From: Arjan van de Ven <arjan@linux.intel.com>
To: paulmck@linux.vnet.ibm.com
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>,
Linux PM <linux-pm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Rafael Wysocki <rafael.j.wysocki@intel.com>,
Len Brown <len.brown@intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@elte.hu>,
Zhang Rui <rui.zhang@intel.com>, Rob Landley <rob@landley.net>
Subject: Re: [PATCH 3/3] PM: Introduce Intel PowerClamp Driver
Date: Tue, 13 Nov 2012 14:45:11 -0800 [thread overview]
Message-ID: <50A2CD77.7000403@linux.intel.com> (raw)
In-Reply-To: <20121113222350.GH2489@linux.vnet.ibm.com>
On 11/13/2012 2:23 PM, Paul E. McKenney wrote:
> On Tue, Nov 13, 2012 at 01:39:22PM -0800, Jacob Pan wrote:
>> On Tue, 13 Nov 2012 13:16:02 -0800
>> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>>
>>>> Please refer to Documentation/thermal/intel_powerclamp.txt for more
>>>> details.
>>>
>>> If I read this correctly, this forces a group of CPUs into idle for
>>> about 600 milliseconds at a time. This would indeed delay grace
>>> periods, which could easily result in user complaints. Also, given
>>> the default RCU_BOOST_DELAY of 500 milliseconds in kernels enabling
>>> RCU_BOOST, you would see needless RCU priority boosting.
>>>
>> the default idle injection duration is 6ms. we adjust the sleep
>> interval to ensure idle ratio. So the idle duration stays the same once
>> set. So would it be safe to delay grace period for this small amount in
>> exchange for less over head in each injection period?
>
> Ah, 6ms of delay is much better than 600ms. Should be OK (famous last
> words!).
well... power clamping is not "free".
You're going to lose performance as a trade off for dropping instantaneous power consumption....
in the measurements we've done comparing various methods.. this one is doing remarkably well.
>
> For most kernel configuration options, it does use softirq. And yes,
> the kthread you are using would yield to softirqs -- but only as long
> as softirq processing hasn't moved over to ksoftirqd. Longer term,
> RCU will be moving from softirq to kthreads, though, and these might be
> prempted by your powerclamp kthread, depending on priorities. It looks
> like you use RT prio 50, which would usually preempt the RCU kthreads
> (unless someone changed the priorities).
we tried to pick a "middle of the road" value, so that usages that really really
want to run, still get to run, but things that are more loose about it, get put on hold.
>
>>> It looks like you could end up with part of the system powerclamped
>>> in some situations, and with all of it powerclamped in other
>>> situations. Is that the case, or am I confused?
>>>
>> could you explain the part that is partially powerclamped?
>
> Suppose that a given system has two sockets. Are the two sockets
> powerclamped independently, or at the same time? My guess was the
> former, but looking at the code again, it looks like the latter.
> So it is a good thing I asked, I guess. ;-)
they are clamped together, and they have to.
you don't get (on the systems where this driver works) any "package" C state unless
all packages are idle completely.
And it's these package C states where the real deep power savings happen, that's
why they are such a juicy target for power clamping ;-)
next prev parent reply other threads:[~2012-11-13 22:45 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-11-12 22:03 [PATCH 0/3] pm: Intel powerclamp driver Jacob Pan
2012-11-12 22:03 ` [PATCH 1/3] tick: export nohz tick idle symbols for module use Jacob Pan
2012-11-12 22:03 ` [PATCH 2/3] x86/nmi: export local_touch_nmi() symbol for modules Jacob Pan
2012-11-12 22:03 ` [PATCH 3/3] PM: Introduce Intel PowerClamp Driver Jacob Pan
2012-11-13 6:33 ` Joe Perches
2012-11-13 6:55 ` Jacob Pan
2012-11-13 21:16 ` Paul E. McKenney
2012-11-13 21:39 ` Jacob Pan
2012-11-13 22:23 ` Paul E. McKenney
2012-11-13 22:45 ` Arjan van de Ven [this message]
2012-11-13 23:02 ` Rafael J. Wysocki
2012-11-14 0:03 ` Paul E. McKenney
2012-11-14 0:03 ` Paul E. McKenney
2012-11-14 0:08 ` Arjan van de Ven
2012-11-14 1:14 ` Jacob Pan
2012-11-14 1:34 ` Paul E. McKenney
2012-11-14 2:59 ` Arjan van de Ven
2012-11-15 3:22 ` Paul E. McKenney
2012-11-14 1:24 ` Jacob Pan
2012-11-13 21:56 ` Arjan van de Ven
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