate value.
> 
> This same issue is probably found in the from_ratio() function, which has a
> similar truncation issue when converting from bandwidth to runtime, but
> since it is working in the nanoseconds range it might not be that big of a
> problem. The value from from_ratio() is used for the setup of the dl_servers
> even when the children bw is zero, so maybe it is possible to add a special
> case?
> 
> Anyways, as it is right now, the cpu.rt.internal may have only a +1/-1us
> error in reporting the actual used values, while the error for the runtime
> value used internally to setup the dl_servers is in the range of tens of
> nanoseconds.

Not a huge problem per se, but it will raise some eyebrows (and generate
questions) if we leave things as is, I fear.

I wonder if, instead of converting to bandwidth ratios and back (losing
precision in both directions), we can compute children's runtime sum directly
in nanoseconds. For children with different periods, we can maybe normalize
(128-bit intermediate?). Parent's internal runtime is then a simple exact
subtraction: parent_runtime - children_runtime_sum. This should reduce
precision loss from double conversions. Also, as you suggest as well, apply
rounding when displaying to user.