Message-Id: <1240826954.8216.8.camel@twins>
Date:	Mon, 27 Apr 2009 12:09:14 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vatsa <vatsa@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Gregory Haskins <gregory.haskins@...il.com>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arun Bharadwaj <arun@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v1 0/3] Saving power by cpu evacuation using
 sched_mc=n

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.

Provide a knob that provides max_thermal_capacity, and schedule
accordingly.

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.

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

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

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