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SubjectRe: [RFC PATCH v1 0/3] Saving power by cpu evacuation using sched_mc=n
* Peter Zijlstra <a.p.zijlstra@chello.nl> [2009-04-27 12:09:14]:

> On Mon, 2009-04-27 at 02:16 +0530, Vaidyanathan Srinivasan wrote:
> > Hi,
> >
> > The sched_mc_powersavings tunable can be set to {0,1,2} to enable
> > aggressive task consolidation to less number of cpu packages and save
> > power. Under certain conditions, sched_mc=2 may provide better
> > performance in a underutilised system by keeping the group of tasks on
> > a single cpu package facilitating cache sharing and reduced off-chip
> > traffic.
> >
> > Extending this concept further, the following patch series tries to
> > implement sched_mc={3,4,5} where CPUs/cores are forced to be idle and
> > thereby save power at the cost of performance. Some of the cpu
> > packages in the system are overloaded with tasks while other packages
> > can have free cpus. This patch is a hack to discuss the idea and
> > requirements.
> >
> > Objective:
> > ----------
> >
> > * Framework to evacuate tasks from cpus in order to force the cpu
> > cores to stay at idle
> >
> > * Interrupts can be moved using user space irqbalancer daemons, while
> > timer migration framework is being discussed:
> > http://lkml.org/lkml/2009/4/16/45
> >
> > * Forcefully idling cpu cores in a system will reduce the power
> > consumption of the system and also cool cpu packages for thermal
> > management
> >
> > Requirements:
> > ------------
> >
> > * Fast response time and low OS overhead to moved tasks away from
> > selected cpu packages. CPU hotplug is too heavyweight for this
> > purpose
> >
> > Use cases:
> > ---------
> >
> > * Enabling the right number of cpus to run the given workload can
> > provide good power vs performance tradeoffs.
> >
> > * Ability to throttle the number of cores uses in the system along
> > with other power saving controls like cpufreq governors can enable
> > the system to operate at a more power efficient operating point and
> > still meet the design objectives.
> >
> > * Facilitate thermal management by evacuating cores from hot cpu packages
> >
> > Alternatives:
> > -------------
> >
> > * CPU hotplug: Heavy weight and slow. Setting up and tear down of
> > data structures involved. May need new fast or light weight
> > notifications
> >
> > * CPUSets: Exclusive CPU sets and partitioned sched domains involve
> > rebuilding sched domains and relatively heavy weight for the purpose
> >
> > The following patch is against 2.6.30-rc3 and will work only in
> > an under utilised system (Tasks <= number of cores).
> >
> > Test results for ebizzy 8 threads at various sched_mc settings has been
> > summarised with relative values below. The test platform is dual socket
> > quad core x86 system (pre-Nehalem).
> >
> > --------------------------------------------------------
> > sched_mc No Cores Performance AvgPower
> > used Records/sec (Watts)
> > --------------------------------------------------------
> > 0 8 1.00x 1.00y
> > 1 8 1.02x 1.01y
> > 2 8 0.83x 1.01y
> > 3 7 0.86x 0.97y
> > 4 6 0.76x 0.92y
> > 5 4 0.72x 0.82y
> > --------------------------------------------------------
> >
> > There were wide run variation with ebizzy. The purpose of the above
> > data is to justify use of core evacuation for power vs performance
> > trade-offs.
> >
> > ToDo:
> > -----
> >
> > * Make the core evacuation predictable under different system load
> > conditions and workload characteristics
> > * Enhance framework to control which packages/cores will be
> > evacuated, this is needed for thermal management
>
>
> I think this is going about it the wrong way.
>
> The whole thing seems to be targeted at thermal management, not power
> saving. Therefore using the power saving stuff is backwards.

The framework is useful for power savings and thermal management.
Actually we can generalise this a framework to throttle cores.

Power savings need only core evacuation, kernel can decide the most
optimum cores to evacuate for best power savings. While in thermal
management we will additional need a 'vector' parameter to direct the
load to different parts of the system and level the heat generated.

> Provide a knob that provides max_thermal_capacity, and schedule
> accordingly.

Yes, we can pick a generic name and use this as a function of total
system capacity to indicate number of cores to evacuate.

> FWIW I utterly hate these force idle things because they cause the
> scheduler to become non-work conserving, but I have to concede that
> software will likely be more suited to handle the thermal overload issue
> than hardware will ever be -- so for that use case I'm willing to go
> along.

Yes, I agree with your opinion. However if we can come up with
a clean framework to take cores out of scheduler's view, then the work
conserving nature of the scheduler can be preserved on the sub-set of
cores. Inserting idle states is more intrusive than leaving out full
cores.

> Also, the user interface should be that single thermal capacity knob,
> more fine grained control is undesired.

For power savings, a single evacuation knob will do. While for
thermal we will need additional parameters to choose the right cores
to evacuate. Some sort of directional/vector parameter.

> Also, before you continue, expand on the interaction with realtime
> processes.

Sure. We will run into complications with respect to realtime
scheduling. You had earlier pointed out a need for variable cpu power
to achieve fairness for non-realtime tasks in the presence of realtime
tasks. We should re-visit that idea.

Thanks for the review comments.

--Vaidy



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