linux-kernel - Re: [RFC v1] Tunable sched_mc_power

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Date:	Thu, 26 Jun 2008 22:17:00 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Andi Kleen <andi@...stfloor.org>, balbir@...ux.vnet.ibm.com,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Vatsa <vatsa@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>
Subject: Re: [RFC v1] Tunable sched_mc_power_savings=n

Vaidyanathan Srinivasan wrote:

Playing devil's advocate here.

> * Andi Kleen <andi@...stfloor.org> [2008-06-26 20:08:41]:
> 
>>> A user could be an application and certain applications can predict their
>>> workload.
>> So you expect the applications to run suid root and change a sysctl?
>> And what happens when two applications run that do that and they have differing
>> requirements? Will they fight over the sysctl?
> 
> System management software and workload monitoring and managing
> software can potentially control the tunable on behalf of the
> applications for best overall power savings and performance.

Does it have the needed information for that? e.g. real time information
on what the system does? I don't think anybody is in a better position
to control that than the kernel.

> Applications with conflicting goals should resolve among themselves.

That sounds wrong to me. Negotiating between conflicting requirements
from different applications is something that kernels are supposed
to do.

> The application with highest performance requirement should win.

That is right, but the kernel can do that based on nice levels
and possibly other information, can't it?

>  The
> power QoS framework set_acceptable_latency() ensures that the lowest
> latency set across the system wins.  

But that only helps kernel drivers, not user space, doesn't it?

> Power management settings affect the entire system.  It may not be
> based on per application priority or nice value.  However if the
> priority of all the applications currently running in the system
> indicate power savings, then the kernel can goto more aggressive power
> saving state.

That's what I meant yes. So if only the file system indexer is running
over night all niced it will run as power efficiently as possible.

> In a small-scale datacenters, peak and off-peak hour settings can be
> potentially done through simple cron jobs.  

Is there any real drawback from only controlling it through nice levels?

Anyways I think the main thing I object to in your proposal is that
your tunable is system global, not per process. I'm also not
sure if a tunable is really a good idea and if the kernel couldn't
do a better job.

-Andi
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