Messages in this thread | | | Date | Mon, 24 Sep 2018 18:19:17 +0100 | From | Patrick Bellasi <> | Subject | Re: [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS_ADMIN by default |
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On 24-Sep 18:26, Peter Zijlstra wrote: > On Mon, Sep 24, 2018 at 04:14:00PM +0100, Patrick Bellasi wrote: > > > ... still it's difficult to give a precise definition of knee point, > > unless you know about platforms which have a sharp change in energy > > efficiency. > > > > The only cases we know about are those where: > > > > A) multiple frequencies uses the same voltage, e.g. > >
On a side note, the following plots represents ee^-1, or eventually, the P on the y axise... my bad.... but you got the meaning anyway ;)
> > > > ^ * > > | Energy O > > | efficiency O+ > > | O | > > | O* | > > | O** | > > | O** O*** | > > | + O** O**** | > > | | O** O***** | > > | | O** | > > | | + | > > | | Same V | Increasing V | > > +---+----------+----------------------+-----------> > > | | | Frequency > > L M H > > > > B) there is a big frequency gap between low frequency OPPs and high > > frequency OPPs, e.g. > > > > O > > ^ **+ > > | Energy ** | > > | efficiency ** | > > | ** | > > | ** | > > | ** | > > | ** | > > | ** | > > | O** | > > | O******+ | > > |O******* | | > > | | | > > ++--------------+------------------+------> > > | | | Frequency > > L M H > > > > > > In case A, all the OPPs left of M are dominated by M in terms > > of energy efficiency and normally they should be never used. > > Unless you are under thermal constraints and you still want to keep > > your code running even if at a lower rate and energy efficiency. > > At this point, however, you already invalidated all the OPPs right of > > M and, on the remaining, you still struggle do define the knee point. > > > > In case B... I'm wondering it such a conf even makes sense ;) > > Is there really some platform out there with such a "non homogeneously > > distributed" set of available frequencies ? > > Well, the curve is a second or third order polynomial (when V~f -> fV^2 > -> f^3), so it shoots up at some point. There's not really anything you > can do about that. But if you're willing to put in active cooling and > lots of energy, you can make it go fast :-)
Sure... until you don't melt the silicon you can push the frequency.
However, if you are going for such aggressive active cooling, perhaps your interest for energy efficiency it's already a very low priority goal.
> Therefore I was thinking: > > > Maybe we can define a threshold > > for a "EE derivative ratio", but it will still be quite arbitrary. > > Because up until de/df=.5 we gain more performance than we loose ee.
You mean up until de < df ?
IOW... the threshold should be de == df => 45deg tangent ?
> But I might not have appreciated the fact that when we work with > imaginary cost units that skews the .5.
The main skew IMO comes from the fact the energy efficiency "tipping point" is very much application / user specific... and it can also change depending on the usage scenario for the same user and platform.
-- #include <best/regards.h>
Patrick Bellasi
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