Messages in this thread | | | Date | Tue, 13 Nov 2012 14:45:11 -0800 | From | Arjan van de Ven <> | Subject | Re: [PATCH 3/3] PM: Introduce Intel PowerClamp Driver |
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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 ;-)
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