Re: [PATCH v5 00/10] track CPU utilization

luca abeni
2018-06-06T21:53:23Z

Hi all,

sorry; I missed the beginning of this thread... Anyway, below I add
some comments:

On Wed, 6 Jun 2018 15:05:58 +0200
Claudio Scordino <claudio@evidence.eu.com> wrote:
[...]
> >> Ok, I see ... Have you guys already tried something like my patch
> >> above (keeping the freq >= this_bw) in real world use cases ? Is
> >> this costing that much energy in practice ? If we fill the gaps
> >> left by DL (when it
> >
> > IIRC, Claudio (now Cc-ed) did experiment a bit with both
> > approaches, so he might add some numbers to my words above. I
> > didn't (yet). But, please consider that I might be reserving (for
> > example) 50% of bandwidth for my heavy and time sensitive task and
> > then have that task wake up only once in a while (but I'll be
> > keeping clock speed up for the whole time). :/
>
> As far as I can remember, we never tested energy consumption of
> running_bw vs this_bw, as at OSPM'17 we had already decided to use
> running_bw implementing GRUB-PA. The rationale is that, as Juri
> pointed out, the amount of spare (i.e. reclaimable) bandwidth in
> this_bw is very user-dependent.

Yes, I agree with this. The appropriateness of using this_bw or
running_bw is highly workload-dependent... If a periodic task consumes
much less than its runtime (or if a sporadic task has inter-activation
times much larger than the SCHED_DEADLINE period), then running_bw has
to be preferred. But if a periodic task consumes almost all of its
runtime before blocking, then this_bw has to be preferred...

But this also depends on the hardware: if the frequency switch time is
small, then running_bw is more appropriate... On the other hand,
this_bw works much better if the frequency switch time is high.
(Talking about this, I remember Claudio measured frequency switch times
large almost 3ms... Is this really due to hardware issues? Or maybe
there is some software issue invoved? 3ms look like a lot of time...)

Anyway, this is why I proposed to use some kind of /sys/... file to
control the kind of deadline utilization used for frequency scaling: in
this way, the system designer / administrator, who hopefully has the
needed information about workload and hardware, can optimize the
frequency scaling behaviour by deciding if running_bw or this_bw will be
used.

Luca

> For example, the user can let this_bw
> be much higher than the measured bandwidth, just to be sure that the
> deadlines are met even in corner cases. In practice, this means that
> the task executes for quite a short time and then blocks (with its
> bandwidth reclaimed, hence the CPU frequency reduced, at the 0lag
> time). Using this_bw rather than running_bw, the CPU frequency would
> remain at the same fixed value even when the task is blocked. I
> understand that on some cases it could even be better (i.e. no waste
> of energy in frequency switch). However, IMHO, these are corner cases
> and in the average case it is better to rely on running_bw and reduce
> the CPU frequency accordingly.
>
> Best regards,
>
> Claudio